When working with optional values, you can write ? before operations like methods, properties, and subscripting. If the value before the ? is nil, everything after the ? is ignored and the value of the whole expression is nil. Otherwise, the optional value is unwrapped, and everything after the ? acts on the unwrapped value. In both cases, the value of the whole expression is an optional value.

let optionalSquare: Square? = Square(sideLength: 2.5, name: "optional square") let sideLength = optionalSquare?.sideLength

Enumerations and Structures

Use enum to create an enumeration. Like classes and all other named types, enumerations can have methods associated with them.

enum Rank: Int { case Ace = 1 case Two, Three, Four, Five, Six, Seven, Eight, Nine, Ten case Jack, Queen, King func simpleDescription() -> String { switch self { case .Ace:

return "ace" case .Jack: return "jack" case .Queen:

return "queen" case .King:

return "king" default:

return String(self.toRaw())

}

e = Rank.Ace eRawValue = ace.toRaw()

EXPERIMENT

Write a function that compares two Rank values by comparing their raw values.

In the example above, the raw value type of the enumeration is Int, so you only have to specify the first raw value. The rest of the raw values are assigned in order. You can also use strings or floating-point numbers as the raw type of an enumeration.

Use the toRaw and fromRaw functions to convert between the raw value and the enumeration value.

if let convertedRank = Rank.fromRaw(3) { let threeDescription = convertedRank.simpleDescription()

}

The member values of an enumeration are actual values, not just another way of writing their raw values. In fact, in cases where there isn锟斤拷t a meaningful raw value, you don锟斤拷t have to provide one.

enum Suit { case Spades, Hearts, Diamonds, Clubs func simpleDescription() -> String { switch self { case .Spades:

return "spades" case .Hearts:

return "hearts" case .Diamonds: return "diamonds" case .Clubs:

return "clubs"

}

arts = Suit.Hearts artsDescription = hearts.simpleDescription()

EXPERIMENT

Add a color method to Suit that returns 锟斤拷black锟斤拷 for spades and clubs, and returns 锟斤拷red锟斤拷 for hearts and diamonds.

Notice the two ways that the Hearts member of the enumeration is referred to above: When assigning a value to the hearts constant, the enumeration member Suit.Hearts is referred to by its full name because the constant doesn锟斤拷t have an explicit type specified. Inside the switch, the enumeration is referred to by the abbreviated form .Hearts because the value of self is already known to be a suit. You can use the abbreviated form anytime the value锟斤拷s type is already known.

Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.

struct Card { var rank: Rank var suit: Suit func simpleDescription() -> String { return "The \(rank.simpleDescription()) of \(suit.simpleDescription())" }

} let threeOfSpades = Card(rank: .Three, suit: .Spades) let threeOfSpadesDescription = threeOfSpades.simpleDescription()

EXPERIMENT

Add a method to Card that creates a full deck of cards, with one card of each combination of rank and suit.

An instance of an enumeration member can have values associated with the instance. Instances of the same enumeration member can have different values associated with them. You provide the associated values when you create the instance. Associated values and raw values are different: The raw value of an enumeration member is the same for all of its instances, and you provide the raw value when you define the enumeration.

For example, consider the case of requesting the sunrise and sunset time from a server. The server either responds with the information or it responds with some error information.

enum ServerResponse {

case Result(String, String) case Error(String)

}

let success = ServerResponse.Result("6:00 am", "8:09 pm") let failure = ServerResponse.Error("Out of cheese.")

switch success { let .Result(sunrise, sunset):

serverResponse = "Sunrise is at \(sunrise) and sunset is at \(sunset)." let .Error(error): serverResponse = "Failure... \(error)"

EXPERIMENT

Add a third case to ServerResponse and to the switch.

Notice how the sunrise and sunset times are extracted from the ServerResponse value as part of matching the value against the switch cases.

Protocols and Extensions

Use protocol to declare a protocol.

protocol ExampleProtocol { var simpleDescription: String { get } mutating func adjust()

}

Classes, enumerations, and structs can all adopt protocols.

class SimpleClass: ExampleProtocol { var simpleDescription: String = "A very simple class." var anotherProperty: Int = 69105 func adjust() { simpleDescription += " Now 100% adjusted."

}

var a = SimpleClass()

a.adjust()

Description = a.simpleDescription

SimpleStructure: ExampleProtocol { r simpleDescription: String = "A simple structure" utating func adjust() { simpleDescription += " (adjusted)"

= SimpleStructure() ust()

Description = b.simpleDescription

EXPERIMENT

Write an enumeration that conforms to this protocol.

Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a method that modifies the structure. The declaration of SimpleClass doesn锟斤拷t need any of its methods marked as mutating because methods on a class can always modify the class.

Use extension to add functionality to an existing type, such as new methods and computed properties. You can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that you imported from a library or framework.

extension Int: ExampleProtocol { var simpleDescription: String { return "The number \(self)"

} mutating func adjust() { self += 42

}

7.simpleDescription

EXPERIMENT

Write an extension for the Double type that adds an absoluteValue property.

You can use a protocol name just like any other named type锟斤拷for example, to create a collection of objects that have different types but that all conform to a single protocol. When you work with values whose type is a protocol type, methods outside the protocol definition are not available.

let protocolValue: ExampleProtocol = a protocolValue.simpleDescription

// protocolValue.anotherProperty // Uncomment to see the error

Even though the variable protocolValue has a runtime type of SimpleClass, the compiler treats it as the given type of ExampleProtocol. This means that you can锟斤拷t accidentally access methods or properties that the class implements in addition to its protocol conformance.

Generics

Write a name inside angle brackets to make a generic function or type.

func repeat<ItemType>(item: ItemType, times: Int) -> ItemType[] { var result = ItemType[]() for i in 0..times { result += item

} return result

} repeat("knock", 4)

You can make generic forms of functions and methods, as well as classes, enumerations, and structures.

// Reimplement the Swift standard library's optional type enum OptionalValue<T> { case None case Some(T)

} var possibleInteger: OptionalValue<Int> = .None possibleInteger = .Some(100)

Use where after the type name to specify a list of requirements锟斤拷for example, to require the type to implement a protocol, to require two types to be the same, or to require a class to have a particular superclass.

func anyCommonElements <T, U where T: Sequence, U: Sequence, T.GeneratorType.Element: Equatable,

T.GeneratorType.Element == U.GeneratorType.Element> (lhs: T, rhs: U) -> Bool {

for lhsItem in lhs { for rhsItem in rhs { if lhsItem == rhsItem { return true

}

} return false

ommonElements([1, 2, 3], [3])

EXPERIMENT

Modify the anyCommonElements function to make a function that returns an array of the elements that any two sequences have in common.

In the simple cases, you can omit where and simply write the protocol or class name after a colon. Writing <T: Equatable> is the same as writing <T where T: Equatable>.

Language Guide

The Basics

Swift is a new programming language for iOS and OS X app development. Nonetheless, many parts of Swift will be familiar from your experience of developing in C and Objective-C.

Swift provides its own versions of all fundamental C and Objective-C types, including Int for integers; Double and Float for floating-point values; Bool for Boolean values; and String for textual data. Swift also provides powerful versions of the two primary collection types, Array and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values cannot be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that do not need to change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C. These include tuples, which enable you to create and pass around groupings of values. Tuples can return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say either 锟斤拷there is a value, and it equals x锟斤拷 or 锟斤拷there isn锟斤拷t a value at all锟斤拷. Optionals are similar to using nil with pointers in Objective-C, but they work for any type, not just classes. Optionals are safer and more expressive than nil pointers in Objective-C and are at the heart of many of Swift锟斤拷s most powerful features.

Optionals are an example of the fact that Swift is a type safe language. Swift helps you to be clear about the types of values your code can work with. If part of your code expects a String, type safety prevents you from passing it an Int by mistake. This enables you to catch and fix errors as early as possible in the development process.

Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or welcomeMessage) with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant cannot be changed once it is set, whereas a variable can be set to a different value in the future.

Declaring Constants and Variables

Constants and variables must be declared before they are used. You declare constants with the let keyword and variables with the var keyword. Here锟斤拷s an example of how constants and variables can be used to track the number of login attempts a user has made:

let maximumNumberOfLoginAttempts = 10 var currentLoginAttempt = 0

This code can be read as:

锟斤拷Declare a new constant called maximumNumberOfLoginAttempts, and give it a value of 10. Then, declare a new variable called currentLoginAttempt, and give it an initial value of 0.锟斤拷

In this example, the maximum number of allowed login attempts is declared as a constant, because the maximum value never changes. The current login attempt counter is declared as a variable, because this value must be incremented after each failed login attempt.

You can declare multiple constants or multiple variables on a single line, separated by commas:

var x = 0.0, y = 0.0, z = 0.0

NOTE

If a stored value in your code is not going to change, always declare it as a constant with the let keyword. Use variables only for storing values that need to be able to change.

Type Annotations

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

var welcomeMessage: String

The colon in the declaration means 锟斤拷锟斤拷of type锟斤拷,锟斤拷 so the code above can be read as:

锟斤拷Declare a variable called welcomeMessage that is of type String.锟斤拷

The phrase 锟斤拷of type String锟斤拷 means 锟斤拷can store any String value.锟斤拷 Think of it as meaning 锟斤拷the type of thing锟斤拷 (or 锟斤拷the kind of thing锟斤拷) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

welcomeMessage = "Hello"

NOTE

It is rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it is defined, Swift can almost always infer the type to be used for that constant or variable, as described in Type Safety and Type Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.

Naming Constants and Variables

You can use almost any character you like for constant and variable names, including Unicode characters:

let 锟斤拷 = 3.14159

let 锟斤拷锟� = "锟斤拷锟斤拷锟斤拷锟�" let = "dogcow"

Constant and variable names cannot contain mathematical symbols, arrows, private-use (or invalid) Unicode code points, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be included elsewhere within the name.

Once you锟斤拷ve declared a constant or variable of a certain type, you can锟斤拷t redeclare it again with the same name, or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a constant.

NOTE

If you need to give a constant or variable the same name as a reserved Swift keyword, you can do so by surrounding the keyword with back ticks (`) when using it as a name. However, you should avoid using keywords as names unless you have absolutely no choice.

You can change the value of an existing variable to another value of a compatible type. In this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

var friendlyWelcome = "Hello!" friendlyWelcome = "Bonjour!"

// friendlyWelcome is now "Bonjour!"

Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do so is reported as an error when your code is compiled:

let languageName = "Swift" languageName = "Swift++"

// this is a compile-time error - languageName cannot be changed

Printing Constants and Variables

You can print the current value of a constant or variable with the println function:

println(friendlyWelcome)

// prints "Bonjour!"

println is a global function that prints a value, followed by a line break, to an appropriate output. If you are working in Xcode, for example, println prints its output in Xcode锟斤拷s

锟斤拷console锟斤拷 pane. (A second function, print, performs the same task without appending a line break to the end of the value to be printed.)

The println function prints any String value you pass to it:

println("This is a string")

// prints "This is a string"

The println function can print more complex logging messages, in a similar manner to Cocoa锟斤拷s NSLog function. These messages can include the current values of constants and variables.

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string, and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

println("The current value of friendlyWelcome is \(friendlyWelcome)") // prints "The current value of friendlyWelcome is Bonjour!"

NOTE

All options you can use with string interpolation are described in String Interpolation.

Comments

Use comments to include non-executable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

// this is a comment

You can also write multiline comments, which start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

/* this is also a comment, but written over multiple lines */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

/* this is the start of the first multiline comment /* this is the second, nested multiline comment */ this is the end of the first multiline comment */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.

Semicolons

Unlike many other languages, Swift does not require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. Semicolons are required, however, if you want to write multiple separate statements on a single line:

let cat = " "; println(cat)

// prints " "

Integers

Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero).

Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming convention similar to C, in that an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names.

Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8 let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.

Int

In most cases, you don锟斤拷t need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform锟斤拷s native word size:

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.

UInt

Swift also provides an unsigned integer type, UInt, which has the same size as the current platform锟斤拷s native word size:

NOTE

Use UInt only when you specifically need an unsigned integer type with the same size as the platform锟斤拷s native word size. If this is not the case, Int is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types, and matches integer type inference, as described in Type Safety and Type Inference.

Floating-Point Numbers

Floating-point numbers are numbers with a fractional component, such as 3.14159, 0.1, and

-273.15.

Floating-point types can represent a much wider range of values than integer types, and can store numbers that are much larger or smaller than can be stored in an Int. Swift provides two signed floating-point number types:

NOTE

Double has a precision of at least 15 decimal digits, whereas the precision of Float can be as little as 6 decimal digits. The appropriate floating-point type to use depends on the nature and range of values you need to work with in your code.

Type Safety and Type Inference

Swift is a type safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can锟斤拷t pass it an Int by mistake.

Because Swift is type safe, it performs type checks when compiling your code and flags any mismatched types as errors. This enables you to catch and fix errors as early as possible in the development process.

Type-checking helps you avoid errors when you锟斤拷re working with different types of values. However, this doesn锟斤拷t mean that you have to specify the type of every constant and variable that you declare. If you don锟斤拷t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

let meaningOfLife = 42

// meaningOfLife is inferred to be of type Int

Likewise, if you don锟斤拷t specify a type for a floating-point literal, Swift infers that you want to create a Double:

let pi = 3.14159

// pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

let anotherPi = 3 + 0.14159

// anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

Numeric Literals

Integer literals can be written as:

All of these integer literals have a decimal value of 17:

let decimalInteger = 17 let binaryInteger = 0b10001 // 17 in binary notation let octalInteger = 0o21 // 17 in octal notation let hexadecimalInteger = 0x11 // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. They can also have an optional exponent, indicated by an uppercase or lowercase e for decimal floats, or an uppercase or lowercase p for hexadecimal floats.

For decimal numbers with an exponent of exp, the base number is multiplied by 10^exp:

For hexadecimal numbers with an exponent of exp, the base number is multiplied by 2^exp:

All of these floating-point literals have a decimal value of 12.1875:

let decimalDouble = 12.1875 let exponentDouble = 1.21875e1 let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeroes and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

let paddedDouble = 000123.456 let oneMillion = 1_000_000 let justOverOneMillion = 1_000_000.000_000_1

Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they are known to be non-negative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they are are specifically needed for the task at hand, because of explicitly-sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly-sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that will not fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

let cannotBeNegative: UInt8 = -1

// UInt8 cannot store negative numbers, and so this will report an error let tooBig: Int8 = Int8.max + 1

// Int8 cannot store a number larger than its maximum value,

// and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They cannot be added together directly, because they are not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

let twoThousand: UInt16 = 2_000 let one: UInt8 = 1 let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it is the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can锟斤拷t pass in any type here, however锟斤拷it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

let three = 3 let pointOneFourOneFiveNine = 0.14159 let pi = Double(three) + pointOneFourOneFiveNine

// pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

The reverse is also true for floating-point to integer conversion, in that an integer type can be initialized with a Double or Float value:

let integerPi = Int(pi)

// integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

NOTE

The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159, because number literals do not have an explicit type in and of themselves. Their type is inferred only at the point that they are evaluated by the compiler.

Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size from an external source:

typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

var maxAmplitudeFound = AudioSample.min

// maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it is an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.

Booleans

Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

let orangesAreOrange = true let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don锟斤拷t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more concise and readable when it initializes constants or variables with other values whose type is already known.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

if turnipsAreDelicious { println("Mmm, tasty turnips!")

} else { println("Eww, turnips are horrible.")

}

// prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in Control Flow.

Swift锟斤拷s type safety prevents non-Boolean values from being be substituted for Bool. The following example reports a compile-time error:

let i = 1

if i {

// this example will not compile, and will report an error

}

However, the alternative example below is valid:

let i = 1 if i == 1 {

// this example will compile successfully

}

The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.

Tuples

Tuples group multiple values into a single compound value. The values within a tuple can be of any type and do not have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn锟斤拷t exist.

let http404Error = (404, "Not Found")

// http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as

锟斤拷a tuple of type (Int, String)锟斤拷.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There锟斤拷s nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple锟斤拷s contents into separate constants or variables, which you then access as usual:

let (statusCode, statusMessage) = http404Error println("The status code is \(statusCode)")

// prints "The status code is 404" println("The status message is \(statusMessage)")

// prints "The status message is Not Found"

If you only need some of the tuple锟斤拷s values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

let (justTheStatusCode, _) = http404Error println("The status code is \(justTheStatusCode)")

// prints "The status code is 404"

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

println("The status code is \(http404Error.0)")

// prints "The status code is 404" println("The status message is \(http404Error.1)")

// prints "The status message is Not Found"

You can name the individual elements in a tuple when the tuple is defined:

let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

println("The status code is \(http200Status.statusCode)")

// prints "The status code is 200" println("The status message is \(http200Status.description)") // prints "The status message is OK"

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.

NOTE

Tuples are useful for temporary groups of related values. They are not suited to the creation of complex data structures. If your data structure is likely to persist beyond a temporary scope, model it as a class or structure, rather than as a tuple. For more information, see Classes and Structures.

Optionals

You use optionals in situations where a value may be absent. An optional says: or

NOTE

The concept of optionals doesn锟斤拷t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object, with nil meaning 锟斤拷the absence of a valid object.锟斤拷 However, this only works for objects锟斤拷it doesn锟斤拷t work for structs, basic C types, or enumeration values. For these types, Objective-C methods typically return a special value (such as NSNotFound) to indicate the absence of a value. This approach assumes that the method锟斤拷s caller knows there is a special value to test against and remembers to check for it. Swift锟斤拷s optionals let you indicate the absence of a value for any type at all, without the need for special constants.

Here锟斤拷s an example. Swift锟斤拷s String type has a method called toInt, which tries to convert a String value into an Int value. However, not every string can be converted into an integer.

The string "123" can be converted into the numeric value 123, but the string "hello, world" does not have an obvious numeric value to convert to.

The example below uses the toInt method to try to convert a String into an Int:

let possibleNumber = "123" let convertedNumber = possibleNumber.toInt()

// convertedNumber is inferred to be of type "Int?", or "optional Int"

Because the toInt method might fail, it returns an optional Int, rather than an Int. An optional Int is written as Int?, not Int. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. (It can锟斤拷t contain anything else, such as a Bool value or a String value. It锟斤拷s either an Int, or it锟斤拷s nothing at all.)

If Statements and Forced Unwrapping

You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false.

Once you锟斤拷re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) to the end of the optional锟斤拷s name. The exclamation mark effectively says, 锟斤拷I know that this optional definitely has a value; please use it.锟斤拷 This is known as forced unwrapping of the optional锟斤拷s value:

if convertedNumber { println("\(possibleNumber) has an integer value of \(convertedNumber!)")

} else { println("\(possibleNumber) could not be converted to an integer") }

// prints "123 has an integer value of 123"

For more on the if statement, see Control Flow.

NOTE

Trying to use ! to access a non-existent optional value triggers a runtime error. Always make sure that an optional contains a non-nil value before using ! to force-unwrap its value.

Optional Binding

You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.

Write optional bindings for the if statement as follows:

}

You can rewrite the possibleNumber example from above to use optional binding rather than forced unwrapping:

if let actualNumber = possibleNumber.toInt() { println("\(possibleNumber) has an integer value of \(actualNumber)")

} else { println("\(possibleNumber) could not be converted to an integer") }

// prints "123 has an integer value of 123"

This can be read as:

锟斤拷If the optional Int returned by possibleNumber.toInt contains a value, set a new constant called actualNumber to the value contained in the optional.锟斤拷

If the conversion is successful, the actualNumber constant becomes available for use within the first branch of the if statement. It has already been initialized with the value contained within the optional, and so there is no need to use the ! suffix to access its value. In this example, actualNumber is simply used to print the result of the conversion.

You can use both constants and variables with optional binding. If you wanted to manipulate the value of actualNumber within the first branch of the if statement, you could write if var actualNumber instead, and the value contained within the optional would be made available as a variable rather than a constant.

nil

You set an optional variable to a valueless state by assigning it the special value nil:

var serverResponseCode: Int? = 404

// serverResponseCode contains an actual Int value of 404 serverResponseCode = nil

// serverResponseCode now contains no value

NOTE

nil cannot be used with non-optional constants and variables. If a constant or variable in your code needs to be able to cope with the absence of a value under certain conditions, always declare it as an optional value of the appropriate type.

If you define an optional constant or variable without providing a default value, the constant or variable is automatically set to nil for you:

var surveyAnswer: String?

// surveyAnswer is automatically set to nil

NOTE

Swift锟斤拷s nil is not the same as nil in Objective-C. In Objective-C, nil is a pointer to a non-existent object. In Swift, nil is not a pointer锟斤拷it is the absence of a value of a certain type. Optionals of any type can be set to nil, not just object types.

Implicitly Unwrapped Optionals

As described above, optionals indicate that a constant or variable is allowed to have 锟斤拷no value锟斤拷. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional锟斤拷s value if it does exist.

Sometimes it is clear from a program锟斤拷s structure that an optional will always have a value, after that value is first set. In these cases, it is useful to remove the need to check and unwrap the optional锟斤拷s value every time it is accessed, because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.

Implicitly unwrapped optionals are useful when an optional锟斤拷s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.

An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a nonoptional value, without the need to unwrap the optional value each time it is accessed. The following example shows the difference in behavior between an optional String and an implicitly unwrapped optional String:

let possibleString: String? = "An optional string." println(possibleString!) // requires an exclamation mark to access its value // prints "An optional string."

let assumedString: String! = "An implicitly unwrapped optional string." println(assumedString) // no exclamation mark is needed to access its value

// prints "An implicitly unwrapped optional string."

You can think of an implicitly unwrapped optional as giving permission for the optional to be unwrapped automatically whenever it is used. Rather than placing an exclamation mark after the optional锟斤拷s name each time you use it, you place an exclamation mark after the optional锟斤拷s type when you declare it.

NOTE

If you try to access an implicitly unwrapped optional when it does not contain a value, you will trigger a runtime error. The result is exactly the same as if you place an exclamation mark after a normal optional that does not contain a value.

You can still treat an implicitly unwrapped optional like a normal optional, to check if it contains a value:

if assumedString {

println(assumedString)

}

// prints "An implicitly unwrapped optional string."

You can also use an implicitly unwrapped optional with optional binding, to check and unwrap its value in a single statement:

if let definiteString = assumedString { println(definiteString)

}

// prints "An implicitly unwrapped optional string."

NOTE

Implicitly unwrapped optionals should not be used when there is a possibility of a variable becoming nil at a later point. Always use a normal optional type if you need to check for a nil value during the lifetime of a variable.

Assertions

Optionals enable you to check for values that may or may not exist, and to write code that copes gracefully with the absence of a value. In some cases, however, it is simply not possible for your code to continue execution if a value does not exist, or if a provided value does not satisfy certain conditions. In these situations, you can trigger an assertion in your code to end code execution and to provide an opportunity to debug the cause of the absent or invalid value.

Debugging with Assertions

An assertion is a runtime check that a logical condition definitely evaluates to true.

Literally put, an assertion 锟斤拷asserts锟斤拷 that a condition is true. You use an assertion to make sure that an essential condition is satisfied before executing any further code. If the condition evaluates to true, code execution continues as usual; if the condition evaluates to false, code execution ends, and your app is terminated.

If your code triggers an assertion while running in a debug environment, such as when you build and run an app in Xcode, you can see exactly where the invalid state occurred and query the state of your app at the time that the assertion was triggered. An assertion also lets you provide a suitable debug message as to the nature of the assert.

You write an assertion by calling the global assert function. You pass the assert function an expression that evaluates to true or false and a message that should be displayed if the result of the condition is false:

let age = -3 assert(age >= 0, "A person's age cannot be less than zero")

// this causes the assertion to trigger, because age is not >= 0

In this example, code execution will continue only if age >= 0 evaluates to true, that is, if the value of age is non-negative. If the value of age is negative, as in the code above, then age >= 0 evaluates to false, and the assertion is triggered, terminating the application.

Assertion messages cannot use string interpolation. The assertion message can be omitted if desired, as in the following example:

assert(age >= 0)

When to Use Assertions

Use an assertion whenever a condition has the potential to be false, but must definitely be true in order for your code to continue execution. Suitable scenarios for an assertion check include:

NOTE

Assertions cause your app to terminate and are not a substitute for designing your code in such a way that invalid conditions are unlikely to arise. Nonetheless, in situations where invalid conditions are possible, an assertion is an effective way to ensure that such conditions are highlighted and noticed during development,

before your app is published.

Basic Operators

An operator is a special symbol or phrase that you use to check, change, or combine values. For example, the addition operator (+) adds two numbers together (as in let i = 1 + 2). More complex examples include the logical AND operator && (as in if enteredDoorCode && passedRetinaScan) and the increment operator ++i, which is a shortcut to increase the value of i by 1.

Swift supports most standard C operators and improves several capabilities to eliminate common coding errors. The assignment operator (=) does not return a value, to prevent it from being mistakenly used when the equal to operator (==) is intended. Arithmetic operators (+, -, *, /, % and so forth) detect and disallow value overflow, to avoid unexpected results when working with numbers that become larger or smaller than the allowed value range of the type that stores them. You can opt in to value overflow behavior by using Swift锟斤拷s overflow operators, as described in Overflow Operators.

Unlike C, Swift lets you perform remainder (%) calculations on floating-point numbers. Swift also provides two range operators (a..b and a...b) not found in C, as a shortcut for expressing a range of values.

This chapter describes the common operators in Swift. Advanced Operators covers Swift锟斤拷s advanced operators, and describes how to define your own custom operators and implement the standard operators for your own custom types.

Terminology

Operators are unary, binary, or ternary:

Unary operators operate on a single target (such as -a). Unary prefix operators appear immediately before their target (such as !b), and unary postfix operators appear immediately after their target (such as i++).

Binary operators operate on two targets (such as 2 + 3) and are infix because they appear in between their two targets.

Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (a ? b : c).

The values that operators affect are operands. In the expression 1 + 2, the + symbol is a binary operator and its two operands are the values 1 and 2.

Assignment Operator

The assignment operator (a = b) initializes or updates the value of a with the value of b:

let b = 10 var a = 5 a = b

// a is now equal to 10

If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple constants or variables at once:

let (x, y) = (1, 2)

// x is equal to 1, and y is equal to 2

Unlike the assignment operator in C and Objective-C, the assignment operator in Swift does not itself return a value. The following statement is not valid:

if x = y {

// this is not valid, because x = y does not return a value

}

This feature prevents the assignment operator (=) from being used by accident when the equal to operator (==) is actually intended. By making if x = y invalid, Swift helps you to avoid these kinds of errors in your code.

Arithmetic Operators

Swift supports the four standard arithmetic operators for all number types:

1 + 2 // equals 3

5 - 3 // equals 2

2 * 3 // equals 6

10.0 / 2.5 // equals 4.0

Unlike the arithmetic operators in C and Objective-C, the Swift arithmetic operators do not allow values to overflow by default. You can opt in to value overflow behavior by using Swift锟斤拷s overflow operators (such as a &+ b). See Overflow Operators.

The addition operator is also supported for String concatenation:

"hello, " + "world" // equals "hello, world"

Two Character values, or one Character value and one String value, can be added together to make a new String value:

let dog: Character = " " let cow: Character = " " let dogCow = dog + cow

// dogCow is equal to " "

See also Concatenating Strings and Characters.

Remainder Operator

The remainder operator (a % b) works out how many multiples of b will fit inside a and returns the value that is left over (known as the remainder).

NOTE

The remainder operator (%) is also known as a modulo operator in other languages. However, its behavior in Swift for negative numbers means that it is, strictly speaking, a remainder rather than a modulo operation.

Here锟斤拷s how the remainder operator works. To calculate 9 % 4, you first work out how many 4s will fit inside 9:

You can fit two 4s inside 9, and the remainder is 1 (shown in orange).

In Swift, this would be written as:

9 % 4 // equals 1

To determine the answer for a % b, the % operator calculates the following equation and returns remainder as its output: a = (b 锟斤拷 some multiplier) + remainder where some multiplier is the largest number of multiples of b that will fit inside a.

Inserting 9 and 4 into this equation yields:

9 = (4 锟斤拷 2) + 1

The same method is applied when calculating the remainder for a negative value of a:

-9 % 4 // equals -1

Inserting -9 and 4 into the equation yields:

-9 = (4 锟斤拷 -2) + -1 giving a remainder value of -1.

The sign of b is ignored for negative values of b. This means that a % b and a % -b always give the same answer.

Floating-Point Remainder Calculations

Unlike the remainder operator in C and Objective-C, Swift锟斤拷s remainder operator can also operate on floating-point numbers:

8 % 2.5 // equals 0.5

In this example, 8 divided by 2.5 equals 3, with a remainder of 0.5, so the remainder operator returns a Double value of 0.5.

Increment and Decrement Operators

Like C, Swift provides an increment operator (++) and a decrement operator (--) as a shortcut to increase or decrease the value of a numeric variable by 1. You can use these operators with variables of any integer or floating-point type.

var i = 0

++i // i now equals 1

Each time you call ++i, the value of i is increased by 1. Essentially, ++i is shorthand for saying i = i + 1. Likewise, --i can be used as shorthand for i = i - 1.

The ++ and -- symbols can be used as prefix operators or as postfix operators. ++i and i++ are both valid ways to increase the value of i by 1. Similarly, --i and i-- are both valid ways to decrease the value of i by 1.

Note that these operators modify i and also return a value. If you only want to increment or decrement the value stored in i, you can ignore the returned value. However, if you do use the returned value, it will be different based on whether you used the prefix or postfix version of the operator, according to the following rules:

For example:

var a = 0 let b = ++a

// a and b are now both equal to 1 let c = a++

// a is now equal to 2, but c has been set to the pre-increment value of 1

In the example above, let b = ++a increments a before returning its value. This is why both

a and b are equal to to the new value of 1.

However, let c = a++ increments a after returning its value. This means that c gets the old value of 1, and a is then updated to equal 2.

Unless you need the specific behavior of i++, it is recommended that you use ++i and --i in all cases, because they have the typical expected behavior of modifying i and returning the result.

Unary Minus Operator

The sign of a numeric value can be toggled using a prefixed -, known as the unary minus operator:

let three = 3 let minusThree = -three // minusThree equals -3 let plusThree = -minusThree // plusThree equals 3, or "minus minus three"

The unary minus operator (-) is prepended directly before the value it operates on, without any white space.

Unary Plus Operator

The unary plus operator (+) simply returns the value it operates on, without any change:

let minusSix = -6 let alsoMinusSix = +minusSix // alsoMinusSix equals -6

Although the unary plus operator doesn锟斤拷t actually do anything, you can use it to provide symmetry in your code for positive numbers when also using the unary minus operator for negative numbers.

Compound Assignment Operators

Like C, Swift provides compound assignment operators that combine assignment (=) with another operation. One example is the addition assignment operator (+=):

var a = 1

a += 2

// a is now equal to 3

The expression a += 2 is shorthand for a = a + 2. Effectively, the addition and the assignment are combined into one operator that performs both tasks at the same time.

NOTE

The compound assignment operators do not return a value. You cannot write let b = a += 2, for example.

This behavior is different from the increment and decrement operators mentioned above.

A complete list of compound assignment operators can be found in Expressions.

Comparison Operators

Swift supports all standard C comparison operators:

NOTE

Swift also provides two identity operators (=== and !==), which you use to test whether two object references both refer to the same object instance. For more information, see Classes and Structures.

Each of the comparison operators returns a Bool value to indicate whether or not the statement is true:

1 == 1 // true, because 1 is equal to 1

2 != 1 // true, because 2 is not equal to 1

2 > 1 // true, because 2 is greater than 1

1 < 2 // true, because 1 is less than 2

1 >= 1 // true, because 1 is greater than or equal to 1

2 <= 1 // false, because 2 is not less than or equal to 1

Comparison operators are often used in conditional statements, such as the if statement:

let name = "world" if name == "world" {

println("hello, world")

} else { println("I'm sorry \(name), but I don't recognize you")

}

// prints "hello, world", because name is indeed equal to "world"

For more on the if statement, see Control Flow.

Ternary Conditional Operator

The ternary conditional operator is a special operator with three parts, which takes the form question ? answer1 : answer2. It is a shortcut for evaluating one of two expressions based on whether question is true or false. If question is true, it evaluates answer1 and returns its value; otherwise, it evaluates answer2 and returns its value.

The ternary conditional operator is shorthand for the code below:

if question { answer1 } else { answer2

}

Here锟斤拷s an example, which calculates the pixel height for a table row. The row height should be 50 pixels taller than the content height if the row has a header, and 20 pixels taller if the row doesn锟斤拷t have a header:

let contentHeight = 40

let hasHeader = true let rowHeight = contentHeight + (hasHeader ? 50 : 20)

// rowHeight is equal to 90

The preceding example is shorthand for the code below:

let contentHeight = 40 let hasHeader = true var rowHeight = contentHeight if hasHeader { rowHeight = rowHeight + 50

} else { rowHeight = rowHeight + 20

}

// rowHeight is equal to 90

The first example锟斤拷s use of the ternary conditional operator means that rowHeight can be set to the correct value on a single line of code. This is more concise than the second example, and removes the need for rowHeight to be a variable, because its value does not need to be modified within an if statement.

The ternary conditional operator provides an efficient shorthand for deciding which of two expressions to consider. Use the ternary conditional operator with care, however. Its conciseness can lead to hard-to-read code if overused. Avoid combining multiple instances of the ternary conditional operator into one compound statement.

Range Operators

Swift includes two range operators, which are shortcuts for expressing a range of values.

Closed Range Operator

The closed range operator (a...b) defines a range that runs from a to b, and includes the values a and b.

The closed range operator is useful when iterating over a range in which you want all of the values to be used, such as with a for-in loop:

for index in 1...5 { println("\(index) times 5 is \(index * 5)")

}

// 1 times 5 is 5

// 2 times 5 is 10

// 3 times 5 is 15

// 4 times 5 is 20

// 5 times 5 is 25

For more on for-in loops, see Control Flow.

Half-Closed Range Operator

The half-closed range operator (a..b) defines a range that runs from a to b, but does not include b. It is said to be half-closed because it contains its first value, but not its final value.

Half-closed ranges are particularly useful when you work with zero-based lists such as arrays, where it is useful to count up to (but not including) the length of the list:

let names = ["Anna", "Alex", "Brian", "Jack"] let count = names.count for i in 0..count { println("Person \(i + 1) is called \(names[i])")

}

// Person 1 is called Anna

// Person 2 is called Alex

// Person 3 is called Brian

// Person 4 is called Jack

Note that the array contains four items, but 0..count only counts as far as 3 (the index of the last item in the array), because it is a half-closed range. For more on arrays, see Arrays.

Logical Operators

Logical operators modify or combine the Boolean logic values true and false. Swift supports the three standard logical operators found in C-based languages:

Logical NOT Operator

The logical NOT operator (!a) inverts a Boolean value so that true becomes false, and false becomes true.

The logical NOT operator is a prefix operator, and appears immediately before the value it operates on, without any white space. It can be read as 锟斤拷not a锟斤拷, as seen in the following example:

let allowedEntry = false if !allowedEntry { println("ACCESS DENIED")

}

// prints "ACCESS DENIED"

The phrase if !allowedEntry can be read as 锟斤拷if not allowed entry.锟斤拷 The subsequent line is only executed if 锟斤拷not allowed entry锟斤拷 is true; that is, if allowedEntry is false.

As in this example, careful choice of Boolean constant and variable names can help to keep code readable and concise, while avoiding double negatives or confusing logic statements.

Logical AND Operator

The logical AND operator (a && b) creates logical expressions where both values must be true for the overall expression to also be true.

If either value is false, the overall expression will also be false. In fact, if the first value is false, the second value won锟斤拷t even be evaluated, because it can锟斤拷t possibly make the overall expression equate to true. This is known as short-circuit evaluation.

This example considers two Bool values and only allows access if both values are true:

let enteredDoorCode = true let passedRetinaScan = false if enteredDoorCode && passedRetinaScan { println("Welcome!")

} else { println("ACCESS DENIED")

}

// prints "ACCESS DENIED"

Logical OR Operator

The logical OR operator (a || b) is an infix operator made from two adjacent pipe characters. You use it to create logical expressions in which only one of the two values has to be true for the overall expression to be true.

Like the Logical AND operator above, the Logical OR operator uses short-circuit evaluation to consider its expressions. If the left side of a Logical OR expression is true, the right side is not evaluated, because it cannot change the outcome of the overall expression.

In the example below, the first Bool value (hasDoorKey) is false, but the second value

(knowsOverridePassword) is true. Because one value is true, the overall expression also evaluates to true, and access is allowed:

let hasDoorKey = false let knowsOverridePassword = true if hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// prints "Welcome!"

Combining Logical Operators

You can combine multiple logical operators to create longer compound expressions:

if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// prints "Welcome!"

This example uses multiple && and || operators to create a longer compound expression. However, the && and || operators still operate on only two values, so this is actually three smaller expressions chained together. It can be read as:

If we锟斤拷ve entered the correct door code and passed the retina scan; or if we have a valid door key; or if we know the emergency override password, then allow access.

Based on the values of enteredDoorCode, passedRetinaScan, and hasDoorKey, the first two miniexpressions are false. However, the emergency override password is known, so the overall compound expression still evaluates to true.

Explicit Parentheses

It is sometimes useful to include parentheses when they are not strictly needed, to make the intention of a complex expression easier to read. In the door access example above, it is useful to add parentheses around the first part of the compound expression to make its intent explicit:

if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword { println("Welcome!")

} else { println("ACCESS DENIED")

}

// prints "Welcome!"

The parentheses make it clear that the first two values are considered as part of a separate possible state in the overall logic. The output of the compound expression doesn锟斤拷t change, but the overall intention is clearer to the reader. Readability is always preferred over brevity; use parentheses where they help to make your intentions clear.

Strings and Characters

A string is an ordered collection of characters, such as "hello, world" or "albatross". Swift strings are represented by the String type, which in turn represents a collection of values of Character type.

Swift锟斤拷s String and Character types provide a fast, Unicode-compliant way to work with text in your code. The syntax for string creation and manipulation is lightweight and readable, with a similar syntax to C strings. String concatenation is as simple as adding together two strings with the + operator, and string mutability is managed by choosing between a constant or a variable, just like any other value in Swift.

Despite this simplicity of syntax, Swift锟斤拷s String type is a fast, modern string implementation. Every string is composed of encoding-independent Unicode characters, and provides support for accessing those characters in various Unicode representations.

Strings can also be used to insert constants, variables, literals, and expressions into longer strings, in a process known as string interpolation. This makes it easy to create custom string values for display, storage, and printing.

NOTE

Swift锟斤拷s String type is bridged seamlessly to Foundation锟斤拷s NSString class. If you are working with the Foundation framework in Cocoa or Cocoa Touch, the entire NSString API is available to call on any String value you create, in addition to the String features described in this chapter. You can also use a String value with any API that requires an NSString instance.

For more information about using String with Foundation and Cocoa, see Using Swift with Cocoa and ObjectiveC.

String Literals

You can include predefined String values within your code as string literals. A string literal is a fixed sequence of textual characters surrounded by a pair of double quotes ("").

A string literal can be used to provide an initial value for a constant or variable:

let someString = "Some string literal value"

Note that Swift infers a type of String for the someString constant, because it is initialized with a string literal value.

String literals can include the following special characters:

The code below shows an example of each kind of special character. The wiseWords constant contains two escaped double quote characters. The dollarSign, blackHeart, and sparklingHeart constants demonstrate the three different Unicode scalar character formats:

let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"

// "Imagination is more important than knowledge" - Einstein let dollarSign = "\x24" // $, Unicode scalar U+0024 let blackHeart = "\u2665" // ♥, Unicode scalar U+2665 let sparklingHeart = "\U0001F496" // , Unicode scalar U+1F496

Initializing an Empty String

To create an empty String value as the starting point for building a longer string, either assign an empty string literal to a variable, or initialize a new String instance with initializer syntax:

var emptyString = "" // empty string literal var anotherEmptyString = String() // initializer syntax

// these two strings are both empty, and are equivalent to each other

You can find out whether a String value is empty by checking its Boolean isEmpty property:

if emptyString.isEmpty { println("Nothing to see here")

}

// prints "Nothing to see here"

String Mutability

You indicate whether a particular String can be modified (or mutated) by assigning it to a variable (in which case it can be modified), or to a constant (in which case it cannot be modified):

var variableString = "Horse" variableString += " and carriage"

// variableString is now "Horse and carriage"

let constantString = "Highlander" constantString += " and another Highlander"

// this reports a compile-time error - a constant string cannot be modified

NOTE

This approach is different from string mutation in Objective-C and Cocoa, where you choose between two classes (NSString and NSMutableString) to indicate whether a string can be mutated.

Strings Are Value Types

Swift锟斤拷s String type is a value type. If you create a new String value, that String value is copied when it is passed to a function or method, or when it is assigned to a constant or variable. In each case, a new copy of the existing String value is created, and the new copy is passed or assigned, not the original version. Value types are described in Structures and Enumerations Are Value Types.

NOTE

This behavior differs from that of NSString in Cocoa. When you create an NSString instance in Cocoa, and pass it to a function or method or assign it to a variable, you are always passing or assigning a reference to the same single NSString. No copying of the string takes place, unless you specifically request it.

Swift锟斤拷s copy-by-default String behavior ensures that when a function or method passes you a String value, it is clear that you own that exact String value, regardless of where it came from. You can be confident that the string you are passed will not be modified unless you modify it yourself.

Behind the scenes, Swift锟斤拷s compiler optimizes string usage so that actual copying takes place only when absolutely necessary. This means you always get great performance when working with strings as value types.

Working with Characters

Swift锟斤拷s String type represents a collection of Character values in a specified order. Each

Character value represents a single Unicode character. You can access the individual Character values in a string by iterating over that string with a for-in loop:

for character in "Dog! " { println(character)

}

// D

// o

// g // ! //

The for-in loop is described in For Loops.

Alternatively, create a stand-alone Character constant or variable from a single-character string literal by providing a Character type annotation:

let yenSign: Character = "¥"

Counting Characters

To retrieve a count of the characters in a string, call the global countElements function and pass in a string as the function锟斤拷s sole parameter:

let unusualMenagerie = "Koala , Snail , Penguin , Dromedary "

println("unusualMenagerie has \(countElements(unusualMenagerie)) characters") // prints "unusualMenagerie has 40 characters"

NOTE

Different Unicode characters and different representations of the same Unicode character can require different amounts of memory to store. Because of this, characters in Swift do not each take up the same amount of memory within a string锟斤拷s representation. As a result, the length of a string cannot be calculated without iterating through the string to consider each of its characters in turn. If you are working with particularly long string values, be aware that the countElements function must iterate over the characters within a string in order to calculate an accurate character count for that string.

Note also that the character count returned by countElements is not always the same as the length property of an NSString that contains the same characters. The length of an NSString is based on the number of 16-bit code units within the string锟斤拷s UTF-16 representation and not the number of Unicode characters within the string. To reflect this fact, the length property from NSString is called utf16count when it is accessed on a Swift String value.

Concatenating Strings and Characters

String and Character values can be added together (or concatenated) with the addition operator (+) to create a new String value:

let string1 = "hello" let string2 = " there" let character1: Character = "!" let character2: Character = "?"

let stringPlusCharacter = string1 + character1 // equals "hello!" let stringPlusString = string1 + string2 // equals "hello there" let characterPlusString = character1 + string1 // equals "!hello" let characterPlusCharacter = character1 + character2 // equals "!?"

You can also append a String or Character value to an existing String variable with the addition assignment operator (+=):

var instruction = "look over" instruction += string2

// instruction now equals "look over there"

var welcome = "good morning" welcome += character1

// welcome now equals "good morning!"

NOTE

You can锟斤拷t append a String or Character to an existing Character variable, because a Character value must contain a single character only.

String Interpolation

String interpolation is a way to construct a new String value from a mix of constants, variables, literals, and expressions by including their values inside a string literal. Each item that you insert into the string literal is wrapped in a pair of parentheses, prefixed by a backslash:

let multiplier = 3 let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"

// message is "3 times 2.5 is 7.5"

In the example above, the value of multiplier is inserted into a string literal as \(multiplier). This placeholder is replaced with the actual value of multiplier when the string interpolation is evaluated to create an actual string.

The value of multiplier is also part of a larger expression later in the string. This expression calculates the value of Double(multiplier) * 2.5 and inserts the result (7.5) into the string. In this case, the expression is written as \(Double(multiplier) * 2.5) when it is included inside the string literal.

NOTE

The expressions you write inside parentheses within an interpolated string cannot contain an unescaped double quote (") or backslash (\), and cannot contain a carriage return or line feed.

Comparing Strings

Swift provides three ways to compare String values: string equality, prefix equality, and suffix equality.

String Equality

Two String values are considered equal if they contain exactly the same characters in the same order:

let quotation = "We're a lot alike, you and I." let sameQuotation = "We're a lot alike, you and I." if quotation == sameQuotation { println("These two strings are considered equal")

}

// prints "These two strings are considered equal"

Prefix and Suffix Equality

To check whether a string has a particular string prefix or suffix, call the string锟斤拷s hasPrefix and hasSuffix methods, both of which take a single argument of type String and return a Boolean value. Both methods perform a character-by-character comparison between the base string and the prefix or suffix string.

The examples below consider an array of strings representing the scene locations from the first two acts of Shakespeare锟斤拷s Romeo and Juliet:

let romeoAndJuliet = [

"Act 1 Scene 1: Verona, A public place",

"Act 1 Scene 2: Capulet's mansion",

"Act 1 Scene 3: A room in Capulet's mansion",

"Act 1 Scene 4: A street outside Capulet's mansion",

"Act 1 Scene 5: The Great Hall in Capulet's mansion",

"Act 2 Scene 1: Outside Capulet's mansion",

"Act 2 Scene 2: Capulet's orchard",

"Act 2 Scene 3: Outside Friar Lawrence's cell", ct 2 Scene 4: A street in Verona", ct 2 Scene 5: Capulet's mansion", ct 2 Scene 6: Friar Lawrence's cell"

You can use the hasPrefix method with the romeoAndJuliet array to count the number of scenes in Act 1 of the play:

var act1SceneCount = 0 for scene in romeoAndJuliet { if scene.hasPrefix("Act 1 ") {

++act1SceneCount

}

} println("There are \(act1SceneCount) scenes in Act 1")

// prints "There are 5 scenes in Act 1"

Similarly, use the hasSuffix method to count the number of scenes that take place in or around Capulet锟斤拷s mansion and Friar Lawrence锟斤拷s cell:

var mansionCount = 0 var cellCount = 0 for scene in romeoAndJuliet { if scene.hasSuffix("Capulet's mansion") {

++mansionCount

} else if scene.hasSuffix("Friar Lawrence's cell") {

++cellCount

}

("\(mansionCount) mansion scenes; \(cellCount) cell scenes")

nts "6 mansion scenes; 2 cell scenes"

Uppercase and Lowercase Strings

You can access an uppercase or lowercase version of a string with its uppercaseString and lowercaseString properties:

let normal = "Could you help me, please?" let shouty = normal.uppercaseString

// shouty is equal to "COULD YOU HELP ME, PLEASE?" let whispered = normal.lowercaseString

// whispered is equal to "could you help me, please?"

Unicode

Unicode is an international standard for encoding and representing text. It enables you to represent almost any character from any language in a standardized form, and to read and write those characters to and from an external source such as a text file or web page.

Swift锟斤拷s String and Character types are fully Unicode-compliant. They support a number of different Unicode encodings, as described below.

Unicode Terminology

Every character in Unicode can be represented by one or more unicode scalars. A unicode scalar is a unique 21-bit number (and name) for a character or modifier, such as U+0061 for LOWERCASE LATIN LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK (" ").

When a Unicode string is written to a text file or some other storage, these unicode scalars are encoded in one of several Unicode-defined formats. Each format encodes the string in small chunks known as code units. These include the UTF-8 format (which encodes a string as 8-bit code units) and the UTF-16 format (which encodes a string as 16-bit code units).

Unicode Representations of Strings

Swift provides several different ways to access Unicode representations of strings.

You can iterate over the string with a for-in statement, to access its individual Character values as Unicode characters. This process is described in Working with Characters.

Alternatively, access a String value in one of three other Unicode-compliant representations:

Each example below shows a different representation of the following string, which is made up of the characters D, o, g, !, and the character ( DOG FACE, or Unicode scalar U+1F436):

let dogString = "Dog! "

UTF-8

You can access a UTF-8 representation of a String by iterating over its utf8 property. This property is of type UTF8View, which is a collection of unsigned 8-bit (UInt8) values, one for each byte in the string锟斤拷s UTF-8 representation:

for codeUnit in dogString.utf8 { print("\(codeUnit) ")

} print("\n")

// 68 111 103 33 240 159 144 182

In the example above, the first four decimal codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-8 representation is the same as their ASCII representation. The last four codeUnit values (240, 159, 144, 182) are a four-byte UTF-8 representation of the DOG FACE character.

UTF-16

You can access a UTF-16 representation of a String by iterating over its utf16 property. This property is of type UTF16View, which is a collection of unsigned 16-bit (UInt16) values, one for each 16-bit code unit in the string锟斤拷s UTF-16 representation:

for codeUnit in dogString.utf16 { print("\(codeUnit) ")

} print("\n")

// 68 111 103 33 55357 56374

Again, the first four codeUnit values (68, 111, 103, 33) represent the characters D, o, g, and !, whose UTF-16 code units have the same values as in the string锟斤拷s UTF-8 representation.

The fifth and sixth codeUnit values (55357 and 56374) are a UTF-16 surrogate pair representation of the DOG FACE character. These values are a lead surrogate value of

U+D83D (decimal value 55357) and a trail surrogate value of U+DC36 (decimal value 56374).

Unicode Scalars

You can access a Unicode scalar representation of a String value by iterating over its unicodeScalars property. This property is of type UnicodeScalarView, which is a collection of values of type UnicodeScalar. A Unicode scalar is any 21-bit Unicode code point that is not a lead surrogate or trail surrogate code point.

Each UnicodeScalar has a value property that returns the scalar锟斤拷s 21-bit value, represented within a UInt32 value:

for scalar in dogString.unicodeScalars { print("\(scalar.value) ")

} print("\n")

// 68 111 103 33 128054

The value properties for the first four UnicodeScalar values (68, 111, 103, 33) once again represent the characters D, o, g, and !. The value property of the fifth and final UnicodeScalar, 128054, is a decimal equivalent of the hexadecimal value 1F436, which is equivalent to the Unicode scalar U+1F436 for the DOG FACE character.

As an alternative to querying their value properties, each UnicodeScalar value can also be used to construct a new String value, such as with string interpolation:

for scalar in dogString.unicodeScalars { println("\(scalar) ")

}

// D

// o

// g // !

Collection Types

Swift provides two collection types, known as arrays and dictionaries, for storing collections of values. Arrays store ordered lists of values of the same type. Dictionaries store unordered collections of values of the same type, which can be referenced and looked up through a unique identifier (also known as a key).

Arrays and dictionaries in Swift are always clear about the types of values and keys that they can store. This means that you cannot insert a value of the wrong type into an array or dictionary by mistake. It also means you can be confident about the types of values you will retrieve from an array or dictionary. Swift锟斤拷s use of explicitly typed collections ensures that your code is always clear about the types of values it can work with and enables you to catch any type mismatches early in your code锟斤拷s development.

NOTE

Swift锟斤拷s Array type exhibits different behavior to other types when assigned to a constant or variable, or when passed to a function or method. For more information, see Mutability of Collections and Assignment and Copy Behavior for Collection Types.

Arrays

An array stores multiple values of the same type in an ordered list. The same value can appear in an array multiple times at different positions.

Swift arrays are specific about the kinds of values they can store. They differ from

Objective-C锟斤拷s NSArray and NSMutableArray classes, which can store any kind of object and do not provide any information about the nature of the objects they return. In Swift, the type of values that a particular array can store is always made clear, either through an explicit type annotation, or through type inference, and does not have to be a class type. If you create an array of Int values, for example, you can锟斤拷t insert any value other than Int values into that array. Swift arrays are type safe, and are always clear about what they may contain.

Array Type Shorthand Syntax

The type of a Swift array is written in full as Array<SomeType>, where SomeType is the type that the array is allowed to store. You can also write the type of an array in shorthand form as SomeType[]. Although the two forms are functionally identical, the shorthand form is preferred, and is used throughout this guide when referring to the type of an array.

Array Literals

You can initialize an array with an array literal, which is a shorthand way to write one or more values as an array collection. An array literal is written as a list of values, separated by commas, surrounded by a pair of square brackets:

[ value 1 , value 2 , value 3 ]

The example below creates an array called shoppingList to store String values:

var shoppingList: String[] = ["Eggs", "Milk"]

// shoppingList has been initialized with two initial items

The shoppingList variable is declared as 锟斤拷an array of String values锟斤拷, written as String[]. Because this particular array has specified a value type of String, it is only allowed to store String values. Here, the shoppingList array is initialized with two String values ("Eggs" and "Milk"), written within an array literal.

NOTE

The shoppingList array is declared as a variable (with the var introducer) and not a constant (with the let introducer) because more items are added to the shopping list in the examples below.

In this case, the array literal contains two String values and nothing else. This matches the type of the shoppingList variable锟斤拷s declaration (an array that can only contain String values), and so the assignment of the array literal is permitted as a way to initialize shoppingList with two initial items.

Thanks to Swift锟斤拷s type inference, you don锟斤拷t have to write the type of the array if you锟斤拷re initializing it with an array literal containing values of the same type. The initialization of shoppingList could have been written in a shorter form instead:

var shoppingList = ["Eggs", "Milk"]

Because all values in the array literal are of the same type, Swift can infer that String[] is the correct type to use for the shoppingList variable.

Accessing and Modifying an Array

You access and modify an array through its methods and properties, or by using subscript syntax.

To find out the number of items in an array, check its read-only count property:

println("The shopping list contains \(shoppingList.count) items.")

// prints "The shopping list contains 2 items."

Use the Boolean isEmpty property as a shortcut for checking whether the count property is equal to 0:

if shoppingList.isEmpty { println("The shopping list is empty.")

} else { println("The shopping list is not empty.")

}

// prints "The shopping list is not empty."

You can add a new item to the end of an array by calling the array锟斤拷s append method:

shoppingList.append("Flour")

// shoppingList now contains 3 items, and someone is making pancakes

Alternatively, add a new item to the end of an array with the addition assignment operator (+=):

shoppingList += "Baking Powder"

// shoppingList now contains 4 items

You can also append an array of compatible items with the addition assignment operator (+=):

shoppingList += ["Chocolate Spread", "Cheese", "Butter"]

// shoppingList now contains 7 items

Retrieve a value from the array by using subscript syntax, passing the index of the value you want to retrieve within square brackets immediately after the name of the array:

var firstItem = shoppingList[0]

// firstItem is equal to "Eggs"

Note that the first item in the array has an index of 0, not 1. Arrays in Swift are always zero-indexed.

You can use subscript syntax to change an existing value at a given index:

shoppingList[0] = "Six eggs"

// the first item in the list is now equal to "Six eggs" rather than "Eggs"

You can also use subscript syntax to change a range of values at once, even if the replacement set of values has a different length than the range you are replacing. The following example replaces "Chocolate Spread", "Cheese", and "Butter" with "Bananas" and "Apples":

shoppingList[4...6] = ["Bananas", "Apples"]

// shoppingList now contains 6 items

NOTE

You can锟斤拷t use subscript syntax to append a new item to the end of an array. If you try to use subscript syntax to retrieve or set a value for an index that is outside of an array锟斤拷s existing bounds, you will trigger a runtime error. However, you can check that an index is valid before using it, by comparing it to the array锟斤拷s count property. Except when count is 0 (meaning the array is empty), the largest valid index in an array will always be count - 1, because arrays are indexed from zero.

To insert an item into the array at a specified index, call the array锟斤拷s insert(atIndex:) method:

shoppingList.insert("Maple Syrup", atIndex: 0)

// shoppingList now contains 7 items

// "Maple Syrup" is now the first item in the list

This call to the insert method inserts a new item with a value of "Maple Syrup" at the very beginning of the shopping list, indicated by an index of 0.

Similarly, you remove an item from the array with the removeAtIndex method. This method removes the item at the specified index and returns the removed item (although you can ignore the returned value if you do not need it):

let mapleSyrup = shoppingList.removeAtIndex(0)

// the item that was at index 0 has just been removed

// shoppingList now contains 6 items, and no Maple Syrup

// the mapleSyrup constant is now equal to the removed "Maple Syrup" string

Any gaps in an array are closed when an item is removed, and so the value at index 0 is once again equal to "Six eggs":

firstItem = shoppingList[0]

// firstItem is now equal to "Six eggs"

If you want to remove the final item from an array, use the removeLast method rather than the removeAtIndex method to avoid the need to query the array锟斤拷s count property. Like the removeAtIndex method, removeLast returns the removed item:

let apples = shoppingList.removeLast()

// the last item in the array has just been removed

// shoppingList now contains 5 items, and no cheese

// the apples constant is now equal to the removed "Apples" string

Iterating Over an Array

You can iterate over the entire set of values in an array with the for-in loop:

for item in shoppingList { println(item)

}

// Six eggs

// Milk

// Flour

// Baking Powder

// Bananas

If you need the integer index of each item as well as its value, use the global enumerate function to iterate over the array instead. The enumerate function returns a tuple for each item in the array composed of the index and the value for that item. You can decompose the tuple into temporary constants or variables as part of the iteration:

for (index, value) in enumerate(shoppingList) { println("Item \(index + 1): \(value)")

}

// Item 1: Six eggs

// Item 2: Milk

// Item 3: Flour

// Item 4: Baking Powder

// Item 5: Bananas

For more about the for-in loop, see For Loops.

Creating and Initializing an Array

You can create an empty array of a certain type (without setting any initial values) using initializer syntax:

var someInts = Int[]() println("someInts is of type Int[] with \(someInts.count) items.")

// prints "someInts is of type Int[] with 0 items."

Note that the type of the someInts variable is inferred to be Int[], because it is set to the output of an Int[] initializer.

Alternatively, if the context already provides type information, such as a function argument or an already-typed variable or constant, you can create an empty array with an empty array literal, which is written as [] (an empty pair of square brackets):

someInts.append(3)

// someInts now contains 1 value of type Int someInts = []

// someInts is now an empty array, but is still of type Int[]

Swift锟斤拷s Array type also provides an initializer for creating an array of a certain size with all of its values set to a provided default value. You pass this initializer the number of items to be added to the new array (called count) and a default value of the appropriate type (called repeatedValue):

var threeDoubles = Double[](count: 3, repeatedValue: 0.0)

// threeDoubles is of type Double[], and equals [0.0, 0.0, 0.0]

Thanks to type inference, you don锟斤拷t need to specify the type to be stored in the array when using this initializer, because it can be inferred from the default value:

var anotherThreeDoubles = Array(count: 3, repeatedValue: 2.5)

// anotherThreeDoubles is inferred as Double[], and equals [2.5, 2.5, 2.5]

Finally, you can create a new array by adding together two existing arrays of compatible type with the addition operator (+). The new array锟斤拷s type is inferred from the type of the two arrays you add together:

var sixDoubles = threeDoubles + anotherThreeDoubles

// sixDoubles is inferred as Double[], and equals [0.0, 0.0, 0.0, 2.5, 2.5, 2.5]

Dictionaries

A dictionary is a container that stores multiple values of the same type. Each value is associated with a unique key, which acts as an identifier for that value within the dictionary. Unlike items in an array, items in a dictionary do not have a specified order. You use a dictionary when you need to look up values based on their identifier, in much the same way that a real-world dictionary is used to look up the definition for a particular word.

Swift dictionaries are specific about the types of keys and values they can store. They differ from Objective-C锟斤拷s NSDictionary and NSMutableDictionary classes, which can use any kind of object as their keys and values and do not provide any information about the nature of these objects. In Swift, the type of keys and values that a particular dictionary can store is always made clear, either through an explicit type annotation or through type inference.

Swift锟斤拷s dictionary type is written as Dictionary<KeyType, ValueType>, where KeyType is the type of value that can be used as a dictionary key, and ValueType is the type of value that the dictionary stores for those keys.

The only restriction is that KeyType must be hashable锟斤拷that is, it must provide a way to make itself uniquely representable. All of Swift锟斤拷s basic types (such as String, Int, Double, and Bool) are hashable by default, and all of these types can be used as the keys of a dictionary. Enumeration member values without associated values (as described in Enumerations) are also hashable by default.

Dictionary Literals

You can initialize a dictionary with with a dictionary literal, which has a similar syntax to the array literal seen earlier. A dictionary literal is a shorthand way to write one or more key-value pairs as a Dictionary collection.

A key-value pair is a combination of a key and a value. In a dictionary literal, the key and value in each key-value pair are separated by a colon. The key-value pairs are written as a list, separated by commas, surrounded by a pair of square brackets:

[ key 1 : value 1 , key 2 : value 2 , key 3 : value 3 ]

The example below creates a dictionary to store the names of international airports. In this dictionary, the keys are three-letter International Air Transport Association codes, and the values are airport names:

var airports: Dictionary<String, String> = ["TYO": "Tokyo", "DUB": "Dublin"]

The airports dictionary is declared as having a type of Dictionary<String, String>, which means 锟斤拷a Dictionary whose keys are of type String, and whose values are also of type String锟斤拷.

NOTE

The airports dictionary is declared as a variable (with the var introducer), and not a constant (with the let introducer), because more airports will be added to the dictionary in the examples below.

The airports dictionary is initialized with a dictionary literal containing two key-value pairs. The first pair has a key of "TYO" and a value of "Tokyo". The second pair has a key of "DUB" and a value of "Dublin".

This dictionary literal contains two String: String pairs. This matches the type of the airports variable declaration (a dictionary with only String keys, and only String values) and so the assignment of the dictionary literal is permitted as a way to initialize the airports dictionary with two initial items.

As with arrays, you don锟斤拷t have to write the type of the dictionary if you锟斤拷re initializing it with a dictionary literal whose keys and values have consistent types. The initialization of airports could have been be written in a shorter form instead:

var airports = ["TYO": "Tokyo", "DUB": "Dublin"]

Because all keys in the literal are of the same type as each other, and likewise all values are of the same type as each other, Swift can infer that Dictionary<String, String> is the correct type to use for the airports dictionary.

Accessing and Modifying a Dictionary

You access and modify a dictionary through its methods and properties, or by using subscript syntax. As with an array, you can find out the number of items in a Dictionary by checking its read-only count property:

println("The dictionary of airports contains \(airports.count) items.")

// prints "The dictionary of airports contains 2 items."

You can add a new item to a dictionary with subscript syntax. Use a new key of the appropriate type as the subscript index, and assign a new value of the appropriate type:

airports["LHR"] = "London"

// the airports dictionary now contains 3 items

You can also use subscript syntax to change the value associated with a particular key:

airports["LHR"] = "London Heathrow"

// the value for "LHR" has been changed to "London Heathrow"

As an alternative to subscripting, use a dictionary锟斤拷s updateValue(forKey:) method to set or

update the value for a particular key. Like the subscript examples above, the

updateValue(forKey:) method sets a value for a key if none exists, or updates the value if that key already exists. Unlike a subscript, however, the updateValue(forKey:) method returns the old value after performing an update. This enables you to check whether or not an update took place.

The updateValue(forKey:) method returns an optional value of the dictionary锟斤拷s value type. For a dictionary that stores String values, for example, the method returns a value of type String?, or 锟斤拷optional String锟斤拷. This optional value contains the old value for that key if one existed before the update, or nil if no value existed:

if let oldValue = airports.updateValue("Dublin International", forKey: "DUB") { println("The old value for DUB was \(oldValue).")

}

// prints "The old value for DUB was Dublin."

You can also use subscript syntax to retrieve a value from the dictionary for a particular key. Because it is possible to request a key for which no value exists, a dictionary锟斤拷s subscript returns an optional value of the dictionary锟斤拷s value type. If the dictionary contains a value for the requested key, the subscript returns an optional value containing the existing value for that key. Otherwise, the subscript returns nil:

if let airportName = airports["DUB"] { println("The name of the airport is \(airportName).")

} else { println("That airport is not in the airports dictionary.")

}

// prints "The name of the airport is Dublin International."

You can use subscript syntax to remove a key-value pair from a dictionary by assigning a value of nil for that key:

airports["APL"] = "Apple International"

// "Apple International" is not the real airport for APL, so delete it airports["APL"] = nil

// APL has now been removed from the dictionary

Alternatively, remove a key-value pair from a dictionary with the removeValueForKey method. This method removes the key-value pair if it exists and returns the removed value, or returns nil if no value existed:

if let removedValue = airports.removeValueForKey("DUB") { println("The removed airport's name is \(removedValue).")

} else { println("The airports dictionary does not contain a value for DUB.") }

// prints "The removed airport's name is Dublin International."

Iterating Over a Dictionary

You can iterate over the key-value pairs in a dictionary with a for-in loop. Each item in the dictionary is returned as a (key, value) tuple, and you can decompose the tuple锟斤拷s members into temporary constants or variables as part of the iteration:

for (airportCode, airportName) in airports { println("\(airportCode): \(airportName)")

}

// TYO: Tokyo

// LHR: London Heathrow

For more about the for-in loop, see For Loops.

You can also retrieve an iteratable collection of a dictionary锟斤拷s keys or values by accessing its keys and values properties:

for airportCode in airports.keys { println("Airport code: \(airportCode)")

}

// Airport code: TYO

// Airport code: LHR

for airportName in airports.values { println("Airport name: \(airportName)")

} port name: Tokyo port name: London Heathrow

If you need to use a dictionary锟斤拷s keys or values with an API that takes an Array instance, initialize a new array with the keys or values property:

let airportCodes = Array(airports.keys)

// airportCodes is ["TYO", "LHR"]

let airportNames = Array(airports.values)

// airportNames is ["Tokyo", "London Heathrow"]

NOTE

Swift锟斤拷s Dictionary type is an unordered collection. The order in which keys, values, and key-value pairs are retrieved when iterating over a dictionary is not specified.

Creating an Empty Dictionary

As with arrays, you can create an empty Dictionary of a certain type by using initializer syntax:

var namesOfIntegers = Dictionary<Int, String>()

// namesOfIntegers is an empty Dictionary<Int, String>

This example creates an empty dictionary of type Int, String to store human-readable names of integer values. Its keys are of type Int, and its values are of type String.

If the context already provides type information, create an empty dictionary with an empty dictionary literal, which is written as [:] (a colon inside a pair of square brackets):

namesOfIntegers[16] = "sixteen"

// namesOfIntegers now contains 1 key-value pair namesOfIntegers = [:]

// namesOfIntegers is once again an empty dictionary of type Int, String

NOTE

Behind the scenes, Swift锟斤拷s array and dictionary types are implemented as generic collections. For more on generic types and collections, see Generics.

Mutability of Collections

Arrays and dictionaries store multiple values together in a single collection. If you create an array or a dictionary and assign it to a variable, the collection that is created will be mutable. This means that you can change (or mutate) the size of the collection after it is created by adding more items to the collection, or by removing existing items from the ones it already contains. Conversely, if you assign an array or a dictionary to a constant, that array or dictionary is immutable, and its size cannot be changed.

For dictionaries, immutability also means that you cannot replace the value for an existing key in the dictionary. An immutable dictionary锟斤拷s contents cannot be changed once they are set.

Immutability has a slightly different meaning for arrays, however. You are still not allowed to perform any action that has the potential to change the size of an immutable array, but you are allowed to set a new value for an existing index in the array. This enables Swift锟斤拷s Array type to provide optimal performance for array operations when the size of an array is fixed.

The mutability behavior of Swift锟斤拷s Array type also affects how array instances are assigned and modified. For more information, see Assignment and Copy Behavior for Collection Types.

NOTE

It is good practice to create immutable collections in all cases where the collection锟斤拷s size does not need to change. Doing so enables the Swift compiler to optimize the performance of the collections you create.

Control Flow

Swift provides all the familiar control flow constructs of C-like languages. These include for and while loops to perform a task multiple times; if and switch statements to execute different branches of code based on certain conditions; and statements such as break and continue to transfer the flow of execution to another point in your code.

In addition to the traditional for-condition-increment loop found in C, Swift adds a for-in loop that makes it easy to iterate over arrays, dictionaries, ranges, strings, and other sequences.

Swift锟斤拷s switch statement is also considerably more powerful than its counterpart in C. The cases of a switch statement do not 锟斤拷fall through锟斤拷 to the next case in Swift, avoiding common C errors caused by missing break statements. Cases can match many different types of pattern, including range matches, tuples, and casts to a specific type. Matched values in a switch case can be bound to temporary constants or variables for use within the case锟斤拷s body, and complex matching conditions can be expressed with a where clause for each case.

For Loops

A for loop performs a set of statements a certain number of times. Swift provides two kinds of for loop:

for-in performs a set of statements for each item in a range, sequence, collection, or progression.

for-condition-increment performs a set of statements until a specific condition is met, typically by incrementing a counter each time the loop ends.

For-In

You use the for-in loop to iterate over collections of items, such as ranges of numbers, items in an array, or characters in a string.

This example prints the first few entries in the five-times-table:

for index in 1...5 {

println("\(index) times 5 is \(index * 5)")

}

// 1 times 5 is 5

// 2 times 5 is 10

// 3 times 5 is 15

// 4 times 5 is 20

// 5 times 5 is 25

The collection of items being iterated is a closed range of numbers from 1 to 5 inclusive, as indicated by the use of the closed range operator (...). The value of index is set to the first number in the range (1), and the statements inside the loop are executed. In this case, the loop contains only one statement, which prints an entry from the five-timestable for the current value of index. After the statement is executed, the value of index is updated to contain the second value in the range (2), and the println function is called again. This process continues until the end of the range is reached.

In the example above, index is a constant whose value is automatically set at the start of each iteration of the loop. As such, it does not have to be declared before it is used. It is implicitly declared simply by its inclusion in the loop declaration, without the need for a let declaration keyword.

NOTE

The index constant exists only within the scope of the loop. If you want to check the value of index after the loop completes, or if you want to work with its value as a variable rather than a constant, you must declare it yourself before its use in the loop.

If you don锟斤拷t need each value from the range, you can ignore the values by using an underscore in place of a variable name:

let base = 3 let power = 10 var answer = 1 for _ in 1...power { answer *= base

} println("\(base) to the power of \(power) is \(answer)") // prints "3 to the power of 10 is 59049"

This example calculates the value of one number to the power of another (in this case, 3 to the power of 10). It multiplies a starting value of 1 (that is, 3 to the power of 0) by 3, ten times, using a half-closed loop that starts with 0 and ends with 9. This calculation doesn锟斤拷t need to know the individual counter values each time through the loop锟斤拷it simply needs to execute the loop the correct number of times. The underscore character _ (used in place of a loop variable) causes the individual values to be ignored and does not provide access to the current value during each iteration of the loop.

Use the for-in loop with an array to iterate over its items:

let names = ["Anna", "Alex", "Brian", "Jack"] for name in names { println("Hello, \(name)!")

}

// Hello, Anna!

// Hello, Alex!

// Hello, Brian!

// Hello, Jack!

You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a (key, value) tuple when the dictionary is iterated, and you can decompose the (key, value) tuple锟斤拷s members as explicitly named constants for use within in the body of the for-in loop. Here, the dictionary锟斤拷s keys are decomposed into a constant called animalName, and the dictionary锟斤拷s values are decomposed into a constant called legCount:

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4] for (animalName, legCount) in numberOfLegs { println("\(animalName)s have \(legCount) legs")

}

// spiders have 8 legs

// ants have 6 legs

// cats have 4 legs

Items in a Dictionary may not necessarily be iterated in the same order as they were inserted. The contents of a Dictionary are inherently unordered, and iterating over them does not guarantee the order in which they will be retrieved. For more on arrays and dictionaries, see Collection Types.)

In addition to arrays and dictionaries, you can also use the for-in loop to iterate over the Character values in a string:

for character in "Hello" { println(character)

}

// H

// e

// l

// o

For-Condition-Increment

In addition to for-in loops, Swift supports traditional C-style for loops with a condition and an incrementer:

for var index = 0; index < 3; ++index { println("index is \(index)")

}

// index is 0

// index is 1

// index is 2

Here锟斤拷s the general form of this loop format:

}

Semicolons separate the three parts of the loop锟斤拷s definition, as in C. However, unlike C,

Swift doesn锟斤拷t need parentheses around the entire 锟斤拷initialization; condition; increment锟斤拷 block.

The loop is executed as follows:

1. When the loop is first entered, the initialization expression is evaluated once, to set up any constants or variables that are needed for the loop.

2. The condition expression is evaluated. If it evaluates to false, the loop ends, and code execution continues after the for loop锟斤拷s closing brace (}). If the expression evaluates to true, code execution continues by executing the statements inside the braces.

3. After all statements are executed, the increment expression is evaluated. It might increase or decrease the value of a counter, or set one of the initialized variables to a new value based on the outcome of the statements. After the increment expression has been evaluated, execution returns to step 2, and the condition expression is evaluated again.

The loop format and execution process described above is shorthand for (and equivalent to) the outline below:

initialization

while condition { statements increment

}

Constants and variables declared within the initialization expression (such as var index = 0) are only valid within the scope of the for loop itself. To retrieve the final value of index after the loop ends, you must declare index before the loop锟斤拷s scope begins:

var index: Int for index = 0; index < 3; ++index { println("index is \(index)")

}

// index is 0

// index is 1 // index is 2 println("The loop statements were executed \(index) times")

// prints "The loop statements were executed 3 times"

Note that the final value of index after this loop is completed is 3, not 2. The last time the increment statement ++index is called, it sets index to 3, which causes index < 3 to equate to false, ending the loop.

While Loops

A while loop performs a set of statements until a condition becomes false. These kinds of loops are best used when the number of iterations is not known before the first iteration begins. Swift provides two kinds of while loop:

While

A while loop starts by evaluating a single condition. If the condition is true, a set of statements is repeated until the condition becomes false.

Here锟斤拷s the general form of a while loop:

while condition { statements

}

This example plays a simple game of Snakes and Ladders (also known as Chutes and Ladders):

The rules of the game are as follows:

The game board is represented by an array of Int values. Its size is based on a constant called finalSquare, which is used to initialize the array and also to check for a win condition later in the example. The board is initialized with 26 zero Int values, not 25 (one each at indices 0 through 25 inclusive):

let finalSquare = 25 var board = Int[](count: finalSquare + 1, repeatedValue: 0)

Some squares are then set to have more specific values for the snakes and ladders. Squares with a ladder base have a positive number to move you up the board, whereas squares with a snake head have a negative number to move you back down the board:

board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08

Square 3 contains the bottom of a ladder that moves you up to square 11. To represent this, board[03] is equal to +08, which is equivalent to an integer value of 8 (the difference between 3 and 11). The unary plus operator (+i) balances with the unary minus operator (i), and numbers lower than 10 are padded with zeros so that all board definitions align. (Neither stylistic tweak is strictly necessary, but they lead to neater code.)

The player锟斤拷s starting square is 锟斤拷square zero锟斤拷, which is just off the bottom left corner of the board. The first dice roll always moves the player on to the board:

var square = 0 var diceRoll = 0 while square < finalSquare {

// roll the dice if ++diceRoll == 7 { diceRoll = 1 } // move by the rolled amount square += diceRoll if square < board.count {

// if we're still on the board, move up or down for a snake or a ladder square += board[square]

("Game over!")

This example uses a very simple approach to dice rolling. Instead of a random number generator, it starts with a diceRoll value of 0. Each time through the while loop, diceRoll is incremented with the prefix increment operator (++i), and is then checked to see if it has become too large. The return value of ++diceRoll is equal to the value of diceRoll after it is incremented. Whenever this return value equals 7, the dice roll has become too large, and is reset to a value of 1. This gives a sequence of diceRoll values that is always 1, 2, 3, 4, 5, 6, 1, 2 and so on.

After rolling the dice, the player moves forward by diceRoll squares. It锟斤拷s possible that the dice roll may have moved the player beyond square 25, in which case the game is over. To cope with this scenario, the code checks that square is less than the board array锟斤拷s count property before adding the value stored in board[square] onto the current square value to move the player up or down any ladders or snakes.

Had this check not been performed, board[square] might try to access a value outside the bounds of the board array, which would trigger an error. If square is now equal to 26, the code would try to check the value of board[26], which is larger than the size of the array.

The current while loop execution then ends, and the loop锟斤拷s condition is checked to see if the loop should be executed again. If the player has moved on or beyond square number 25, the loop锟斤拷s condition evaluates to false, and the game ends.

A while loop is appropriate in this case because the length of the game is not clear at the start of the while loop. Instead, the loop is executed until a particular condition is satisfied.

Do-While

The other variation of the while loop, known as the do-while loop, performs a single pass through the loop block first, before considering the loop锟斤拷s condition. It then continues to repeat the loop until the condition is false.

Here锟斤拷s the general form of a do-while loop:

do { statements

} while condition

Here锟斤拷s the Snakes and Ladders example again, written as a do-while loop rather than a while loop. The values of finalSquare, board, square, and diceRoll are initialized in exactly the same way as with a while loop:

let finalSquare = 25 var board = Int[](count: finalSquare + 1, repeatedValue: 0) board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08 var square = 0 var diceRoll = 0

In this version of the game, the first action in the loop is to check for a ladder or a snake. No ladder on the board takes the player straight to square 25, and so it is not possible to win the game by moving up a ladder. Therefore, it is safe to check for a snake or a ladder as the first action in the loop.

At the start of the game, the player is on 锟斤拷square zero锟斤拷. board[0] always equals 0, and has no effect:

do {

// move up or down for a snake or ladder

square += board[square]

// roll the dice if ++diceRoll == 7 { diceRoll = 1 } // move by the rolled amount square += diceRoll } while square < finalSquare println("Game over!")

After the code checks for snakes and ladders, the dice is rolled, and the player is moved forward by diceRoll squares. The current loop execution then ends.

The loop锟斤拷s condition (while square < finalSquare) is the same as before, but this time it is not evaluated until the end of the first run through the loop. The structure of the do-while loop is better suited to this game than the while loop in the previous example. In the do-while loop above, square += board[square] is always executed immediately after the loop锟斤拷s while condition confirms that square is still on the board. This behavior removes the need for the array bounds check seen in the earlier version of the game.

Conditional Statements

It is often useful to execute different pieces of code based on certain conditions. You might want to run an extra piece of code when an error occurs, or to display a message when a value becomes too high or too low. To do this, you make parts of your code conditional.

Swift provides two ways to add conditional branches to your code, known as the if statement and the switch statement. Typically, you use the if statement to evaluate simple conditions with only a few possible outcomes. The switch statement is better suited to more complex conditions with multiple possible permutations, and is useful in situations where pattern-matching can help select an appropriate code branch to execute.

If

In its simplest form, the if statement has a single if condition. It executes a set of statements only if that condition is true:

var temperatureInFahrenheit = 30

if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

}

// prints "It's very cold. Consider wearing a scarf."

The preceding example checks whether the temperature is less than or equal to 32 degrees Fahrenheit (the freezing point of water). If it is, a message is printed. Otherwise, no message is printed, and code execution continues after the if statement锟斤拷s closing brace.

The if statement can provide an alternative set of statements, known as an else clause, for when the if condition is false. These statements are indicated by the else keyword:

temperatureInFahrenheit = 40 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else { println("It's not that cold. Wear a t-shirt.")

}

// prints "It's not that cold. Wear a t-shirt."

One of these two branches is always executed. Because the temperature has increased to 40 degrees Fahrenheit, it is no longer cold enough to advise wearing a scarf, and so the else branch is triggered instead.

You can chain multiple if statements together, to consider additional clauses:

temperatureInFahrenheit = 90 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else if temperatureInFahrenheit >= 86 { println("It's really warm. Don't forget to wear sunscreen.")

} else { println("It's not that cold. Wear a t-shirt.")

}

// prints "It's really warm. Don't forget to wear sunscreen."

Here, an additional if statement is added to respond to particularly warm temperatures. The final else clause remains, and prints a response for any temperatures that are neither too warm nor too cold.

The final else clause is optional, however, and can be excluded if the set of conditions does not need to be complete:

temperatureInFahrenheit = 72 if temperatureInFahrenheit <= 32 { println("It's very cold. Consider wearing a scarf.")

} else if temperatureInFahrenheit >= 86 { println("It's really warm. Don't forget to wear sunscreen.")

}

In this example, the temperature is neither too cold nor too warm to trigger the if or else if conditions, and so no message is printed.

Switch

A switch statement considers a value and compares it against several possible matching patterns. It then executes an appropriate block of code, based on the first pattern that matches successfully. A switch statement provides an alternative to the if statement for responding to multiple potential states.

In its simplest form, a switch statement compares a value against one or more values of the same type:

otherwise, do something else

}

Every switch statement consists of multiple possible cases, each of which begins with the case keyword. In addition to comparing against specific values, Swift provides several ways for each case to specify more complex matching patterns. These options are described later in this section.

The body of each switch case is a separate branch of code execution, in a similar manner to the branches of an if statement. The switch statement determines which branch should be selected. This is known as switching on the value that is being considered.

Every switch statement must be exhaustive. That is, every possible value of the type being considered must be matched by one of the switch cases. If it is not appropriate to provide a switch case for every possible value, you can define a default catch-all case to cover any values that are not addressed explicitly. This catch-all case is indicated by the keyword default, and must always appear last.

This example uses a switch statement to consider a single lowercase character called someCharacter:

let someCharacter: Character = "e" switch someCharacter { case "a", "e", "i", "o", "u":

println("\(someCharacter) is a vowel") case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m", "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":

println("\(someCharacter) is a consonant") default: println("\(someCharacter) is not a vowel or a consonant")

nts "e is a vowel"

The switch statement锟斤拷s first case matches all five lowercase vowels in the English language. Similarly, its second case matches all lowercase English consonants.

It is not practical to write all other possible characters as part of a switch case, and so this switch statement provides a default case to match all other characters that are not vowels or consonants. This provision ensures that the switch statement is exhaustive.

No Implicit Fallthrough

In contrast with switch statements in C and Objective-C, switch statements in Swift do not fall through the bottom of each case and into the next one by default. Instead, the entire switch statement finishes its execution as soon as the first matching switch case is completed, without requiring an explicit break statement. This makes the switch statement safer and easier to use than in C, and avoids executing more than one switch case by mistake.

NOTE

You can still break out of a matched switch case before that case has completed its execution if you need to.

See Break in a Switch Statement for details.

The body of each case must contain at least one executable statement. It is not valid to write the following code, because the first case is empty:

let anotherCharacter: Character = "a" switch anotherCharacter { case "a": case "A": println("The letter A") default:

println("Not the letter A")

}

// this will report a compile-time error

Unlike a switch statement in C, this switch statement does not match both "a" and "A". Rather, it reports a compile-time error that case "a": does not contain any executable statements. This approach avoids accidental fallthrough from one case to another, and makes for safer code that is clearer in its intent.

Multiple matches for a single switch case can be separated by commas, and can be written over multiple lines if the list is long:

}

NOTE

To opt in to fallthrough behavior for a particular switch case, use the fallthrough keyword, as described in Fallthrough.

Range Matching

Values in switch cases can be checked for their inclusion in a range. This example uses number ranges to provide a natural-language count for numbers of any size:

let count = 3_000_000_000_000 let countedThings = "stars in the Milky Way" var naturalCount: String switch count { case 0:

naturalCount = "no" case 1...3:

naturalCount = "a few" case 4...9: turalCount = "several" 10...99: turalCount = "tens of"

100...999: turalCount = "hundreds of" 1000...999_999:

turalCount = "thousands of"

lt:

turalCount = "millions and millions of"

("There are \(naturalCount) \(countedThings).")

nts "There are millions and millions of stars in the Milky Way."

Tuples

You can use tuples to test multiple values in the same switch statement. Each element of the tuple can be tested against a different value or range of values. Alternatively, use the underscore (_) identifier to match any possible value.

The example below takes an (x, y) point, expressed as a simple tuple of type (Int, Int), and categorizes it on the graph that follows the example:

let somePoint = (1, 1) switch somePoint { case (0, 0): println("(0, 0) is at the origin") case (_, 0):

println("(\(somePoint.0), 0) is on the x-axis") case (0, _):

println("(0, \(somePoint.1)) is on the y-axis") case (-2...2, -2...2):

ntln("(\(somePoint.0), \(somePoint.1)) is inside the box")

lt:

ntln("(\(somePoint.0), \(somePoint.1)) is outside of the box")

nts "(1, 1) is inside the box"

The switch statement determines if the point is at the origin (0, 0); on the red x-axis; on the orange y-axis; inside the blue 4-by-4 box centered on the origin; or outside of the box.

Unlike C, Swift allows multiple switch cases to consider the same value or values. In fact, the point (0, 0) could match all four of the cases in this example. However, if multiple matches are possible, the first matching case is always used. The point (0, 0) would match case (0, 0) first, and so all other matching cases would be ignored.

Value Bindings

A switch case can bind the value or values it matches to temporary constants or variables, for use in the body of the case. This is known as value binding, because the values are 锟斤拷bound锟斤拷 to temporary constants or variables within the case锟斤拷s body.

The example below takes an (x, y) point, expressed as a tuple of type (Int, Int) and categorizes it on the graph that follows:

let anotherPoint = (2, 0) switch anotherPoint { case (let x, 0):

println("on the x-axis with an x value of \(x)") case (0, let y):

println("on the y-axis with a y value of \(y)") case let (x, y):

println("somewhere else at (\(x), \(y))")

} nts "on the x-axis with an x value of 2"

The switch statement determines if the point is on the red x-axis, on the orange y-axis, or elsewhere, on neither axis.

The three switch cases declare placeholder constants x and y, which temporarily take on one or both tuple values from anotherPoint. The first case, case (let x, 0), matches any point with a y value of 0 and assigns the point锟斤拷s x value to the temporary constant x. Similarly, the second case, case (0, let y), matches any point with an x value of 0 and assigns the point锟斤拷s y value to the temporary constant y.

Once the temporary constants are declared, they can be used within the case锟斤拷s code block. Here, they are used as shorthand for printing the values with the println function.

Note that this switch statement does not have a default case. The final case, case let (x, y), declares a tuple of two placeholder constants that can match any value. As a result, it matches all possible remaining values, and a default case is not needed to make the switch statement exhaustive.

In the example above, x and y are declared as constants with the let keyword, because there is no need to modify their values within the body of the case. However, they could have been declared as variables instead, with the var keyword. If this had been done, a temporary variable would have been created and initialized with the appropriate value. Any changes to that variable would only have an effect within the body of the case.

Where

A switch case can use a where clause to check for additional conditions.

The example below categorizes an (x, y) point on the following graph:

let yetAnotherPoint = (1, -1) switch yetAnotherPoint { case let (x, y) where x == y:

println("(\(x), \(y)) is on the line x == y") case let (x, y) where x == -y:

println("(\(x), \(y)) is on the line x == -y") case let (x, y):

println("(\(x), \(y)) is just some arbitrary point")

} nts "(1, -1) is on the line x == -y"

The switch statement determines if the point is on the green diagonal line where x == y, on the purple diagonal line where x == -y, or neither.

The three switch cases declare placeholder constants x and y, which temporarily take on the two tuple values from point. These constants are used as part of a where clause, to create a dynamic filter. The switch case matches the current value of point only if the where clause锟斤拷s condition evaluates to true for that value.

As in the previous example, the final case matches all possible remaining values, and so a default case is not needed to make the switch statement exhaustive.

Control Transfer Statements

Control transfer statements change the order in which your code is executed, by transferring control from one piece of code to another. Swift has four control transfer statements:

The control, break and fallthrough statements are described below. The return statement is described in Functions.

Continue

The continue statement tells a loop to stop what it is doing and start again at the beginning of the next iteration through the loop. It says 锟斤拷I am done with the current loop iteration锟斤拷 without leaving the loop altogether.

NOTE

In a for-condition-increment loop, the incrementer is still evaluated after calling the continue statement. The loop itself continues to work as usual; only the code within the loop锟斤拷s body is skipped.

The following example removes all vowels and spaces from a lowercase string to create a cryptic puzzle phrase:

let puzzleInput = "great minds think alike" var puzzleOutput = "" for character in puzzleInput { switch character { case "a", "e", "i", "o", "u", " ":

continue default:

puzzleOutput += character

}

(puzzleOutput) nts "grtmndsthnklk"

The code above calls the continue keyword whenever it matches a vowel or a space, causing the current iteration of the loop to end immediately and to jump straight to the start of the next iteration. This behavior enables the switch block to match (and ignore) only the vowel and space characters, rather than requiring the block to match every character that should get printed.

Break

The break statement ends execution of an entire control flow statement immediately. The break statement can be used inside a switch statement or loop statement when you want to terminate the execution of the switch or loop statement earlier than would otherwise be the case.

Break in a Loop Statement

When used inside a loop statement, break ends the loop锟斤拷s execution immediately, and transfers control to the first line of code after the loop锟斤拷s closing brace (}). No further code from the current iteration of the loop is executed, and no further iterations of the loop are started.

Break in a Switch Statement

When used inside a switch statement, break causes the switch statement to end its execution immediately, and to transfer control to the first line of code after the switch statement锟斤拷s closing brace (}).

This behavior can be used to match and ignore one or more cases in a switch statement. Because Swift锟斤拷s switch statement is exhaustive and does not allow empty cases, it is sometimes necessary to deliberately match and ignore a case in order to make your intentions explicit. You do this by writing the break statement as the entire body of the case you want to ignore. When that case is matched by the switch statement, the break statement inside the case ends the switch statement锟斤拷s execution immediately.

NOTE

A switch case that only contains a comment is reported as a compile-time error. Comments are not statements and do not cause a switch case to be ignored. Always use a break statement to ignore a switch case.

The following example switches on a Character value and determines whether it represents a number symbol in one of four languages. Multiple values are covered in a single switch case for brevity:

let numberSymbol: Character = "锟斤拷" // Simplified Chinese for the number 3 var possibleIntegerValue: Int?

switch numberSymbol { case "1", "١", "一", "๑":

possibleIntegerValue = 1 case "2", "٢", "锟斤拷", "๒":

possibleIntegerValue = 2 case "3", "٣", "锟斤拷", "๓":

possibleIntegerValue = 3

"4", "٤", "锟斤拷", "๔": ssibleIntegerValue = 4

lt:

eak

ntegerValue = possibleIntegerValue { ntln("The integer value of \(numberSymbol) is \(integerValue).")

{ ntln("An integer value could not be found for \(numberSymbol).")

nts "The integer value of 锟斤拷 is 3."

This example checks numberSymbol to determine whether it is a Latin, Arabic, Chinese, or Thai symbol for the numbers 1 to 4. If a match is found, one of the switch statement锟斤拷s cases sets an optional Int? variable called possibleIntegerValue to an appropriate integer value.

After the switch statement completes its execution, the example uses optional binding to determine whether a value was found. The possibleIntegerValue variable has an implicit initial value of nil by virtue of being an optional type, and so the optional binding will succeed only if possibleIntegerValue was set to an actual value by one of the switch statement锟斤拷s first four cases.

It is not practical to list every possible Character value in the example above, so a default case provides a catchall for any characters that are not matched. This default case does not need to perform any action, and so it is written with a single break statement as its body. As soon as the default statement is matched, the break statement ends the switch statement锟斤拷s execution, and code execution continues from the if let statement.

Fallthrough

Switch statements in Swift do not fall through the bottom of each case and into the next one. Instead, the entire switch statement completes its execution as soon as the first matching case is completed. By contrast, C requires you to insert an explicit break statement at the end of every switch case to prevent fallthrough. Avoiding default fallthrough means that Swift switch statements are much more concise and predictable than their counterparts in C, and thus they avoid executing multiple switch cases by mistake.

If you really need C-style fallthrough behavior, you can opt in to this behavior on a caseby-case basis with the fallthrough keyword. The example below uses fallthrough to create a textual description of a number:

let integerToDescribe = 5 var description = "The number \(integerToDescribe) is" switch integerToDescribe { case 2, 3, 5, 7, 11, 13, 17, 19:

description += " a prime number, and also" fallthrough default:

description += " an integer."

}

(description) nts "The number 5 is a prime number, and also an integer."

This example declares a new String variable called description and assigns it an initial value.

The function then considers the value of integerToDescribe using a switch statement. If the

value of integerToDescribe is one of the prime numbers in the list, the function appends text to the end of description, to note that the number is prime. It then uses the fallthrough keyword to 锟斤拷fall into锟斤拷 the default case as well. The default case adds some extra text to the end of the description, and the switch statement is complete.

If the value of integerToDescribe is not in the list of known prime numbers, it is not matched by the first switch case at all. There are no other specific cases, and so integerToDescribe is matched by the catchall default case.

After the switch statement has finished executing, the number锟斤拷s description is printed using the println function. In this example, the number 5 is correctly identified as a prime number.

NOTE

The fallthrough keyword does not check the case conditions for the switch case that it causes execution to fall into. The fallthrough keyword simply causes code execution to move directly to the statements inside the next case (or default case) block, as in C锟斤拷s standard switch statement behavior.

Labeled Statements

You can nest loops and switch statements inside other loops and switch statements in Swift to create complex control flow structures. However, loops and switch statements can both use the break statement to end their execution prematurely. Therefore, it is sometimes useful to be explicit about which loop or switch statement you want a break statement to terminate. Similarly, if you have multiple nested loops, it can be useful to be explicit about which loop the continue statement should affect.

To achieve these aims, you can mark a loop statement or switch statement with a statement label, and use this label with the break statement or continue statement to end or continue the execution of the labeled statement.

A labeled statement is indicated by placing a label on the same line as the statement锟斤拷s introducer keyword, followed by a colon. Here锟斤拷s an example of this syntax for a while loop, although the principle is the same for all loops and switch statements:

label name : while condition { statements

}

The following example uses the break and continue statements with a labeled while loop for an adapted version of the Snakes and Ladders game that you saw earlier in this chapter. This time around, the game has an extra rule:

If a particular dice roll would take you beyond square 25, you must roll again until you roll the exact number needed to land on square 25.

The game board is the same as before:

The values of finalSquare, board, square, and diceRoll are initialized in the same way as before:

This version of the game uses a while loop and a switch statement to implement the game锟斤拷s logic. The while loop has a statement label called gameLoop, to indicate that it is the main game loop for the Snakes and Ladders game.

The while loop锟斤拷s condition is while square != finalSquare, to reflect that you must land exactly on square 25:

gameLoop: while square != finalSquare {

if ++diceRoll == 7 { diceRoll = 1 } switch square + diceRoll { case finalSquare:

// diceRoll will move us to the final square, so the game is over break gameLoop case let newSquare where newSquare > finalSquare:

// diceRoll will move us beyond the final square, so roll again continue gameLoop fault:

// this is a valid move, so find out its effect square += diceRoll square += board[square]

("Game over!")

The dice is rolled at the start of each loop. Rather than moving the player immediately, a switch statement is used to consider the result of the move, and to work out if the move is allowed:

NOTE

If the break statement above did not use the gameLoop label, it would break out of the switch statement, not the while statement. Using the gameLoop label makes it clear which control statement should be terminated.

Note also that it is not strictly necessary to use the gameLoop label when calling continue gameLoop to jump

to the next iteration of the loop. There is only one loop in the game, and so there is no ambiguity as to which loop the continue statement will affect. However, there is no harm in using the gameLoop label with the continue statement. Doing so is consistent with the label锟斤拷s use alongside the break statement, and helps make the game锟斤拷s logic clearer to read and understand.

Functions

Functions are self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to 锟斤拷call锟斤拷 the function to perform its task when needed.

Swift锟斤拷s unified function syntax is flexible enough to express anything from a simple C-style function with no parameter names to a complex Objective-C-style method with local and external parameter names for each parameter. Parameters can provide default values to simplify function calls and can be passed as in-out parameters, which modify a passed variable once the function has completed its execution.

Every function in Swift has a type, consisting of the function锟斤拷s parameter types and return type. You can use this type like any other type in Swift, which makes it easy to pass functions as parameters to other functions, and to return functions from functions. Functions can also be written within other functions to encapsulate useful functionality within a nested function scope.

Defining and Calling Functions

When you define a function, you can optionally define one or more named, typed values that the function takes as input (known as parameters), and/or a type of value that the function will pass back as output when it is done (known as its return type).

Every function has a function name, which describes the task that the function performs. To use a function, you 锟斤拷call锟斤拷 that function with its name and pass it input values (known as arguments) that match the types of the function锟斤拷s parameters. A function锟斤拷s arguments must always be provided in the same order as the function锟斤拷s parameter list.

The function in the example below is called greetingForPerson, because that锟斤拷s what it does锟斤拷it takes a person锟斤拷s name as input and returns a greeting for that person. To accomplish this, you define one input parameter锟斤拷a String value called personName锟斤拷and a return type of String, which will contain a greeting for that person:

func sayHello(personName: String) -> String { let greeting = "Hello, " + personName + "!" return greeting

}

All of this information is rolled up into the function锟斤拷s definition, which is prefixed with the func keyword. You indicate the function锟斤拷s return type with the return arrow -> (a hyphen followed by a right angle bracket), which is followed by the name of the type to return.

The definition describes what the function does, what it expects to receive, and what it returns when it is done. The definition makes it easy for the function to be called elsewhere in your code in a clear and unambiguous way:

println(sayHello("Anna")) // prints "Hello, Anna!" println(sayHello("Brian"))

// prints "Hello, Brian!"

You call the sayHello function by passing it a String argument value in parentheses, such as sayHello("Anna"). Because the function returns a String value, sayHello can be wrapped in a call to the println function to print that string and see its return value, as shown above.

The body of the sayHello function starts by defining a new String constant called greeting and setting it to a simple greeting message for personName. This greeting is then passed back out of the function using the return keyword. As soon as return greeting is called, the function finishes its execution and returns the current value of greeting.

You can call the sayHello function multiple times with different input values. The example above shows what happens if it is called with an input value of "Anna", and an input value of "Brian". The function returns a tailored greeting in each case.

To simplify the body of this function, combine the message creation and the return statement into one line:

func sayHelloAgain(personName: String) -> String { return "Hello again, " + personName + "!"

} println(sayHelloAgain("Anna"))

// prints "Hello again, Anna!"

Function Parameters and Return Values

Function parameters and return values are extremely flexible in Swift. You can define anything from a simple utility function with a single unnamed parameter to a complex function with expressive parameter names and different parameter options.

Multiple Input Parameters

Functions can have multiple input parameters, which are written within the function锟斤拷s parentheses, separated by commas.

This function takes a start and an end index for a half-open range, and works out how many elements the range contains:

func halfOpenRangeLength(start: Int, end: Int) -> Int { return end - start

} println(halfOpenRangeLength(1, 10))

// prints "9"

Functions Without Parameters

Functions are not required to define input parameters. Here锟斤拷s a function with no input parameters, which always returns the same String message whenever it is called:

func sayHelloWorld() -> String { return "hello, world"

} println(sayHelloWorld())

// prints "hello, world"

The function definition still needs parentheses after the function锟斤拷s name, even though it does not take any parameters. The function name is also followed by an empty pair of parentheses when the function is called.

Functions Without Return Values

Functions are not required to define a return type. Here锟斤拷s a version of the sayHello function, called waveGoodbye, which prints its own String value rather than returning it:

func sayGoodbye(personName: String) { println("Goodbye, \(personName)!")

} sayGoodbye("Dave")

// prints "Goodbye, Dave!"

Because it does not need to return a value, the function锟斤拷s definition does not include the return arrow (->) or a return type.

NOTE

Strictly speaking, the sayGoodbye function does still return a value, even though no return value is defined. Functions without a defined return type return a special value of type Void. This is simply an empty tuple, in effect a tuple with zero elements, which can be written as ().

The return value of a function can be ignored when it is called:

func printAndCount(stringToPrint: String) -> Int { println(stringToPrint) return countElements(stringToPrint)

} func printWithoutCounting(stringToPrint: String) { printAndCount(stringToPrint)

} printAndCount("hello, world")

// prints "hello, world" and returns a value of 12

WithoutCounting("hello, world") nts "hello, world" but does not return a value

The first function, printAndCount, prints a string, and then returns its character count as an Int. The second function, printWithoutCounting, calls the first function, but ignores its return value. When the second function is called, the message is still printed by the first function, but the returned value is not used.

NOTE

Return values can be ignored, but a function that says it will return a value must always do so. A function with a defined return type cannot allow control to fall out of the bottom of the function without returning a value, and attempting to do so will result in a compile-time error.

Functions with Multiple Return Values

You can use a tuple type as the return type for a function to return multiple values as part of one compound return value.

The example below defines a function called count, which counts the number of vowels, consonants, and other characters in a string, based on the standard set of vowels and consonants used in American English:

func count(string: String) -> (vowels: Int, consonants: Int, others: Int) { var vowels = 0, consonants = 0, others = 0 for character in string { switch String(character).lowercaseString { case "a", "e", "i", "o", "u":

++vowels case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",

"n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":

++consonants default:

++others

}

turn (vowels, consonants, others)

You can use this count function to count the characters in an arbitrary string, and to retrieve the counted totals as a tuple of three named Int values:

let total = count("some arbitrary string!") println("\(total.vowels) vowels and \(total.consonants) consonants") // prints "6 vowels and 13 consonants"

Note that the tuple锟斤拷s members do not need to be named at the point that the tuple is returned from the function, because their names are already specified as part of the function锟斤拷s return type.

Function Parameter Names

All of the above functions define parameter names for their parameters:

func someFunction(parameterName: Int) {

// function body goes here, and can use parameterName

// to refer to the argument value for that parameter

}

However, these parameter names are only used within the body of the function itself, and cannot be used when calling the function. These kinds of parameter names are known as local parameter names, because they are only available for use within the function锟斤拷s body.

External Parameter Names

Sometimes it锟斤拷s useful to name each parameter when you call a function, to indicate the purpose of each argument you pass to the function.

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

func someFunction(externalParameterName localParameterName: Int) {

// function body goes here, and can use localParameterName

// to refer to the argument value for that parameter

}

NOTE

If you provide an external parameter name for a parameter, that external name must always be used when calling the function.

As an example, consider the following function, which joins two strings by inserting a third 锟斤拷joiner锟斤拷 string between them:

func join(s1: String, s2: String, joiner: String) -> String { return s1 + joiner + s2

}

When you call this function, the purpose of the three strings that you pass to the function is unclear:

join("hello", "world", ", ")

// returns "hello, world"

To make the purpose of these String values clearer, define external parameter names for each join function parameter:

func join(string s1: String, toString s2: String, withJoiner joiner: String)

-> String { return s1 + joiner + s2

}

In this version of the join function, the first parameter has an external name of string and a local name of s1; the second parameter has an external name of toString and a local name of s2; and the third parameter has an external name of withJoiner and a local name of joiner.

You can now use these external parameter names to call the function in a clear and unambiguous way:

join(string: "hello", toString: "world", withJoiner: ", ")

// returns "hello, world"

The use of external parameter names enables this second version of the join function to be called in an expressive, sentence-like manner by users of the function, while still providing a function body that is readable and clear in intent.

NOTE

Consider using external parameter names whenever the purpose of a function锟斤拷s arguments would be unclear to someone reading your code for the first time. You do not need to specify external parameter names if the purpose of each parameter is clear and unambiguous when the function is called.

Shorthand External Parameter Names

If you want to provide an external parameter name for a function parameter, and the local parameter name is already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead, write the name once, and prefix the name with a hash symbol (#). This tells Swift to use that name as both the local parameter name and the external parameter name.

This example defines a function called containsCharacter, which defines external parameter names for both of its parameters by placing a hash symbol before their local parameter names:

func containsCharacter(#string: String, #characterToFind: Character) -> Bool { for character in string { if character == characterToFind { return true

} } return false

}

This function锟斤拷s choice of parameter names makes for a clear, readable function body, while also enabling the function to be called without ambiguity:

let containsAVee = containsCharacter(string: "aardvark", characterToFind: "v")

// containsAVee equals true, because "aardvark" contains a "v"

Default Parameter Values

You can define a default value for any parameter as part of a function锟斤拷s definition. If a default value is defined, you can omit that parameter when calling the function.

NOTE

Place parameters with default values at the end of a function锟斤拷s parameter list. This ensures that all calls to the function use the same order for their non-default arguments, and makes it clear that the same function is being called in each case.

Here锟斤拷s a version of the join function from earlier, which provides a default value for its joiner parameter:

func join(string s1: String, toString s2: String, withJoiner joiner: String = " ") -> String { return s1 + joiner + s2

}

If a string value for joiner is provided when the join function is called, that string value is used to join the two strings together, as before:

join(string: "hello", toString: "world", withJoiner: "-")

// returns "hello-world"

However, if no value of joiner is provided when the function is called, the default value of a single space (" ") is used instead:

join(string: "hello", toString: "world")

// returns "hello world"

External Names for Parameters with Default Values

In most cases, it is useful to provide (and therefore require) an external name for any parameter with a default value. This ensures that the argument for that parameter is clear in purpose if a value is provided when the function is called.

To make this process easier, Swift provides an automatic external name for any defaulted parameter you define, if you do not provide an external name yourself. The automatic external name is the same as the local name, as if you had written a hash symbol before the local name in your code.

Here锟斤拷s a version of the join function from earlier, which does not provide external names for any of its parameters, but still provides a default value for its joiner parameter:

NOTE

In-out parameters cannot have default values, and variadic parameters cannot be marked as inout. If you mark a parameter as inout, it cannot also be marked as var or let.

Here锟斤拷s an example of a function called swapTwoInts, which has two in-out integer parameters called a and b:

func swapTwoInts(inout a: Int, inout b: Int) { let temporaryA = a a = b b = temporaryA

}

The swapTwoInts function simply swaps the value of b into a, and the value of a into b. The function performs this swap by storing the value of a in a temporary constant called temporaryA, assigning the value of b to a, and then assigning temporaryA to b.

You can call the swapTwoInts function with two variables of type Int to swap their values. Note that the names of someInt and anotherInt are prefixed with an ampersand when they are passed to the swapTwoInts function:

var someInt = 3 var anotherInt = 107 swapTwoInts(&someInt, &anotherInt) println("someInt is now \(someInt), and anotherInt is now \(anotherInt)")

// prints "someInt is now 107, and anotherInt is now 3"

The example above shows that the original values of someInt and anotherInt are modified by the swapTwoInts function, even though they were originally defined outside of the function.

NOTE

In-out parameters are not the same as returning a value from a function. The swapTwoInts example above does not define a return type or return a value, but it still modifies the values of someInt and anotherInt. Inout parameters are an alternative way for a function to have an effect outside of the scope of its function body.

Function Types

Every function has a specific function type, made up of the parameter types and the return type of the function.

For example:

func addTwoInts(a: Int, b: Int) -> Int { return a + b

} func multiplyTwoInts(a: Int, b: Int) -> Int { return a * b

}

This example defines two simple mathematical functions called addTwoInts and multiplyTwoInts. These functions each take two Int values, and return an Int value, which is the result of performing an appropriate mathematical operation.

The type of both of these functions is (Int, Int) -> Int. This can be read as:

锟斤拷A function type that has two parameters, both of type Int, and that returns a value of type Int.锟斤拷

Here锟斤拷s another example, for a function with no parameters or return value:

func printHelloWorld() { println("hello, world")

}

The type of this function is () -> (), or 锟斤拷a function that has no parameters, and returns Void.锟斤拷 Functions that don锟斤拷t specify a return value always return Void, which is equivalent to an empty tuple in Swift, shown as ().

Using Function Types

You use function types just like any other types in Swift. For example, you can define a constant or variable to be of a function type and assign an appropriate function to that variable:

var mathFunction: (Int, Int) -> Int = addTwoInts

This can be read as:

锟斤拷Define a variable called mathFunction, which has a type of 锟斤拷a function that takes two Int values, and returns an Int value.锟斤拷 Set this new variable to refer to the function called addTwoInts.锟斤拷

The addTwoInts function has the same type as the mathFunction variable, and so this assignment is allowed by Swift锟斤拷s type-checker.

You can now call the assigned function with the name mathFunction:

println("Result: \(mathFunction(2, 3))")

// prints "Result: 5"

A different function with the same matching type can be assigned to the same variable, in the same way as for non-function types:

mathFunction = multiplyTwoInts println("Result: \(mathFunction(2, 3))")

// prints "Result: 6"

As with any other type, you can leave it to Swift to infer the function type when you assign a function to a constant or variable:

let anotherMathFunction = addTwoInts

// anotherMathFunction is inferred to be of type (Int, Int) -> Int

Function Types as Parameter Types

You can use a function type such as (Int, Int) -> Int as a parameter type for another function. This enables you to leave some aspects of a function锟斤拷s implementation for the function锟斤拷s caller to provide when the function is called.

Here锟斤拷s an example to print the results of the math functions from above:

func printMathResult(mathFunction: (Int, Int) -> Int, a: Int, b: Int) { println("Result: \(mathFunction(a, b))")

}

printMathResult(addTwoInts, 3, 5)

// prints "Result: 8"

This example defines a function called printMathResult, which has three parameters. The first parameter is called mathFunction, and is of type (Int, Int) -> Int. You can pass any function of that type as the argument for this first parameter. The second and third parameters are called a and b, and are both of type Int. These are used as the two input values for the provided math function.

When printMathResult is called, it is passed the addTwoInts function, and the integer values 3 and 5. It calls the provided function with the values 3 and 5, and prints the result of 8.

The role of printMathResult is to print the result of a call to a math function of an appropriate type. It doesn锟斤拷t matter what that function锟斤拷s implementation actually does锟斤拷it matters only that the function is of the correct type. This enables printMathResult to hand off some of its functionality to the caller of the function in a type-safe way.

Function Types as Return Types

You can use a function type as the return type of another function. You do this by writing a complete function type immediately after the return arrow (->) of the returning function. The next example defines two simple functions called stepForward and stepBackward. The stepForward function returns a value one more than its input value, and the stepBackward function returns a value one less than its input value. Both functions have a type of (Int) ->

Int:

func stepForward(input: Int) -> Int { return input + 1

} func stepBackward(input: Int) -> Int { return input - 1

}

Here锟斤拷s a function called chooseStepFunction, whose return type is 锟斤拷a function of type (Int) -> Int锟斤拷.

chooseStepFunction returns the stepForward function or the stepBackward function based on a Boolean parameter called backwards:

func chooseStepFunction(backwards: Bool) -> (Int) -> Int {

return backwards ? stepBackward : stepForward

}

You can now use chooseStepFunction to obtain a function that will step in one direction or the other:

var currentValue = 3 let moveNearerToZero = chooseStepFunction(currentValue > 0)

// moveNearerToZero now refers to the stepBackward() function

The preceding example works out whether a positive or negative step is needed to move a variable called currentValue progressively closer to zero. currentValue has an initial value of 3, which means that currentValue > 0 returns true, causing chooseStepFunction to return the stepBackward function. A reference to the returned function is stored in a constant called moveNearerToZero.

Now that moveNearerToZero refers to the correct function, it can be used to count to zero:

println("Counting to zero:") // Counting to zero:

while currentValue != 0 { println("\(currentValue)... ") currentValue = moveNearerToZero(currentValue)

}

println("zero!")

// 3...

// 2...

Nested Functions

All of the functions you have encountered so far in this chapter have been examples of global functions, which are defined at a global scope. You can also define functions inside the bodies of other functions, known as nested functions.

Nested functions are hidden from the outside world by default, but can still be called and used by their enclosing function. An enclosing function can also return one of its nested functions to allow the nested function to be used in another scope.

You can rewrite the chooseStepFunction example above to use and return nested functions:

func chooseStepFunction(backwards: Bool) -> (Int) -> Int { func stepForward(input: Int) -> Int { return input + 1 } func stepBackward(input: Int) -> Int { return input - 1 } return backwards ? stepBackward : stepForward

} var currentValue = -4 let moveNearerToZero = chooseStepFunction(currentValue > 0) // moveNearerToZero now refers to the nested stepForward() function while currentValue != 0 { ntln("\(currentValue)... ") rrentValue = moveNearerToZero(currentValue)

("zero!")

Closures

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.

Closures can capture and store references to any constants and variables from the context in which they are defined. This is known as closing over those constants and variables, hence the name 锟斤拷closures锟斤拷. Swift handles all of the memory management of capturing for you.

NOTE

Don锟斤拷t worry if you are not familiar with the concept of 锟斤拷capturing锟斤拷. It is explained in detail below in Capturing Values.

Global and nested functions, as introduced in Functions, are actually special cases of closures. Closures take one of three forms:

Global functions are closures that have a name and do not capture any values.

Nested functions are closures that have a name and can capture values from their enclosing function.

Closure expressions are unnamed closures written in a lightweight syntax that can capture values from their surrounding context.

Swift锟斤拷s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free syntax in common scenarios. These optimizations include:

Inferring parameter and return value types from context

Implicit returns from single-expression closures

Shorthand argument names

Trailing closure syntax

Closure Expressions

Nested functions, as introduced in Nested Functions, are a convenient means of naming and defining self-contained blocks of code as part of a larger function. However, it is sometimes useful to write shorter versions of function-like constructs without a full declaration and name. This is particularly true when you work with functions that take other functions as one or more of their arguments.

Closure expressions are a way to write inline closures in a brief, focused syntax. Closure expressions provide several syntax optimizations for writing closures in their simplest form without loss of clarity or intent. The closure expression examples below illustrate these optimizations by refining a single example of the sort function over several iterations, each of which expresses the same functionality in a more succinct way.

The Sort Function

Swift锟斤拷s standard library provides a function called sort, which sorts an array of values of a known type, based on the output of a sorting closure that you provide. Once it completes the sorting process, the sort function returns a new array of the same type and size as the old one, with its elements in the correct sorted order.

The closure expression examples below use the sort function to sort an array of String values in reverse alphabetical order. Here锟斤拷s the initial array to be sorted:

let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]

The sort function takes two arguments:

This example is sorting an array of String values, and so the sorting closure needs to be a function of type (String, String) -> Bool.

One way to provide the sorting closure is to write a normal function of the correct type, and to pass it in as the sort function锟斤拷s second parameter:

func backwards(s1: String, s2: String) -> Bool { return s1 > s2

}

var reversed = sort(names, backwards)

// reversed is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"]

If the first string (s1) is greater than the second string (s2), the backwards function will return true, indicating that s1 should appear before s2 in the sorted array. For characters in strings, 锟斤拷greater than锟斤拷 means 锟斤拷appears later in the alphabet than锟斤拷. This means that the letter "B" is 锟斤拷greater than锟斤拷 the letter "A", and the string "Tom" is greater than the string "Tim". This gives a reverse alphabetical sort, with "Barry" being placed before "Alex", and so on.

However, this is a rather long-winded way to write what is essentially a single-expression function (a > b). In this example, it would be preferable to write the sorting closure inline, using closure expression syntax.

Closure Expression Syntax

Closure expression syntax has the following general form:

}

Closure expression syntax can use constant parameters, variable parameters, and inout parameters. Default values cannot be provided. Variadic parameters can be used if you name the variadic parameter and place it last in the parameter list. Tuples can also be used as parameter types and return types.

The example below shows a closure expression version of the backwards function from earlier:

reversed = sort(names, { (s1: String, s2: String) -> Bool in return s1 > s2

})

Note that the declaration of parameters and return type for this inline closure is identical to the declaration from the backwards function. In both cases, it is written as (s1: String, s2: String) -> Bool. However, for the inline closure expression, the parameters and return type are written inside the curly braces, not outside of them.

The start of the closure锟斤拷s body is introduced by the in keyword. This keyword indicates

that the definition of the closure锟斤拷s parameters and return type has finished, and the body of the closure is about to begin.

Because the body of the closure is so short, it can even be written on a single line:

reversed = sort(names, { (s1: String, s2: String) -> Bool in return s1 > s2 } )

This illustrates that the overall call to the sort function has remained the same. A pair of parentheses still wrap the entire set of arguments for the function. However, one of those arguments is now an inline closure.

Inferring Type From Context

Because the sorting closure is passed as an argument to a function, Swift can infer the types of its parameters and the type of the value it returns from the type of the sort function锟斤拷s second parameter. This parameter is expecting a function of type (String, String) -> Bool. This means that the String, String, and Bool types do not need to be written as part of the closure expression锟斤拷s definition. Because all of the types can be inferred, the return arrow (->) and the parentheses around the names of the parameters can also be omitted:

reversed = sort(names, { s1, s2 in return s1 > s2 } )

It is always possible to infer parameter types and return type when passing a closure to a function as an inline closure expression. As a result, you rarely need to write an inline closure in its fullest form.

Nonetheless, you can make the types explicit if you wish, and doing so is encouraged if it avoids ambiguity for readers of your code. In the case of the sort function, the purpose of the closure is clear from the fact that sorting is taking place, and it is safe for a reader to assume that the closure is likely to be working with String values, because it is assisting with the sorting of an array of strings.

Implicit Returns from Single-Expression Closures

Single-expression closures can implicitly return the result of their single expression by omitting the return keyword from their declaration, as in this version of the previous example:

reversed = sort(names, { s1, s2 in s1 > s2 } )

Here, the function type of the sort function锟斤拷s second argument makes it clear that a Bool value must be returned by the closure. Because the closure锟斤拷s body contains a single expression (s1 > s2) that returns a Bool value, there is no ambiguity, and the return keyword can be omitted.

Shorthand Argument Names

Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure锟斤拷s arguments by the names $0, $1, $2, and so on.

If you use these shorthand argument names within your closure expression, you can omit the closure锟斤拷s argument list from its definition, and the number and type of the shorthand argument names will be inferred from the expected function type. The in keyword can also be omitted, because the closure expression is made up entirely of its body:

reversed = sort(names, { $0 > $1 } )

Here, $0 and $1 refer to the closure锟斤拷s first and second String arguments.

Operator Functions

There锟斤拷s actually an even shorter way to write the closure expression above. Swift锟斤拷s String type defines its string-specific implementation of the greater-than operator (>) as a function that has two parameters of type String, and returns a value of type Bool. This exactly matches the function type needed for the sort function锟斤拷s second parameter. Therefore, you can simply pass in the greater-than operator, and Swift will infer that you want to use its string-specific implementation:

reversed = sort(names, >)

For more about operator functions, see Operator Functions.

Trailing Closures

If you need to pass a closure expression to a function as the function锟斤拷s final argument and the closure expression is long, it can be useful to write it as a trailing closure instead. A trailing closure is a closure expression that is written outside of (and after) the parentheses of the function call it supports:

func someFunctionThatTakesAClosure(closure: () -> ()) { // function body goes here

}

// here's how you call this function without using a trailing closure:

someFunctionThatTakesAClosure({

// closure's body goes here

})

re's how you call this function with a trailing closure instead:

FunctionThatTakesAClosure() { trailing closure's body goes here

NOTE

If a closure expression is provided as the function锟斤拷s only argument and you provide that expression as a trailing closure, you do not need to write a pair of parentheses () after the function锟斤拷s name when you call the function.

The string-sorting closure from the Closure Expression Syntax section above can be written outside of the sort function锟斤拷s parentheses as a trailing closure: reversed = sort(names) { $0 > $1 }

Trailing closures are most useful when the closure is sufficiently long that it is not possible to write it inline on a single line. As an example, Swift锟斤拷s Array type has a map method which takes a closure expression as its single argument. The closure is called once for each item in the array, and returns an alternative mapped value (possibly of some other type) for that item. The nature of the mapping and the type of the returned value is left up to the closure to specify.

After applying the provided closure to each array element, the map method returns a new array containing all of the new mapped values, in the same order as their corresponding values in the original array.

Here锟斤拷s how you can use the map method with a trailing closure to convert an array of Int values into an array of String values. The array [16, 58, 510] is used to create the new array

["OneSix", "FiveEight", "FiveOneZero"]:

let digitNames = [

0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four",

5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine"

] let numbers = [16, 58, 510]

The code above creates a dictionary of mappings between the integer digits and Englishlanguage versions of their names. It also defines an array of integers, ready to be converted into strings.

You can now use the numbers array to create an array of String values, by passing a closure expression to the array锟斤拷s map method as a trailing closure. Note that the call to numbers.map does not need to include any parentheses after map, because the map method has only one parameter, and that parameter is provided as a trailing closure:

let strings = numbers.map { (var number) -> String in var output = "" while number > 0 { output = digitNames[number % 10]! + output number /= 10

} return output

} ngs is inferred to be of type String[] value is ["OneSix", "FiveEight", "FiveOneZero"]

The map function calls the closure expression once for each item in the array. You do not need to specify the type of the closure锟斤拷s input parameter, number, because the type can be inferred from the values in the array to be mapped.

In this example, the closure锟斤拷s number parameter is defined as a variable parameter, as described in Constant and Variable Parameters, so that the parameter锟斤拷s value can be modified within the closure body, rather than declaring a new local variable and assigning the passed number value to it. The closure expression also specifies a return type of String, to indicate the type that will be stored in the mapped output array.

The closure expression builds a string called output each time it is called. It calculates the last digit of number by using the remainder operator (number % 10), and uses this digit to look up an appropriate string in the digitNames dictionary.

NOTE

The call to the digitNames dictionary锟斤拷s subscript is followed by an exclamation mark (!), because dictionary subscripts return an optional value to indicate that the dictionary lookup can fail if the key does not exist. In the example above, it is guaranteed that number % 10 will always be a valid subscript key for the digitNames dictionary, and so an exclamation mark is used to force-unwrap the String value stored in the subscript锟斤拷s optional return value.

The string retrieved from the digitNames dictionary is added to the front of output, effectively building a string version of the number in reverse. (The expression number % 10 gives a value of 6 for 16, 8 for 58, and 0 for 510.)

The number variable is then divided by 10. Because it is an integer, it is rounded down during the division, so 16 becomes 1, 58 becomes 5, and 510 becomes 51.

The process is repeated until number /= 10 is equal to 0, at which point the output string is returned by the closure, and is added to the output array by the map function.

The use of trailing closure syntax in the example above neatly encapsulates the closure锟斤拷s functionality immediately after the function that closure supports, without needing to wrap the entire closure within the map function锟斤拷s outer parentheses.

Capturing Values

A closure can capture constants and variables from the surrounding context in which it is defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.

The simplest form of a closure in Swift is a nested function, written within the body of another function. A nested function can capture any of its outer function锟斤拷s arguments and can also capture any constants and variables defined within the outer function.

Here锟斤拷s an example of a function called makeIncrementor, which contains a nested function called incrementor. The nested incrementor function captures two values, runningTotal and amount, from its surrounding context. After capturing these values, incrementor is returned by makeIncrementor as a closure that increments runningTotal by amount each time it is called.

func makeIncrementor(forIncrement amount: Int) -> () -> Int { var runningTotal = 0 func incrementor() -> Int { runningTotal += amount return runningTotal

} return incrementor

}

The return type of makeIncrementor is () -> Int. This means that it returns a function, rather than a simple value. The function it returns has no parameters, and returns an Int value each time it is called. To learn how functions can return other functions, see Function Types as Return Types.

The makeIncrementor function defines an integer variable called runningTotal, to store the current running total of the incrementor that will be returned. This variable is initialized with a value of 0.

The makeIncrementor function has a single Int parameter with an external name of forIncrement, and a local name of amount. The argument value passed to this parameter specifies how much runningTotal should be incremented by each time the returned incrementor function is called.

makeIncrementor defines a nested function called incrementor, which performs the actual incrementing. This function simply adds amount to runningTotal, and returns the result.

When considered in isolation, the nested incrementor function might seem unusual:

func incrementor() -> Int { runningTotal += amount return runningTotal

}

The incrementor function doesn锟斤拷t have any parameters, and yet it refers to runningTotal and amount from within its function body. It does this by capturing the existing values of runningTotal and amount from its surrounding function and using them within its own function body.

Because it does not modify amount, incrementor actually captures and stores a copy of the value stored in amount. This value is stored along with the new incrementor function.

However, because it modifies the runningTotal variable each time it is called, incrementor captures a reference to the current runningTotal variable, and not just a copy of its initial value. Capturing a reference ensures sure that runningTotal does not disappear when the call to makeIncrementor ends, and ensures that runningTotal will continue to be available the next time that the incrementor function is called.

NOTE

Swift determines what should be captured by reference and what should be copied by value. You don锟斤拷t need to annotate amount or runningTotal to say that they can be used within the nested incrementor function. Swift also handles all memory management involved in disposing of runningTotal when it is no longer needed by the incrementor function.

Here锟斤拷s an example of makeIncrementor in action: let incrementByTen = makeIncrementor(forIncrement: 10)

This example sets a constant called incrementByTen to refer to an incrementor function that adds 10 to its runningTotal variable each time it is called. Calling the function multiple times shows this behavior in action:

incrementByTen()

// returns a value of 10 incrementByTen()

// returns a value of 20 incrementByTen()

// returns a value of 30

If you create another incrementor, it will have its own stored reference to a new, separate runningTotal variable. In the example below, incrementBySeven captures a reference to a new runningTotal variable, and this variable is unconnected to the one captured by incrementByTen:

let incrementBySeven = makeIncrementor(forIncrement: 7) incrementBySeven()

// returns a value of 7 incrementByTen()

// returns a value of 40

NOTE

If you assign a closure to a property of a class instance, and the closure captures that instance by referring to the instance or its members, you will create a strong reference cycle between the closure and the instance. Swift uses capture lists to break these strong reference cycles. For more information, see Strong Reference Cycles for Closures.

Closures Are Reference Types

In the example above, incrementBySeven and incrementByTen are constants, but the closures these constants refer to are still able to increment the runningTotal variables that they have captured. This is because functions and closures are reference types.

Whenever you assign a function or a closure to a constant or a variable, you are actually setting that constant or variable to be a reference to the function or closure. In the example above, it is the choice of closure that incrementByTen refers to that is constant, and not the contents of the closure itself.

This also means that if you assign a closure to two different constants or variables, both of those constants or variables will refer to the same closure:

let alsoIncrementByTen = incrementByTen alsoIncrementByTen()

// returns a value of 50

Enumerations

An enumeration defines a common type for a group of related values and enables you to work with those values in a type-safe way within your code.

If you are familiar with C, you will know that C enumerations assign related names to a set of integer values. Enumerations in Swift are much more flexible, and do not have to provide a value for each member of the enumeration. If a value (known as a 锟斤拷raw锟斤拷 value) is provided for each enumeration member, the value can be a string, a character, or a value of any integer or floating-point type.

Alternatively, enumeration members can specify associated values of any type to be stored along with each different member value, much as unions or variants do in other languages. You can define a common set of related members as part of one enumeration, each of which has a different set of values of appropriate types associated with it.

Enumerations in Swift are first-class types in their own right. They adopt many features traditionally supported only by classes, such as computed properties to provide additional information about the enumeration锟斤拷s current value, and instance methods to provide functionality related to the values the enumeration represents. Enumerations can also define initializers to provide an initial member value; can be extended to expand their functionality beyond their original implementation; and can conform to protocols to provide standard functionality.

For more on these capabilities, see Properties, Methods, Initialization, Extensions, and Protocols.

Enumeration Syntax

You introduce enumerations with the enum keyword and place their entire definition within a pair of braces:

enum SomeEnumeration {

// enumeration definition goes here

}

Here锟斤拷s an example for the four main points of a compass:

enum CompassPoint {

case North case South case East case West

}

The values defined in an enumeration (such as North, South, East, and West) are the member values (or members) of that enumeration. The case keyword indicates that a new line of member values is about to be defined.

NOTE

Unlike C and Objective-C, Swift enumeration members are not assigned a default integer value when they are created. In the CompassPoints example above, North, South, East and West do not implicitly equal 0, 1, 2 and 3. Instead, the different enumeration members are fully-fledged values in their own right, with an explicitly-defined type of CompassPoint.

Multiple member values can appear on a single line, separated by commas:

enum Planet { case Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

}

Each enumeration definition defines a brand new type. Like other types in Swift, their names (such as CompassPoint and Planet) should start with a capital letter. Give enumeration types singular rather than plural names, so that they read as self-evident:

var directionToHead = CompassPoint.West

The type of directionToHead is inferred when it is initialized with one of the possible values of

CompassPoint. Once directionToHead is declared as a CompassPoint, you can set it to a different CompassPoint value using a shorter dot syntax:

directionToHead = .East

The type of directionToHead is already known, and so you can drop the type when setting its value. This makes for highly readable code when working with explicitly-typed enumeration values.

Matching Enumeration Values with a Switch Statement

You can match individual enumeration values with a switch statement:

directionToHead = .South switch directionToHead { case .North:

println("Lots of planets have a north") case .South:

println("Watch out for penguins") case .East:

println("Where the sun rises") case .West: ntln("Where the skies are blue")

nts "Watch out for penguins"

You can read this code as:

锟斤拷Consider the value of directionToHead. In the case where it equals .North, print "Lots of planets have a north". In the case where it equals .South, print "Watch out for penguins".锟斤拷锟斤拷and so on.

As described in Control Flow, a switch statement must be exhaustive when considering an enumeration锟斤拷s members. If the case for .West is omitted, this code does not compile, because it does not consider the complete list of CompassPoint members. Requiring exhaustiveness ensures that enumeration members are not accidentally omitted.

When it is not appropriate to provide a case for every enumeration member, you can provide a default case to cover any members that are not addressed explicitly:

let somePlanet = Planet.Earth switch somePlanet { case .Earth:

println("Mostly harmless") default:

println("Not a safe place for humans")

}

// prints "Mostly harmless"

Associated Values

The examples in the previous section show how the members of an enumeration are a defined (and typed) value in their own right. You can set a constant or variable to Planet.Earth, and check for this value later. However, it is sometimes useful to be able to store associated values of other types alongside these member values. This enables you to store additional custom information along with the member value, and permits this information to vary each time you use that member in your code.

You can define Swift enumerations to store associated values of any given type, and the value types can be different for each member of the enumeration if needed.

Enumerations similar to these are known as discriminated unions, tagged unions, or variants in other programming languages.

For example, suppose an inventory tracking system needs to track products by two different types of barcode. Some products are labeled with 1D barcodes in UPC-A format, which uses the numbers 0 to 9. Each barcode has a 锟斤拷number system锟斤拷 digit, followed by ten 锟斤拷identifier锟斤拷 digits. These are followed by a 锟斤拷check锟斤拷 digit to verify that the code has been scanned correctly:

Other products are labeled with 2D barcodes in QR code format, which can use any ISO 8859-1 character and can encode a string up to 2,953 characters long:

It would be convenient for an inventory tracking system to be able to store UPC-A barcodes as a tuple of three integers, and QR code barcodes as a string of any length.

In Swift, an enumeration to define product barcodes of either type might look like this:

enum Barcode { case UPCA(Int, Int, Int) case QRCode(String)

}

This can be read as:

锟斤拷Define an enumeration type called Barcode, which can take either a value of UPCA with an associated value of type (Int, Int, Int), or a value of QRCode with an associated value of type

String.锟斤拷

This definition does not provide any actual Int or String values锟斤拷it just defines the type of associated values that Barcode constants and variables can store when they are equal to Barcode.UPCA or Barcode.QRCode.

New barcodes can then be created using either type:

var productBarcode = Barcode.UPCA(8, 85909_51226, 3)

This example creates a new variable called productBarcode and assigns it a value of

Barcode.UPCA with an associated tuple value of (8, 8590951226, 3). The provided 锟斤拷identifier锟斤拷 value has an underscore within its integer literal锟斤拷85909_51226锟斤拷to make it easier to read as a barcode.

The same product can be assigned a different type of barcode: productBarcode = .QRCode("ABCDEFGHIJKLMNOP")

At this point, the original Barcode.UPCA and its integer values are replaced by the new

Barcode.QRCode and its string value. Constants and variables of type Barcode can store either a .UPCA or a .QRCode (together with their associated values), but they can only store one of them at any given time.

The different barcode types can be checked using a switch statement, as before. This time, however, the associated values can be extracted as part of the switch statement. You extract each associated value as a constant (with the let prefix) or a variable (with the var prefix) for use within the switch case锟斤拷s body:

switch productBarcode {

case .UPCA(let numberSystem, let identifier, let check):

println("UPC-A with value of \(numberSystem), \(identifier), \(check).") case .QRCode(let productCode):

println("QR code with value of \(productCode).")

}

// prints "QR code with value of ABCDEFGHIJKLMNOP."

If all of the associated values for a enumeration member are extracted as constants, or if all are extracted as variables, you can place a single var or let annotation before the member name, for brevity:

switch productBarcode { case let .UPCA(numberSystem, identifier, check):

println("UPC-A with value of \(numberSystem), \(identifier), \(check).") case let .QRCode(productCode):

println("QR code with value of \(productCode).")

}

// prints "QR code with value of ABCDEFGHIJKLMNOP."

Raw Values

The barcode example in Associated Values shows how members of an enumeration can declare that they store associated values of different types. As an alternative to associated values, enumeration members can come prepopulated with default values (called raw values), which are all of the same type.

Here锟斤拷s an example that stores raw ASCII values alongside named enumeration members:

enum ASCIIControlCharacter: Character { case Tab = "\t" case LineFeed = "\n" case CarriageReturn = "\r"

}

Here, the raw values for an enumeration called ASCIIControlCharacter are defined to be of type Character, and are set to some of the more common ASCII control characters. Character values are described in Strings and Characters.

Note that raw values are not the same as associated values. Raw values are set to prepopulated values when you first define the enumeration in your code, like the three ASCII codes above. The raw value for a particular enumeration member is always the same. Associated values are set when you create a new constant or variable based on one of the enumeration锟斤拷s members, and can be different each time you do so.

Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration. When integers are used for raw values, they auto-increment if no value is specified for some of the enumeration members.

The enumeration below is a refinement of the earlier Planet enumeration, with raw integer values to represent each planet锟斤拷s order from the sun:

enum Planet: Int { case Mercury = 1, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

}

Auto-incrementation means that Planet.Venus has a raw value of 2, and so on.

Access the raw value of an enumeration member with its toRaw method:

let earthsOrder = Planet.Earth.toRaw()

// earthsOrder is 3

Use an enumeration锟斤拷s fromRaw method to try to find an enumeration member with a particular raw value. This example identifies Uranus from its raw value of 7:

let possiblePlanet = Planet.fromRaw(7)

// possiblePlanet is of type Planet? and equals Planet.Uranus

Not all possible Int values will find a matching planet, however. Because of this, the fromRaw method returns an optional enumeration member. In the example above, possiblePlanet is of type Planet?, or 锟斤拷optional Planet.锟斤拷

If you try to find a Planet with a position of 9, the optional Planet value returned by fromRaw will be nil:

let positionToFind = 9 if let somePlanet = Planet.fromRaw(positionToFind) { switch somePlanet { case .Earth:

println("Mostly harmless") default:

println("Not a safe place for humans")

}

} else { ntln("There isn't a planet at position \(positionToFind)")

nts "There isn't a planet at position 9"

This example uses optional binding to try to access a planet with a raw value of 9. The statement if let somePlanet = Planet.fromRaw(9) retrieves an optional Planet, and sets somePlanet to the contents of that optional Planet if it can be retrieved. In this case, it is not possible to retrieve a planet with a position of 9, and so the else branch is executed instead.

Classes and Structures

Classes and structures are general-purpose, flexible constructs that become the building blocks of your program锟斤拷s code. You define properties and methods to add functionality to your classes and structures by using exactly the same syntax as for constants, variables, and functions.

Unlike other programming languages, Swift does not require you to create separate interface and implementation files for custom classes and structures. In Swift, you define a class or a structure in a single file, and the external interface to that class or structure is automatically made available for other code to use.

NOTE

An instance of a class is traditionally known as an object. However, Swift classes and structures are much closer in functionality than in other languages, and much of this chapter describes functionality that can apply to instances of either a class or a structure type. Because of this, the more general term instance is used.

Comparing Classes and Structures

Classes and structures in Swift have many things in common. Both can:

Define properties to store values

Define methods to provide functionality

Define subscripts to provide access to their values using subscript syntax

Define initializers to set up their initial state

Be extended to expand their functionality beyond a default implementation

Conform to protocols to provide standard functionality of a certain kind

For more information, see Properties, Methods, Subscripts, Initialization, Extensions, and Protocols.

Classes have additional capabilities that structures do not:

Inheritance enables one class to inherit the characteristics of another.

For more information, see Inheritance, Type Casting, Initialization, and Automatic Reference Counting.

NOTE

Structures are always copied when they are passed around in your code, and do not use reference counting.

Definition Syntax

Classes and structures have a similar definition syntax. You introduce classes with the class keyword and structures with the struct keyword. Both place their entire definition within a pair of braces:

class SomeClass {

// class definition goes here

} struct SomeStructure {

// structure definition goes here

}

NOTE

Whenever you define a new class or structure, you effectively define a brand new Swift type. Give types

UpperCamelCase names (such as SomeClass and SomeStructure here) to match the capitalization of standard Swift types (such as String, Int, and Bool). Conversely, always give properties and methods lowerCamelCase names (such as frameRate and incrementCount) to differentiate them from type names.

Here锟斤拷s an example of a structure definition and a class definition:

struct Resolution {

var width = 0 var height = 0

} class VideoMode { var resolution = Resolution() var interlaced = false var frameRate = 0.0 var name: String?

The example above defines a new structure called Resolution, to describe a pixel-based display resolution. This structure has two stored properties called width and height. Stored properties are constants or variables that are bundled up and stored as part of the class or structure. These two properties are inferred to be of type Int by setting them to an initial integer value of 0.

The example above also defines a new class called VideoMode, to describe a specific video mode for video display. This class has four variable stored properties. The first, resolution, is initialized with a new Resolution structure instance, which infers a property type of Resolution. For the other three properties, new VideoMode instances will be initialized with an interlaced setting of false (meaning 锟斤拷non-interlaced video锟斤拷), a playback frame rate of 0.0, and an optional String value called name. The name property is automatically given a default value of nil, or 锟斤拷no name value锟斤拷, because it is of an optional type.

Class and Structure Instances

The Resolution structure definition and the VideoMode class definition only describe what a Resolution or VideoMode will look like. They themselves do not describe a specific resolution or video mode. To do that, you need to create an instance of the structure or class.

The syntax for creating instances is very similar for both structures and classes:

let someResolution = Resolution() let someVideoMode = VideoMode()

Structures and classes both use initializer syntax for new instances. The simplest form of initializer syntax uses the type name of the class or structure followed by empty parentheses, such as Resolution() or VideoMode(). This creates a new instance of the class or structure, with any properties initialized to their default values. Class and structure initialization is described in more detail in Initialization.

Accessing Properties

You can access the properties of an instance using dot syntax. In dot syntax, you write the property name immediately after the instance name, separated by a period (.), without any spaces:

println("The width of someResolution is \(someResolution.width)")

// prints "The width of someResolution is 0"

In this example, someResolution.width refers to the width property of someResolution, and returns its default initial value of 0.

You can drill down into sub-properties, such as the width property in the resolution property of a VideoMode:

println("The width of someVideoMode is \(someVideoMode.resolution.width)")

// prints "The width of someVideoMode is 0"

You can also use dot syntax to assign a new value to a variable property:

someVideoMode.resolution.width = 1280 println("The width of someVideoMode is now \(someVideoMode.resolution.width)") // prints "The width of someVideoMode is now 1280"

NOTE

Unlike Objective-C, Swift enables you to set sub-properties of a structure property directly. In the last example above, the width property of the resolution property of someVideoMode is set directly, without your needing to set the entire resolution property to a new value.

Memberwise Initializers for Structure Types

All structures have an automatically-generated memberwise initializer, which you can use to initialize the member properties of new structure instances. Initial values for the properties of the new instance can be passed to the memberwise initializer by name:

let vga = Resolution(width: 640, height: 480)

Unlike structures, class instances do not receive a default memberwise initializer. Initializers are described in more detail in Initialization.

Structures and Enumerations Are Value Types

A value type is a type that is copied when it is assigned to a variable or constant, or when it is passed to a function.

You锟斤拷ve actually been using value types extensively throughout the previous chapters. In fact, all of the basic types in Swift锟斤拷integers, floating-point numbers, Booleans, strings, arrays and dictionaries锟斤拷are value types, and are implemented as structures behind the scenes.

All structures and enumerations are value types in Swift. This means that any structure and enumeration instances you create锟斤拷and any value types they have as properties锟斤拷are always copied when they are passed around in your code.

Consider this example, which uses the Resolution structure from the previous example:

let hd = Resolution(width: 1920, height: 1080) var cinema = hd

This example declares a constant called hd and sets it to a Resolution instance initialized with the width and height of full HD video (1920 pixels wide by 1080 pixels high).

It then declares a variable called cinema and sets it to the current value of hd. Because Resolution is a structure, a copy of the existing instance is made, and this new copy is assigned to cinema. Even though hd and cinema now have the same width and height, they are two completely different instances behind the scenes.

Next, the width property of cinema is amended to be the width of the slightly-wider 2K standard used for digital cinema projection (2048 pixels wide and 1080 pixels high): cinema.width = 2048

Checking the width property of cinema shows that it has indeed changed to be 2048:

println("cinema is now \(cinema.width) pixels wide")

// prints "cinema is now 2048 pixels wide"

However, the width property of the original hd instance still has the old value of 1920:

println("hd is still \(hd.width) pixels wide")

// prints "hd is still 1920 pixels wide"

When cinema was given the current value of hd, the values stored in hd were copied into the new cinema instance. The end result is two completely separate instances, which just happened to contain the same numeric values. Because they are separate instances, setting the width of cinema to 2048 doesn锟斤拷t affect the width stored in hd.

The same behavior applies to enumerations:

enum CompassPoint { case North, South, East, West

} var currentDirection = CompassPoint.West let rememberedDirection = currentDirection currentDirection = .East if rememberedDirection == .West { println("The remembered direction is still .West")

} nts "The remembered direction is still .West"

When rememberedDirection is assigned the value of currentDirection, it is actually set to a copy of that value. Changing the value of currentDirection thereafter does not affect the copy of the original value that was stored in rememberedDirection.

Classes Are Reference Types

Unlike value types, reference types are not copied when they are assigned to a variable or constant, or when they are passed to a function. Rather than a copy, a reference to the same existing instance is used instead.

Here锟斤拷s an example, using the VideoMode class defined above:

let tenEighty = VideoMode()

tenEighty.resolution = hd tenEighty.interlaced = true tenEighty.name = "1080i" tenEighty.frameRate = 25.0

This example declares a new constant called tenEighty and sets it to refer to a new instance of the VideoMode class. The video mode is assigned a copy of the HD resolution of 1920 by 1080 from before. It is set to be interlaced, and is given a name of "1080i". Finally, it is set to a frame rate of 25.0 frames per second.

Next, tenEighty is assigned to a new constant, called alsoTenEighty, and the frame rate of alsoTenEighty is modified:

let alsoTenEighty = tenEighty alsoTenEighty.frameRate = 30.0

Because classes are reference types, tenEighty and alsoTenEighty actually both refer to the same VideoMode instance. Effectively, they are just two different names for the same single instance.

Checking the frameRate property of tenEighty shows that it correctly reports the new frame rate of 30.0 from the underlying VideoMode instance:

println("The frameRate property of tenEighty is now \(tenEighty.frameRate)")

// prints "The frameRate property of tenEighty is now 30.0"

Note that tenEighty and alsoTenEighty are declared as constants, rather than variables.

However, you can still change tenEighty.frameRate and alsoTenEighty.frameRate because the values of the tenEighty and alsoTenEighty constants themselves do not actually change. tenEighty and alsoTenEighty themselves do not 锟斤拷store锟斤拷 the VideoMode instance锟斤拷instead, they both refer to a VideoMode instance behind the scenes. It is the frameRate property of the underlying VideoMode that is changed, not the values of the constant references to that VideoMode.

Identity Operators

Because classes are reference types, it is possible for multiple constants and variables to refer to the same single instance of a class behind the scenes. (The same is not true for structures and enumerations, because they are value types and are always copied when they are assigned to a constant or variable, or passed to a function.)

It can sometimes be useful to find out if two constants or variables refer to exactly the same instance of a class. To enable this, Swift provides two identity operators:

Use these operators to check whether two constants or variables refer to the same single instance:

if tenEighty === alsoTenEighty { println("tenEighty and alsoTenEighty refer to the same Resolution instance.") }

// prints "tenEighty and alsoTenEighty refer to the same Resolution instance."

Note that 锟斤拷identical to锟斤拷 (represented by three equals signs, or ===) does not mean the same thing as 锟斤拷equal to锟斤拷 (represented by two equals signs, or ==):

When you define your own custom classes and structures, it is your responsibility to decide what qualifies as two instances being 锟斤拷equal锟斤拷. The process of defining your own

implementations of the 锟斤拷equal to锟斤拷 and 锟斤拷not equal to锟斤拷 operators is described in Equivalence Operators.

Pointers

If you have experience with C, C++, or Objective-C, you may know that these languages use pointers to refer to addresses in memory. A Swift constant or variable that refers to an instance of some reference type is similar to a pointer in C, but is not a direct pointer to an address in memory, and does not require you to write an asterisk (*) to indicate that you are creating a reference. Instead, these references are defined like any other constant or variable in Swift.

Choosing Between Classes and Structures

You can use both classes and structures to define custom data types to use as the building blocks of your program锟斤拷s code.

However, structure instances are always passed by value, and class instances are always passed by reference. This means that they are suited to different kinds of tasks. As you consider the data constructs and functionality that you need for a project, decide whether each data construct should be defined as a class or as a structure.

As a general guideline, consider creating a structure when one or more of these conditions apply:

Examples of good candidates for structures include:

In all other cases, define a class, and create instances of that class to be managed and passed by reference. In practice, this means that most custom data constructs should be classes, not structures.

Assignment and Copy Behavior for Collection Types

Swift锟斤拷s Array and Dictionary types are implemented as structures. However, arrays have slightly different copying behavior from dictionaries and other structures when they are assigned to a constant or variable, and when they are passed to a function or method.

The behavior described for Array and Dictionary below is different again from the behavior of NSArray and NSDictionary in Foundation, which are implemented as classes, not structures. NSArray and NSDictionary instances are always assigned and passed around as a reference to an existing instance, rather than as a copy.

NOTE

The descriptions below refer to the 锟斤拷copying锟斤拷 of arrays, dictionaries, strings, and other values. Where copying is mentioned, the behavior you see in your code will always be as if a copy took place. However, Swift only performs an actual copy behind the scenes when it is absolutely necessary to do so. Swift manages all value copying to ensure optimal performance, and you should not avoid assignment to try to preempt this optimization.

Assignment and Copy Behavior for Dictionaries

Whenever you assign a Dictionary instance to a constant or variable, or pass a Dictionary instance as an argument to a function or method call, the dictionary is copied at the point that the assignment or call takes place. This process is described in Structures and Enumerations Are Value Types.

If the keys and/or values stored in the Dictionary instance are value types (structures or enumerations), they too are copied when the assignment or call takes place. Conversely, if the keys and/or values are reference types (classes or functions), the references are copied, but not the class instances or functions that they refer to. This copy behavior for a dictionary锟斤拷s keys and values is the same as the copy behavior for a structure锟斤拷s stored properties when the structure is copied.

The example below defines a dictionary called ages, which stores the names and ages of four people. The ages dictionary is then assigned to a new variable called copiedAges and is copied when this assignment takes place. After the assignment, ages and copiedAges are two separate dictionaries.

var ages = ["Peter": 23, "Wei": 35, "Anish": 65, "Katya": 19] var copiedAges = ages

The keys for this dictionary are of type String, and the values are of type Int. Both types are value types in Swift, and so the keys and values are also copied when the dictionary copy takes place.

You can prove that the ages dictionary has been copied by changing an age value in one of the dictionaries and checking the corresponding value in the other. If you set the value for

"Peter" in the copiedAges dictionary to 24, the ages dictionary still returns the old value of 23 from before the copy took place:

copiedAges["Peter"] = 24 println(ages["Peter"])

// prints "23"

Assignment and Copy Behavior for Arrays

The assignment and copy behavior for Swift锟斤拷s Array type is more complex than for its Dictionary type. Array provides C-like performance when you work with an array锟斤拷s contents and copies an array锟斤拷s contents only when copying is necessary.

If you assign an Array instance to a constant or variable, or pass an Array instance as an argument to a function or method call, the contents of the array are not copied at the point that the assignment or call takes place. Instead, both arrays share the same sequence of element values. When you modify an element value through one array, the result is observable through the other.

For arrays, copying only takes place when you perform an action that has the potential to modify the length of the array. This includes appending, inserting, or removing items, or using a ranged subscript to replace a range of items in the array. If and when array copying does take place, the copy behavior for an array锟斤拷s contents is the same as for a dictionary锟斤拷s keys and values, as described in Assignment and Copy Behavior for Dictionaries.

The example below assigns a new array of Int values to a variable called a. This array is also assigned to two further variables called b and c:

var a = [1, 2, 3] var b = a var c = a

You can retrieve the first value in the array with subscript syntax on either a, b, or c:

println(a[0])

// 1 println(b[0]) // 1 println(c[0])

// 1

If you set an item in the array to a new value with subscript syntax, all three of a, b, and c will return the new value. Note that the array is not copied when you set a new value with subscript syntax, because setting a single value with subscript syntax does not have the potential to change the array锟斤拷s length:

a[0] = 42

println(a[0]) // 42 println(b[0]) // 42 println(c[0])

// 42

However, if you append a new item to a, you do modify the array锟斤拷s length. This prompts Swift to create a new copy of the array at the point that you append the new value. Henceforth, a is a separate, independent copy of the array.

If you change a value in a after the copy is made, a will return a different value from b and c, which both still reference the original array contents from before the copy took place: a.append(4) a[0] = 777

println(a[0]) // 777 println(b[0]) // 42 println(c[0])

// 42

Ensuring That an Array Is Unique

It can be useful to ensure that you have a unique copy of an array before performing an action on that array锟斤拷s contents, or before passing that array to a function or method. You ensure the uniqueness of an array reference by calling the unshare method on a variable of array type. (The unshare method cannot be called on a constant array.)

If multiple variables currently refer to the same array, and you call the unshare method on one of those variables, the array is copied, so that the variable has its own independent copy of the array. However, no copying takes place if the variable is already the only reference to the array.

At the end of the previous example, b and c both reference the same array. Call the unshare method on b to make it become a unique copy: b.unshare()

If you change the first value in b after calling the unshare method, all three arrays will now report a different value:

b[0] = -105 println(a[0]) // 777 println(b[0]) // -105 println(c[0])

// 42

Checking Whether Two Arrays Share the Same Elements

Check whether two arrays or subarrays share the same storage and elements by comparing them with the identity operators (=== and !==).

The example below uses the 锟斤拷identical to锟斤拷 operator (===) to check whether b and c still share the same array elements:

if b === c { println("b and c still share the same array elements.")

} else { println("b and c now refer to two independent sets of array elements.") }

// prints "b and c now refer to two independent sets of array elements."

Alternatively, use the identity operators to check whether two subarrays share the same elements. The example below compares two identical subarrays from b and confirms that they refer to the same elements:

if b[0...1] === b[0...1] { println("These two subarrays share the same elements.")

} else { println("These two subarrays do not share the same elements.") }

// prints "These two subarrays share the same elements."

Forcing a Copy of an Array

Force an explicit copy of an array by calling the array锟斤拷s copy method. This method performs a shallow copy of the array and returns a new array containing the copied items.

The example below defines an array called names, which stores the names of seven people. A new variable called copiedNames is set to the result of calling the copy method on the names array:

var names = ["Mohsen", "Hilary", "Justyn", "Amy", "Rich", "Graham", "Vic"] var copiedNames = names.copy()

You can prove that the names array has been copied by changing an item in one of the arrays and checking the corresponding item in the other. If you set the first item in the copiedNames array to "Mo" rather than "Mohsen", the names array still returns the old value of "Mohsen" from before the copy took place:

copiedNames[0] = "Mo" println(names[0])

// prints "Mohsen"

NOTE

If you simply need to be sure that your reference to an array锟斤拷s contents is the only reference in existence, call the unshare method, not the copy method. The unshare method does not make a copy of the array unless it is necessary to do so. The copy method always copies the array, even if it is already unshared.

Properties

Properties associate values with a particular class, structure, or enumeration. Stored properties store constant and variable values as part of an instance, whereas computed properties calculate (rather than store) a value. Computed properties are provided by classes, structures, and enumerations. Stored properties are provided only by classes and structures.

Stored and computed properties are usually associated with instances of a particular type. However, properties can also be associated with the type itself. Such properties are known as type properties.

In addition, you can define property observers to monitor changes in a property锟斤拷s value, which you can respond to with custom actions. Property observers can be added to stored properties you define yourself, and also to properties that a subclass inherits from its superclass.

Stored Properties

In its simplest form, a stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

You can provide a default value for a stored property as part of its definition, as described in Default Property Values. You can also set and modify the initial value for a stored property during initialization. This is true even for constant stored properties, as described in Modifying Constant Properties During Initialization.

The example below defines a structure called FixedLengthRange, which describes a range of integers whose range length cannot be changed once it is created:

struct FixedLengthRange { var firstValue: Int let length: Int

} var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)

// the range represents integer values 0, 1, and 2

rangeOfThreeItems.firstValue = 6

// the range now represents integer values 6, 7, and 8

Instances of FixedLengthRange have a variable stored property called firstValue and a constant stored property called length. In the example above, length is initialized when the new range is created and cannot be changed thereafter, because it is a constant property.

Stored Properties of Constant Structure Instances

If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance锟斤拷s properties, even if they were declared as variable properties:

let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)

// this range represents integer values 0, 1, 2, and 3 rangeOfFourItems.firstValue = 6

// this will report an error, even thought firstValue is a variable property

Because rangeOfFourItems is declared as a constant (with the let keyword), it is not possible to change its firstValue property, even though firstValue is a variable property.

This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties.

The same is not true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance锟斤拷s variable properties.

Lazy Stored Properties

A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the @lazy attribute before its declaration.

NOTE

You must always declare a lazy property as a variable (with the var keyword), because its initial value may not be retrieved until after instance initialization completes. Constant properties must always have a value before initialization completes, and therefore cannot be declared as lazy.

Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are not known until after an instance锟斤拷s initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that should not be performed unless or until it is needed.

The example below uses a lazy stored property to avoid unnecessary initialization of a complex class. This example defines two classes called DataImporter and DataManager, neither of which is shown in full:

class DataImporter {

DataImporter is a class to import data from an external file.

The class is assumed to take a non-trivial amount of time to initialize.

*/ var fileName = "data.txt"

// the DataImporter class would provide data importing functionality here }

DataManager { azy var importer = DataImporter() r data = String[]() the DataManager class would provide data management functionality here anager = DataManager() ger.data += "Some data" ger.data += "Some more data"

DataImporter instance for the importer property has not yet been created

The DataManager class has a stored property called data, which is initialized with a new, empty array of String values. Although the rest of its functionality is not shown, the purpose of this DataManager class is to manage and provide access to this array of String data.

Part of the functionality of the DataManager class is the ability to import data from a file.

This functionality is provided by the DataImporter class, which is assumed to take a nontrivial amount of time to initialize. This might be because a DataImporter instance needs to open a file and read its contents into memory when the DataImporter instance is initialized.

It is possible for a DataManager instance to manage its data without ever importing data from a file, so there is no need to create a new DataImporter instance when the DataManager itself is created. Instead, it makes more sense to create the DataImporter instance if and when it is first used.

Because it is marked with the @lazy attribute, the DataImporter instance for the importer property is only created when the importer property is first accessed, such as when its fileName property is queried:

println(manager.importer.fileName)

// the DataImporter instance for the importer property has now been created

// prints "data.txt"

Stored Properties and Instance Variables

If you have experience with Objective-C, you may know that it provides two ways to store values and references as part of a class instance. In addition to properties, you can use instance variables as a backing store for the values stored in a property.

Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly. This approach avoids confusion about how the value is accessed in different contexts and simplifies the property锟斤拷s declaration into a single, definitive statement. All information about the property锟斤拷including its name, type, and memory management characteristics锟斤拷is defined in a single location as part of the type锟斤拷s definition.

Computed Properties

In addition to stored properties, classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

struct Point { var x = 0.0, y = 0.0

}

struct Size { var width = 0.0, height = 0.0

} struct Rect { var origin = Point() var size = Size() r center: Point { t { let centerX = origin.x + (size.width / 2) let centerY = origin.y + (size.height / 2) return Point(x: centerX, y: centerY)

t(newCenter) { origin.x = newCenter.x - (size.width / 2) origin.y = newCenter.y - (size.height / 2)

quare = Rect(origin: Point(x: 0.0, y: 0.0), e: Size(width: 10.0, height: 10.0)) ialSquareCenter = square.center

e.center = Point(x: 15.0, y: 15.0)

("square.origin is now at (\(square.origin.x), \(square.origin.y))") nts "square.origin is now at (10.0, 10.0)"

This example defines three structures for working with geometric shapes:

The Rect structure also provides a computed property called center. The current center position of a Rect can always be determined from its origin and size, and so you don锟斤拷t need to store the center point as an explicit Point value. Instead, Rect defines a custom getter and setter for a computed variable called center, to enable you to work with the rectangle锟斤拷s center as if it were a real stored property.

The preceding example creates a new Rect variable called square. The square variable is initialized with an origin point of (0, 0), and a width and height of 10. This square is represented by the blue square in the diagram below.

The square variable锟斤拷s center property is then accessed through dot syntax (square.center), which causes the getter for center to be called, to retrieve the current property value. Rather than returning an existing value, the getter actually calculates and returns a new Point to represent the center of the square. As can be seen above, the getter correctly returns a center point of (5, 5).

The center property is then set to a new value of (15, 15), which moves the square up and to the right, to the new position shown by the orange square in the diagram below. Setting the center property calls the setter for center, which modifies the x and y values of the stored origin property, and moves the square to its new position.

Shorthand Setter Declaration

If a computed property锟斤拷s setter does not define a name for the new value to be set, a default name of newValue is used. Here锟斤拷s an alternative version of the Rect structure, which takes advantage of this shorthand notation:

struct AlternativeRect { var origin = Point() var size = Size() var center: Point { get { let centerX = origin.x + (size.width / 2) let centerY = origin.y + (size.height / 2) return Point(x: centerX, y: centerY)

} t { origin.x = newValue.x - (size.width / 2) origin.y = newValue.y - (size.height / 2)

Read-Only Computed Properties

A computed property with a getter but no setter is known as a read-only computed property. A read-only computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a different value.

NOTE

You must declare computed properties锟斤拷including read-only computed properties锟斤拷as variable properties with the var keyword, because their value is not fixed. The let keyword is only used for constant properties, to indicate that their values cannot be changed once they are set as part of instance initialization.

You can simplify the declaration of a read-only computed property by removing the get keyword and its braces:

struct Cuboid { var width = 0.0, height = 0.0, depth = 0.0 var volume: Double { return width * height * depth

}

} let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0) println("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)")

// prints "the volume of fourByFiveByTwo is 40.0"

This example defines a new structure called Cuboid, which represents a 3D rectangular box with width, height, and depth properties. This structure also has a read-only computed property called volume, which calculates and returns the current volume of the cuboid. It doesn锟斤拷t make sense for volume to be settable, because it would be ambiguous as to which values of width, height, and depth should be used for a particular volume value. Nonetheless, it is useful for a Cuboid to provide a read-only computed property to enable external users to discover its current calculated volume.

Property Observers

Property observers observe and respond to changes in a property锟斤拷s value. Property observers are called every time a property锟斤拷s value is set, even if the new value is the same as the property锟斤拷s current value.

You can add property observers to any stored properties you define, apart from lazy stored properties. You can also add property observers to any inherited property (whether stored or computed) by overriding the property within a subclass. Property overriding is described in Overriding.

NOTE

You don锟斤拷t need to define property observers for non-overridden computed properties, because you can observe and respond to changes to their value from directly within the computed property锟斤拷s setter.

You have the option to define either or both of these observers on a property:

If you implement a willSet observer, it is passed the new property value as a constant parameter. You can specify a name for this parameter as part of your willSet implementation. If you choose not to write the parameter name and parentheses within your implementation, the parameter will still be made available with a default parameter name of newValue.

Similarly, if you implement a didSet observer, it will be passed a constant parameter containing the old property value. You can name the parameter if you wish, or use the default parameter name of oldValue.

NOTE

willSet and didSet observers are not called when a property is first initialized. They are only called when the property锟斤拷s value is set outside of an initialization context.

Here锟斤拷s an example of willSet and didSet in action. The example below defines a new class called StepCounter, which tracks the total number of steps that a person takes while walking. This class might be used with input data from a pedometer or other step counter to keep track of a person锟斤拷s exercise during their daily routine.

class StepCounter { var totalSteps: Int = 0 { willSet(newTotalSteps) { println("About to set totalSteps to \(newTotalSteps)")

} didSet { if totalSteps > oldValue { println("Added \(totalSteps - oldValue) steps")

}

epCounter = StepCounter() ounter.totalSteps = 200 out to set totalSteps to 200 ded 200 steps ounter.totalSteps = 360 out to set totalSteps to 360 ded 160 steps ounter.totalSteps = 896 out to set totalSteps to 896 ded 536 steps

The StepCounter class declares a totalSteps property of type Int. This is a stored property with willSet and didSet observers.

The willSet and didSet observers for totalSteps are called whenever the property is assigned a new value. This is true even if the new value is the same as the current value.

This example锟斤拷s willSet observer uses a custom parameter name of newTotalSteps for the upcoming new value. In this example, it simply prints out the value that is about to be set.

The didSet observer is called after the value of totalSteps is updated. It compares the new value of totalSteps against the old value. If the total number of steps has increased, a message is printed to indicate how many new steps have been taken. The didSet observer does not provide a custom parameter name for the old value, and the default name of oldValue is used instead.

NOTE

If you assign a value to a property within its own didSet observer, the new value that you assign will replace the one that was just set.

Global and Local Variables

The capabilities described above for computing and observing properties are also available to global variables and local variables. Global variables are variables that are defined outside of any function, method, closure, or type context. Local variables are variables that are defined within a function, method, or closure context.

The global and local variables you have encountered in previous chapters have all been stored variables. Stored variables, like stored properties, provide storage for a value of a certain type and allow that value to be set and retrieved.

However, you can also define computed variables and define observers for stored variables, in either a global or local scope. Computed variables calculate rather than store a value, and are written in the same way as computed properties.

NOTE

Global constants and variables are always computed lazily, in a similar manner to Lazy Stored Properties. Unlike lazy stored properties, global constants and variables do not need to be marked with the @lazy attribute.

Local constants and variables are never computed lazily.

Type Properties

Instance properties are properties that belong to an instance of a particular type. Every time you create a new instance of that type, it has its own set of property values, separate from any other instance.

You can also define properties that belong to the type itself, not to any one instance of that type. There will only ever be one copy of these properties, no matter how many instances of that type you create. These kinds of properties are called type properties.

Type properties are useful for defining values that are universal to all instances of a particular type, such as a constant property that all instances can use (like a static constant in C), or a variable property that stores a value that is global to all instances of that type (like a static variable in C).

For value types (that is, structures and enumerations), you can define stored and computed type properties. For classes, you can define computed type properties only.

Stored type properties for value types can be variables or constants. Computed type properties are always declared as variable properties, in the same way as computed instance properties.

NOTE

Unlike stored instance properties, you must always give stored type properties a default value. This is because the type itself does not have an initializer that can assign a value to a stored type property at initialization time.

Type Property Syntax

In C and Objective-C, you define static constants and variables associated with a type as global static variables. In Swift, however, type properties are written as part of the type锟斤拷s definition, within the type锟斤拷s outer curly braces, and each type property is explicitly scoped to the type it supports.

You define type properties for value types with the static keyword, and type properties for class types with the class keyword. The example below shows the syntax for stored and computed type properties:

struct SomeStructure { static var storedTypeProperty = "Some value." static var computedTypeProperty: Int {

// return an Int value here

}

} enum SomeEnumeration { static var storedTypeProperty = "Some value." static var computedTypeProperty: Int { return an Int value here

SomeClass { ss var computedTypeProperty: Int { return an Int value here

NOTE

The computed type property examples above are for read-only computed type properties, but you can also define read-write computed type properties with the same syntax as for computed instance properties.

Querying and Setting Type Properties

Type properties are queried and set with dot syntax, just like instance properties. However, type properties are queried and set on the type, not on an instance of that type. For example:

println(SomeClass.computedTypeProperty)

// prints "42"

println(SomeStructure.storedTypeProperty)

// prints "Some value."

SomeStructure.storedTypeProperty = "Another value." println(SomeStructure.storedTypeProperty)

// prints "Another value."

The examples that follow use two stored type properties as part of a structure that models an audio level meter for a number of audio channels. Each channel has an integer audio level between 0 and 10 inclusive.

The figure below illustrates how two of these audio channels can be combined to model a stereo audio level meter. When a channel锟斤拷s audio level is 0, none of the lights for that channel are lit. When the audio level is 10, all of the lights for that channel are lit. In this figure, the left channel has a current level of 9, and the right channel has a current level of 7:

The audio channels described above are represented by instances of the AudioChannel structure:

struct AudioChannel { static let thresholdLevel = 10 static var maxInputLevelForAllChannels = 0 var currentLevel: Int = 0 { didSet { if currentLevel > AudioChannel.thresholdLevel { // cap the new audio level to the threshold level currentLevel = AudioChannel.thresholdLevel

}

if currentLevel > AudioChannel.maxInputLevelForAllChannels { // store this as the new overall maximum input level

AudioChannel.maxInputLevelForAllChannels = currentLevel

}

The AudioChannel structure defines two stored type properties to support its functionality. The first, thresholdLevel, defines the maximum threshold value an audio level can take. This is a constant value of 10 for all AudioChannel instances. If an audio signal comes in with a higher value than 10, it will be capped to this threshold value (as described below).

The second type property is a variable stored property called maxInputLevelForAllChannels. This keeps track of the maximum input value that has been received by any AudioChannel instance. It starts with an initial value of 0.

The AudioChannel structure also defines a stored instance property called currentLevel, which represents the channel锟斤拷s current audio level on a scale of 0 to 10.

The currentLevel property has a didSet property observer to check the value of currentLevel whenever it is set. This observer performs two checks:

NOTE

In the first of these two checks, the didSet observer sets currentLevel to a different value. This does not, however, cause the observer to be called again.

You can use the AudioChannel structure to create two new audio channels called leftChannel and rightChannel, to represent the audio levels of a stereo sound system:

var leftChannel = AudioChannel() var rightChannel = AudioChannel()

If you set the currentLevel of the left channel to 7, you can see that the maxInputLevelForAllChannels type property is updated to equal 7:

leftChannel.currentLevel = 7 println(leftChannel.currentLevel)

// prints "7" println(AudioChannel.maxInputLevelForAllChannels)

// prints "7"

If you try to set the currentLevel of the right channel to 11, you can see that the right channel锟斤拷s currentLevel property is capped to the maximum value of 10, and the maxInputLevelForAllChannels type property is updated to equal 10:

rightChannel.currentLevel = 11 println(rightChannel.currentLevel)

// prints "10" println(AudioChannel.maxInputLevelForAllChannels)

// prints "10"

Methods

Methods are functions that are associated with a particular type. Classes, structures, and enumerations can all define instance methods, which encapsulate specific tasks and functionality for working with an instance of a given type. Classes, structures, and enumerations can also define type methods, which are associated with the type itself. Type methods are similar to class methods in Objective-C.

The fact that structures and enumerations can define methods in Swift is a major difference from C and Objective-C. In Objective-C, classes are the only types that can define methods. In Swift, you can choose whether to define a class, structure, or enumeration, and still have the flexibility to define methods on the type you create.

Instance Methods

Instance methods are functions that belong to instances of a particular class, structure, or enumeration. They support the functionality of those instances, either by providing ways to access and modify instance properties, or by providing functionality related to the instance锟斤拷s purpose. Instance methods have exactly the same syntax as functions, as described in Functions.

You write an instance method within the opening and closing braces of the type it belongs to. An instance method has implicit access to all other instance methods and properties of that type. An instance method can be called only on a specific instance of the type it belongs to. It cannot be called in isolation without an existing instance.

Here锟斤拷s an example that defines a simple Counter class, which can be used to count the number of times an action occurs:

class Counter { var count = 0 func increment() { count++

} func incrementBy(amount: Int) { count += amount

}

func reset() { count = 0

The Counter class defines three instance methods:

The Counter class also declares a variable property, count, to keep track of the current counter value.

You call instance methods with the same dot syntax as properties:

let counter = Counter()

// the initial counter value is 0 counter.increment()

// the counter's value is now 1 counter.incrementBy(5) // the counter's value is now 6 counter.reset()

// the counter's value is now 0

Local and External Parameter Names for Methods

Function parameters can have both a local name (for use within the function锟斤拷s body) and an external name (for use when calling the function), as described in External Parameter Names. The same is true for method parameters, because methods are just functions that are associated with a type. However, the default behavior of local names and external names is different for functions and methods.

Methods in Swift are very similar to their counterparts in Objective-C. As in Objective-C, the name of a method in Swift typically refers to the method锟斤拷s first parameter using a preposition such as with, for, or by, as seen in the incrementBy method from the preceding Counter class example. The use of a preposition enables the method to be read as a sentence when it is called. Swift makes this established method naming convention easy to write by using a different default approach for method parameters than it uses for function parameters.

Specifically, Swift gives the first parameter name in a method a local parameter name by default, and gives the second and subsequent parameter names both local and external parameter names by default. This convention matches the typical naming and calling convention you will be familiar with from writing Objective-C methods, and makes for expressive method calls without the need to qualify your parameter names.

Consider this alternative version of the Counter class, which defines a more complex form of the incrementBy method:

class Counter { var count: Int = 0 func incrementBy(amount: Int, numberOfTimes: Int) { count += amount * numberOfTimes

}

This incrementBy method has two parameters锟斤拷amount and numberOfTimes. By default, Swift treats amount as a local name only, but treats numberOfTimes as both a local and an external name. You call the method as follows:

let counter = Counter() counter.incrementBy(5, numberOfTimes: 3)

// counter value is now 15

You don锟斤拷t need to define an external parameter name for the first argument value, because its purpose is clear from the function name incrementBy. The second argument, however, is qualified by an external parameter name to make its purpose clear when the method is called.

This default behavior effectively treats the method as if you had written a hash symbol (#) before the numberOfTimes parameter:

func incrementBy(amount: Int, #numberOfTimes: Int) { count += amount * numberOfTimes

}

The default behavior described above mean that method definitions in Swift are written with the same grammatical style as Objective-C, and are called in a natural, expressive way.

Modifying External Parameter Name Behavior for Methods

Sometimes it锟斤拷s useful to provide an external parameter name for a method锟斤拷s first parameter, even though this is not the default behavior. You can either add an explicit external name yourself, or you can prefix the first parameter锟斤拷s name with a hash symbol to use the local name as an external name too.

Conversely, if you do not want to provide an external name for the second or subsequent parameter of a method, override the default behavior by using an underscore character (_) as an explicit external parameter name for that parameter.

The self Property

Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself. You use this implicit self property to refer to the current instance within its own instance methods.

The increment method in the example above could have been written like this:

func increment() { self.count++

}

In practice, you don锟斤拷t need to write self in your code very often. If you don锟斤拷t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method. This assumption is demonstrated by the use of count (rather than self.count) inside the three instance methods for Counter.

The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the implicit self property to distinguish between the parameter name and the property name.

Here, self disambiguates between a method parameter called x and an instance property that is also called x:

struct Point { var x = 0.0, y = 0.0 func isToTheRightOfX(x: Double) -> Bool { return self.x > x

}

} let somePoint = Point(x: 4.0, y: 5.0) if somePoint.isToTheRightOfX(1.0) { println("This point is to the right of the line where x == 1.0")

nts "This point is to the right of the line where x == 1.0"

Without the self prefix, Swift would assume that both uses of x referred to the method parameter called x.

Modifying Value Types from Within Instance Methods

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit self property, and this new instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the mutating keyword before the func keyword for that method:

struct Point { var x = 0.0, y = 0.0 mutating func moveByX(deltaX: Double, y deltaY: Double) { x += deltaX

y += deltaY

}

} var somePoint = Point(x: 1.0, y: 1.0) somePoint.moveByX(2.0, y: 3.0)

("The point is now at (\(somePoint.x), \(somePoint.y))") nts "The point is now at (3.0, 4.0)"

The Point structure above defines a mutating moveByX method, which moves a Point instance by a certain amount. Instead of returning a new point, this method actually modifies the point on which it is called. The mutating keyword is added to its definition to enable it to modify its properties.

Note that you cannot call a mutating method on a constant of structure type, because its properties cannot be changed, even if they are variable properties, as described in Stored Properties of Constant Structure Instances:

let fixedPoint = Point(x: 3.0, y: 3.0) fixedPoint.moveByX(2.0, y: 3.0)

// this will report an error

Assigning to self Within a Mutating Method

Mutating methods can assign an entirely new instance to the implicit self property. The Point example shown above could have been written in the following way instead:

struct Point { var x = 0.0, y = 0.0 mutating func moveByX(deltaX: Double, y deltaY: Double) { self = Point(x: x + deltaX, y: y + deltaY) }

}

This version of the mutating moveByX method creates a brand new structure whose x and y values are set to the target location. The end result of calling this alternative version of the method will be exactly the same as for calling the earlier version.

Mutating methods for enumerations can set the implicit self parameter to be a different member from the same enumeration:

enum TriStateSwitch { case Off, Low, High mutating func next() { switch self { case Off:

self = Low case Low:

self = High case High:

self = Off

} venLight = TriStateSwitch.Low Light.next() enLight is now equal to .High Light.next() enLight is now equal to .Off

This example defines an enumeration for a three-state switch. The switch cycles between three different power states (Off, Low and High) every time its next method is called.

Type Methods

Instance methods, as described above, are methods that are called on an instance of a particular type. You can also define methods that are called on the type itself. These kinds of methods are called type methods. You indicate type methods for classes by writing the keyword class before the method锟斤拷s func keyword, and type methods for structures and enumerations by writing the keyword static before the method锟斤拷s func keyword.

NOTE

In Objective-C, you can define type-level methods only for Objective-C classes. In Swift, you can define typelevel methods for all classes, structures, and enumerations. Each type method is explicitly scoped to the type it supports.

Type methods are called with dot syntax, like instance methods. However, you call type methods on the type, not on an instance of that type. Here锟斤拷s how you call a type method on a class called SomeClass:

class SomeClass { class func someTypeMethod() {

// type method implementation goes here }

}

SomeClass.someTypeMethod()

Within the body of a type method, the implicit self property refers to the type itself, rather than an instance of that type. For structures and enumerations, this means that you can use self to disambiguate between static properties and static method parameters, just as you do for instance properties and instance method parameters.

More generally, any unqualified method and property names that you use within the body of a type method will refer to other type-level methods and properties. A type method can call another type method with the other method锟斤拷s name, without needing to prefix it with the type name. Similarly, type methods on structures and enumerations can access static properties by using the static property锟斤拷s name without a type name prefix.

The example below defines a structure called LevelTracker, which tracks a player锟斤拷s progress through the different levels or stages of a game. It is a single-player game, but can store information for multiple players on a single device.

All of the game锟斤拷s levels (apart from level one) are locked when the game is first played. Every time a player finishes a level, that level is unlocked for all players on the device. The LevelTracker structure uses static properties and methods to keep track of which levels of the game have been unlocked. It also tracks the current level for an individual player.

struct LevelTracker { static var highestUnlockedLevel = 1 static func unlockLevel(level: Int) { if level > highestUnlockedLevel { highestUnlockedLevel = level } } static func levelIsUnlocked(level: Int) -> Bool { return level <= highestUnlockedLevel

} var currentLevel = 1 utating func advanceToLevel(level: Int) -> Bool { if LevelTracker.levelIsUnlocked(level) { currentLevel = level return true } else { return false

}

The LevelTracker structure keeps track of the highest level that any player has unlocked. This value is stored in a static property called highestUnlockedLevel.

LevelTracker also defines two type functions to work with the highestUnlockedLevel property. The first is a type function called unlockLevel, which updates the value of highestUnlockedLevel whenever a new level is unlocked. The second is a convenience type function called levelIsUnlocked, which returns true if a particular level number is already unlocked. (Note that these type methods can access the highestUnlockedLevel static property without your needing to write it as LevelTracker.highestUnlockedLevel.)

In addition to its static property and type methods, LevelTracker tracks an individual player锟斤拷s progress through the game. It uses an instance property called currentLevel to track the level that a player is currently playing.

To help manage the currentLevel property, LevelTracker defines an instance method called advanceToLevel. Before updating currentLevel, this method checks whether the requested new level is already unlocked. The advanceToLevel method returns a Boolean value to indicate whether or not it was actually able to set currentLevel.

The LevelTracker structure is used with the Player class, shown below, to track and update the progress of an individual player:

class Player {

var tracker = LevelTracker() let playerName: String func completedLevel(level: Int) { LevelTracker.unlockLevel(level + 1) tracker.advanceToLevel(level + 1)

} init(name: String) { playerName = name

The Player class creates a new instance of LevelTracker to track that player锟斤拷s progress. It also provides a method called completedLevel, which is called whenever a player completes a particular level. This method unlocks the next level for all players and updates the player锟斤拷s progress to move them to the next level. (The Boolean return value of advanceToLevel is ignored, because the level is known to have been unlocked by the call to LevelTracker.unlockLevel on the previous line.)

You can create a instance of the Player class for a new player, and see what happens when the player completes level one:

var player = Player(name: "Argyrios") player.completedLevel(1) println("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)")

// prints "highest unlocked level is now 2"

If you create a second player, whom you try to move to a level that is not yet unlocked by any player in the game, the attempt to set the player锟斤拷s current level fails:

player = Player(name: "Beto") if player.tracker.advanceToLevel(6) { println("player is now on level 6")

} else { println("level 6 has not yet been unlocked")

}

// prints "level 6 has not yet been unlocked"

If you want your custom value type to be initializable with the default initializer and memberwise initializer, and also with your own custom initializers, write your custom initializers in an extension rather than as part of the value type锟斤拷s original implementation. For more information, see Extensions.

The following example defines a custom Rect structure to represent a geometric rectangle. The example requires two supporting structures called Size and Point, both of which provide default values of 0.0 for all of their properties:

struct Size { var width = 0.0, height = 0.0

} struct Point { var x = 0.0, y = 0.0

}

You can initialize the Rect structure below in one of three ways锟斤拷by using its default zeroinitialized origin and size property values, by providing a specific origin point and size, or by providing a specific center point and size. These initialization options are represented by three custom initializers that are part of the Rect structure锟斤拷s definition:

struct Rect { var origin = Point() var size = Size() init() {} init(origin: Point, size: Size) { self.origin = origin self.size = size

} init(center: Point, size: Size) { let originX = center.x - (size.width / 2) let originY = center.y - (size.height / 2) self.init(origin: Point(x: originX, y: originY), size: size)

The first Rect initializer, init(), is functionally the same as the default initializer that the structure would have received if it did not have its own custom initializers. This initializer has an empty body, represented by an empty pair of curly braces {}, and does not perfom

any initialization. Calling this initializer returns a Rect instance whose origin and size properties are both initialized with the default values of Point(x: 0.0, y: 0.0) and Size(width: 0.0, height: 0.0) from their property definitions:

let basicRect = Rect()

// basicRect's origin is (0.0, 0.0) and its size is (0.0, 0.0)

The second Rect initializer, init(origin:size:), is functionally the same as the memberwise initializer that the structure would have received if it did not have its own custom initializers. This initializer simply assigns the origin and size argument values to the appropriate stored properties:

let originRect = Rect(origin: Point(x: 2.0, y: 2.0), size: Size(width: 5.0, height: 5.0))

// originRect's origin is (2.0, 2.0) and its size is (5.0, 5.0)

The third Rect initializer, init(center:size:), is slightly more complex. It starts by calculating an appropriate origin point based on a center point and a size value. It then calls (or delegates) to the init(origin:size:) initializer, which stores the new origin and size values in the appropriate properties:

let centerRect = Rect(center: Point(x: 4.0, y: 4.0), size: Size(width: 3.0, height: 3.0))

// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

The init(center:size:) initializer could have assigned the new values of origin and size to the appropriate properties itself. However, it is more convenient (and clearer in intent) for the init(center:size:) initializer to take advantage of an existing initializer that already provides exactly that functionality.

NOTE

For an alternative way to write this example without defining the init() and init(origin:size:) initializers yourself, see Extensions.

Class Inheritance and Initialization

All of a class锟斤拷s stored properties锟斤拷including any properties the class inherits from its superclass锟斤拷must be assigned an initial value during initialization.

Swift defines two kinds of initializers for class types to help ensure all stored properties receive an initial value. These are known as designated initializers and convenience initializers.

Designated Initializers and Convenience Initializers

Designated initializers are the primary initializers for a class. A designated initializer fully initializes all properties introduced by that class and calls an appropriate superclass initializer to continue the initialization process up the superclass chain.

Classes tend to have very few designated initializers, and it is quite common for a class to have only one. Designated initializers are 锟斤拷funnel锟斤拷 points through which initialization takes place, and through which the initialization process continues up the superclass chain.

Every class must have at least one designated initializer. In some cases, this requirement is satisfied by inheriting one or more designated initializers from a superclass, as described in Automatic Initializer Inheritance below.

Convenience initializers are secondary, supporting initializers for a class. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer锟斤拷s parameters set to default values. You can also define a convenience initializer to create an instance of that class for a specific use case or input value type.

You do not have to provide convenience initializers if your class does not require them. Create convenience initializers whenever a shortcut to a common initialization pattern will save time or make initialization of the class clearer in intent.

Initializer Chaining

To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers:

Designated initializers must call a designated initializer from their immediate superclass.

Convenience initializers must call another initializer available in the same class.

Convenience initializers must ultimately end up calling a designated initializer.

A simple way to remember this is:

These rules are illustrated in the figure below:

Here, the superclass has a single designated initializer and two convenience initializers. One convenience initializer calls another convenience initializer, which in turn calls the single designated initializer. This satisfies rules 2 and 3 from above. The superclass does not itself have a further superclass, and so rule 1 does not apply.

The subclass in this figure has two designated initializers and one convenience initializer. The convenience initializer must call one of the two designated initializers, because it can only call another initializer from the same class. This satisfies rules 2 and 3 from above. Both designated initializers must call the single designated initializer from the superclass, to satisfy rule 1 from above.

NOTE

These rules don锟斤拷t affect how users of your classes create instances of each class. Any initializer in the diagram above can be used to create a fully-initialized instance of the class they belong to. The rules only affect how you write the class锟斤拷s implementation.

The figure below shows a more complex class hierarchy for four classes. It illustrates how the designated initializers in this hierarchy act as 锟斤拷funnel锟斤拷 points for class initialization, simplifying the interrelationships among classes in the chain:

Two-Phase Initialization

Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial value by the class that introduced it. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use.

The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to each class in a class hierarchy. Two-phase initialization prevents property values from being accessed before they are initialized, and prevents property values from being set to a different value by another initializer unexpectedly.

NOTE

Swift锟斤拷s two-phase initialization process is similar to initialization in Objective-C. The main difference is that during phase 1, Objective-C assigns zero or null values (such as 0 or nil) to every property. Swift锟斤拷s initialization flow is more flexible in that it lets you set custom initial values, and can cope with types for which 0 or nil is not a valid default value.

Swift锟斤拷s compiler performs four helpful safety-checks to make sure that two-phase initialization is completed without error:

A designated initializer must ensure that all of the properties introduced by its class are initialized before it delegates up to a superclass initializer.

As mentioned above, the memory for an object is only considered fully initialized once the initial state of all of its stored properties is known. In order for this rule to be satisfied, a designated initializer must make sure that all its own properties are initialized before it hands off up the chain.

A designated initializer must delegate up to a superclass initializer before assigning a value to an inherited property. If it doesn锟斤拷t, the new value the designated initializer assigns will be overwritten by the superclass as part of its own initialization.

A convenience initializer must delegate to another initializer before assigning a value to any property (including properties defined by the same class). If it doesn锟斤拷t, the new value the convenience initializer assigns will be overwritten by its own class锟斤拷s designated initializer.

An initializer cannot call any instance methods, read the values of any instance properties, or refer to self as a value until after the first phase of initialization is complete.

The class instance is not fully valid until the first phase ends. Properties can only be accessed, and methods can only be called, once the class instance is known to be valid at the end of the first phase.

Here锟斤拷s how two-phase initialization plays out, based on the four safety checks above:

Phase 1

Phase 2

Here锟斤拷s how phase 1 looks for an initialization call for a hypothetical subclass and superclass:

In this example, initialization begins with a call to a convenience initializer on the subclass. This convenience initializer cannot yet modify any properties. It delegates across to a designated initializer from the same class.

The designated initializer makes sure that all of the subclass锟斤拷s properties have a value, as per safety check 1. It then calls a designated initializer on its superclass to continue the initialization up the chain.

The superclass锟斤拷s designated initializer makes sure that all of the superclass properties have a value. There are no further superclasses to initialize, and so no further delegation is needed.

As soon as all properties of the superclass have an initial value, its memory is considered fully initialized, and Phase 1 is complete.

Here锟斤拷s how phase 2 looks for the same initialization call:

The superclass锟斤拷s designated initializer now has an opportunity to customize the instance further (although it does not have to).

Once the superclass锟斤拷s designated initializer is finished, the subclass锟斤拷s designated initializer can perform additional customization (although again, it does not have to).

Finally, once the subclass锟斤拷s designated initializer is finished, the convenience initializer that was originally called can perform additional customization.

Initializer Inheritance and Overriding

Unlike subclasses in Objective-C, Swift subclasses do not not inherit their superclass initializers by default. Swift锟斤拷s approach prevents a situation in which a simple initializer from a superclass is automatically inherited by a more specialized subclass and is used to create a new instance of the subclass that is not fully or correctly initialized.

If you want your custom subclass to present one or more of the same initializers as its superclass锟斤拷perhaps to perform some customization during initialization锟斤拷you can provide an overriding implementation of the same initializer within your custom subclass.

If the initializer you are overriding is a designated initializer, you can override its implementation in your subclass and call the superclass version of the initializer from within your overriding version.

If the initializer you are overriding is a convenience initializer, your override must call another designated initializer from its own subclass, as per the rules described above in Initializer Chaining.

NOTE

Unlike methods, properties, and subscripts, you do not need to write the override keyword when overriding an initializer.

Automatic Initializer Inheritance

As mentioned above, subclasses do not not inherit their superclass initializers by default.

However, superclass initializers are automatically inherited if certain conditions are met. In practice, this means that you do not need to write initializer overrides in many common scenarios, and can inherit your superclass initializers with minimal effort whenever it is safe to do so.

Assuming that you provide default values for any new properties you introduce in a subclass, the following two rules apply:

If your subclass doesn锟斤拷t define any designated initializers, it automatically inherits all of its superclass designated initializers.

If your subclass provides an implementation of all of its superclass designated initializers锟斤拷either by inheriting them as per rule 1, or by providing a custom implementation as part of its definition锟斤拷then it automatically inherits all of the superclass convenience initializers.

These rules apply even if your subclass adds further convenience initializers.

NOTE

A subclass can implement a superclass designated initializer as a subclass convenience initializer as part of satisfying rule 2.

Syntax for Designated and Convenience Initializers

Designated initializers for classes are written in the same way as simple initializers for value types:

init( parameters ) { statements

}

Convenience initializers are written in the same style, but with the convenience keyword placed before the init keyword, separated by a space:

convenience init( parameters ) { statements

}

Designated and Convenience Initializers in Action

The following example shows designated initializers, convenience initializers, and automatic initializer inheritance in action. This example defines a hierarchy of three classes called Food, RecipeIngredient, and ShoppingListItem, and demonstrates how their initializers interact.

The base class in the hierarchy is called Food, which is a simple class to encapsulate the name of a foodstuff. The Food class introduces a single String property called name and provides two initializers for creating Food instances:

class Food { var name: String init(name: String) { self.name = name

} convenience init() { self.init(name: "[Unnamed]")

}

The figure below shows the initializer chain for the Food class:

Classes do not have a default memberwise initializer, and so the Food class provides a designated initializer that takes a single argument called name. This initializer can be used to create a new Food instance with a specific name:

let namedMeat = Food(name: "Bacon")

// namedMeat's name is "Bacon"

The init(name: String) initializer from the Food class is provided as a designated initializer, because it ensures that all stored properties of a new Food instance are fully initialized. The Food class does not have a superclass, and so the init(name: String) initializer does not need to call super.init() to complete its initialization.

The Food class also provides a convenience initializer, init(), with no arguments. The init() initializer provides a default placeholder name for a new food by delegating across to the Food class锟斤拷s init(name: String) with a name value of [Unnamed]:

let mysteryMeat = Food()

// mysteryMeat's name is "[Unnamed]"

The second class in the hierarchy is a subclass of Food called RecipeIngredient. The

RecipeIngredient class models an ingredient in a cooking recipe. It introduces an Int property called quantity (in addition to the name property it inherits from Food) and defines two initializers for creating RecipeIngredient instances:

class RecipeIngredient: Food { var quantity: Int init(name: String, quantity: Int) { self.quantity = quantity super.init(name: name)

} convenience init(name: String) { self.init(name: name, quantity: 1)

}

The figure below shows the initializer chain for the RecipeIngredient class:

The RecipeIngredient class has a single designated initializer, init(name: String, quantity: Int), which can be used to populate all of the properties of a new RecipeIngredient instance. This initializer starts by assigning the passed quantity argument to the quantity property, which is the only new property introduced by RecipeIngredient. After doing so, the initializer delegates up to the init(name: String) initializer of the Food class. This process satisfies safety check 1 from Two-Phase Initialization above.

RecipeIngredient also defines a convenience initializer, init(name: String), which is used to create a RecipeIngredient instance by name alone. This convenience initializer assumes a quantity of 1 for any RecipeIngredient instance that is created without an explicit quantity. The definition of this convenience initializer makes RecipeIngredient instances quicker and more convenient to create, and avoids code duplication when creating several single-quantity RecipeIngredient instances. This convenience initializer simply delegates across to the class锟斤拷s designated initializer.

Note that the init(name: String) convenience initializer provided by RecipeIngredient takes the same parameters as the init(name: String) designated initializer from Food. Even though RecipeIngredient provides this initializer as a convenience initializer, RecipeIngredient has nonetheless provided an implementation of all of its superclass锟斤拷s designated initializers. Therefore, RecipeIngredient automatically inherits all of its superclass锟斤拷s convenience initializers too.

In this example, the superclass for RecipeIngredient is Food, which has a single convenience initializer called init(). This initializer is therefore inherited by RecipeIngredient. The inherited version of init() functions in exactly the same way as the Food version, except that it delegates to the RecipeIngredient version of init(name: String) rather than the Food version.

All three of these initializers can be used to create new RecipeIngredient instances:

let oneMysteryItem = RecipeIngredient() let oneBacon = RecipeIngredient(name: "Bacon") let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6)

The third and final class in the hierarchy is a subclass of RecipeIngredient called ShoppingListItem. The ShoppingListItem class models a recipe ingredient as it appears in a shopping list.

Every item in the shopping list starts out as 锟斤拷unpurchased锟斤拷. To represent this fact,

ShoppingListItem introduces a Boolean property called purchased, with a default value of false. ShoppingListItem also adds a computed description property, which provides a textual description of a ShoppingListItem instance:

class ShoppingListItem: RecipeIngredient { var purchased = false var description: String { var output = "\(quantity) x \(name.lowercaseString)" output += purchased ? " ✔" : " ✘" return output

}

NOTE

ShoppingListItem does not define an initializer to provide an initial value for purchased, because items in a shopping list (as modeled here) always start out unpurchased.

Because it provides a default value for all of the properties it introduces and does not define any initializers itself, ShoppingListItem automatically inherits all of the designated and convenience initializers from its superclass.

The figure below shows the overall initializer chain for all three classes:

You can use all three of the inherited initializers to create a new ShoppingListItem instance:

var breakfastList = [

ShoppingListItem(),

ShoppingListItem(name: "Bacon"),

ShoppingListItem(name: "Eggs", quantity: 6),

] breakfastList[0].name = "Orange juice" breakfastList[0].purchased = true for item in breakfastList { println(item.description)

orange juice ✔ bacon ✘ eggs ✘

Here, a new array called breakfastList is created from an array literal containing three new ShoppingListItem instances. The type of the array is inferred to be ShoppingListItem[]. After the array is created, the name of the ShoppingListItem at the start of the array is changed from " [Unnamed]" to "Orange juice" and it is marked as having been purchased. Printing the description of each item in the array shows that their default states have been set as expected.

Setting a Default Property Value with a Closure or Function

If a stored property锟斤拷s default value requires some customization or setup, you can use a closure or global function to provide a customized default value for that property. Whenever a new instance of the type that the property belongs to is initialized, the closure or function is called, and its return value is assigned as the property锟斤拷s default value.

These kinds of closures or functions typically create a temporary value of the same type as the property, tailor that value to represent the desired initial state, and then return that temporary value to be used as the property锟斤拷s default value.

Here锟斤拷s a skeleton outline of how a closure can be used to provide a default property value:

class SomeClass { let someProperty: SomeType = {

// create a default value for someProperty inside this closure // someValue must be of the same type as SomeType return someValue

}()

}

Note that the closure锟斤拷s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately. If you omit these parentheses, you are trying to assign the closure itself to the property, and not the return value of the closure.

NOTE

If you use a closure to initialize a property, remember that the rest of the instance has not yet been initialized at the point that the closure is executed. This means that you cannot access any other property values from within your closure, even if those properties have default values. You also cannot use the implicit self property, or call any of the instance锟斤拷s methods.

The example below defines a structure called Checkerboard, which models a board for the game of Checkers (also known as Draughts):

The game of Checkers is played on a ten-by-ten board, with alternating black and white squares. To represent this game board, the Checkerboard structure has a single property called boardColors, which is an array of 100 Bool values. A value of true in the array represents a black square and a value of false represents a white square. The first item in the array represents the top left square on the board and the last item in the array represents the bottom right square on the board.

The boardColors array is initialized with a closure to set up its color values:

struct Checkerboard { let boardColors: Bool[] = { var temporaryBoard = Bool[]() var isBlack = false for i in 1...10 { for j in 1...10 { temporaryBoard.append(isBlack) isBlack = !isBlack

} isBlack = !isBlack

} return temporaryBoard

}() nc squareIsBlackAtRow(row: Int, column: Int) -> Bool { return boardColors[(row * 10) + column]

Whenever a new Checkerboard instance is created, the closure is executed, and the default value of boardColors is calculated and returned. The closure in the example above calculates and sets the appropriate color for each square on the board in a temporary array called temporaryBoard, and returns this temporary array as the closure锟斤拷s return value once its setup is complete. The returned array value is stored in boardColors and can be queried with the squareIsBlackAtRow utility function:

let board = Checkerboard() println(board.squareIsBlackAtRow(0, column: 1))

// prints "true" println(board.squareIsBlackAtRow(9, column: 9))

// prints "false"

Deinitialization

A deinitializer is called immediately before a class instance is deallocated. You write deinitializers with the deinit keyword, similar to how intializers are written with the init keyword. Deinitializers are only available on class types.

How Deinitialization Works

Swift automatically deallocates your instances when they are no longer needed, to free up resources. Swift handles the memory management of instances through automatic reference counting (ARC), as described in Automatic Reference Counting. Typically you don锟斤拷t need to perform manual clean-up when your instances are deallocated. However, when you are working with your own resources, you might need to perform some additional clean-up yourself. For example, if you create a custom class to open a file and write some data to it, you might need to close the file before the class instance is deallocated.

Class definitions can have at most one deinitializer per class. The deinitializer does not take any parameters and is written without parentheses:

deinit {

// perform the deinitialization

}

Deinitializers are called automatically, just before instance deallocation takes place. You are not allowed to call a deinitializer yourself. Superclass deinitializers are inherited by their subclasses, and the superclass deinitializer is called automatically at the end of a subclass deinitializer implementation. Superclass deinitializers are always called, even if a subclass does not provide its own deinitializer.

Because an instance is not deallocated until after its deinitializer is called, a deinitializer can access all properties of the instance it is called on and can modify its behavior based on those properties (such as looking up the name of a file that needs to be closed).

Deinitializers in Action

Here锟斤拷s an example of a deinitializer in action. This example defines two new types, Bank and Player, for a simple game. The Bank structure manages a made-up currency, which can never have more than 10,000 coins in circulation. There can only ever be one Bank in the game, and so the Bank is implemented as a structure with static properties and methods to store and manage its current state:

struct Bank { static var coinsInBank = 10_000 static func vendCoins(var numberOfCoinsToVend: Int) -> Int { numberOfCoinsToVend = min(numberOfCoinsToVend, coinsInBank) coinsInBank -= numberOfCoinsToVend return numberOfCoinsToVend

} static func receiveCoins(coins: Int) { coinsInBank += coins

Bank keeps track of the current number of coins it holds with its coinsInBank property. It also offers two methods锟斤拷vendCoins and receiveCoins锟斤拷to handle the distribution and collection of coins.

vendCoins checks that there are enough coins in the bank before distributing them. If there are not enough coins, Bank returns a smaller number than the number that was requested

(and returns zero if no coins are left in the bank). vendCoins declares numberOfCoinsToVend as a variable parameter, so that the number can be modified within the method锟斤拷s body without the need to declare a new variable. It returns an integer value to indicate the actual number of coins that were provided.

The receiveCoins method simply adds the received number of coins back into the bank锟斤拷s coin store.

The Player class describes a player in the game. Each player has a certain number of coins stored in their purse at any time. This is represented by the player锟斤拷s coinsInPurse property:

class Player { var coinsInPurse: Int init(coins: Int) { coinsInPurse = Bank.vendCoins(coins) } func winCoins(coins: Int) { coinsInPurse += Bank.vendCoins(coins)

} deinit {

Bank.receiveCoins(coinsInPurse)

Each Player instance is initialized with a starting allowance of a specified number of coins from the bank during initialization, although a Player instance may receive fewer than that number if not enough coins are available.

The Player class defines a winCoins method, which retrieves a certain number of coins from the bank and adds them to the player锟斤拷s purse. The Player class also implements a deinitializer, which is called just before a Player instance is deallocated. Here, the deinitializer simply returns all of the player锟斤拷s coins to the bank:

var playerOne: Player? = Player(coins: 100) println("A new player has joined the game with \(playerOne!.coinsInPurse) coins")

// prints "A new player has joined the game with 100 coins" println("There are now \(Bank.coinsInBank) coins left in the bank")

// prints "There are now 9900 coins left in the bank"

A new Player instance is created, with a request for 100 coins if they are available. This

Player instance is stored in an optional Player variable called playerOne. An optional variable is used here, because players can leave the game at any point. The optional lets you track whether there is currently a player in the game.

Because playerOne is an optional, it is qualified with an exclamation mark (!) when its coinsInPurse property is accessed to print its default number of coins, and whenever its winCoins method is called:

playerOne!.winCoins(2_000) println("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins")

// prints "PlayerOne won 2000 coins & now has 2100 coins" println("The bank now only has \(Bank.coinsInBank) coins left") // prints "The bank now only has 7900 coins left"

Here, the player has won 2,000 coins. The player锟斤拷s purse now contains 2,100 coins, and the bank has only 7,900 coins left.

playerOne = nil println("PlayerOne has left the game") // prints "PlayerOne has left the game" println("The bank now has \(Bank.coinsInBank) coins")

// prints "The bank now has 10000 coins"

The player has now left the game. This is indicated by setting the optional playerOne variable to nil, meaning 锟斤拷no Player instance.锟斤拷 At the point that this happens, the playerOne variable锟斤拷s reference to the Player instance is broken. No other properties or variables are still referring to the Player instance, and so it is deallocated in order to free up its memory. Just before this happens, its deinitializer is called automatically, and its coins are returned to the bank.

Automatic Reference Counting

Swift uses Automatic Reference Counting (ARC) to track and manage your app锟斤拷s memory usage. In most cases, this means that memory management 锟斤拷just works锟斤拷 in Swift, and you do not need to think about memory management yourself. ARC automatically frees up the memory used by class instances when those instances are no longer needed.

However, in a few cases ARC requires more information about the relationships between parts of your code in order to manage memory for you. This chapter describes those situations and shows how you enable ARC to manage all of your app锟斤拷s memory.

NOTE

Reference counting only applies to instances of classes. Structures and enumerations are value types, not reference types, and are not stored and passed by reference.

How ARC Works

Every time you create a new instance of a class, ARC allocates a chunk of memory to store information about that instance. This memory holds information about the type of the instance, together with the values of any stored properties associated with that instance.

Additionally, when an instance is no longer needed, ARC frees up the memory used by that instance so that the memory can be used for other purposes instead. This ensures that class instances do not take up space in memory when they are no longer needed.

However, if ARC were to deallocate an instance that was still in use, it would no longer be possible to access that instance锟斤拷s properties, or call that instance锟斤拷s methods. Indeed, if you tried to access the instance, your app would most likely crash.

To make sure that instances don锟斤拷t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.

To make this possible, whenever you assign a class instance to a property, constant, or variable, that property, constant, or variable makes a strong reference to the instance. The reference is called a 锟斤拷strong锟斤拷 reference because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong reference remains.

ARC in Action

Here锟斤拷s an example of how Automatic Reference Counting works. This example starts with a simple class called Person, which defines a stored constant property called name:

class Person { let name: String init(name: String) { self.name = name println("\(name) is being initialized")

} deinit { println("\(name) is being deinitialized")

}

The Person class has an initializer that sets the instance锟斤拷s name property and prints a message to indicate that initialization is underway. The Person class also has a deinitializer that prints a message when an instance of the class is deallocated.

The next code snippet defines three variables of type Person?, which are used to set up multiple references to a new Person instance in subsequent code snippets. Because these variables are of an optional type (Person?, not Person), they are automatically initialized with a value of nil, and do not currently reference a Person instance.

var reference1: Person? var reference2: Person? var reference3: Person?

You can now create a new Person instance and assign it to one of these three variables:

reference1 = Person(name: "John Appleseed")

// prints "John Appleseed is being initialized"

Note that the message "John Appleseed is being initialized" is printed at the point that you call the Person class锟斤拷s initializer. This confirms that initialization has taken place.

Because the new Person instance has been assigned to the reference1 variable, there is now a strong reference from reference1 to the new Person instance. Because there is at least one strong reference, ARC makes sure that this Person is kept in memory and is not deallocated.

If you assign the same Person instance to two more variables, two more strong references to that instance are established:

reference2 = reference1 reference3 = reference1

There are now three strong references to this single Person instance.

If you break two of these strong references (including the original reference) by assigning nil to two of the variables, a single strong reference remains, and the Person instance is not deallocated:

reference1 = nil reference2 = nil

ARC does not deallocate the Person instance until the third and final strong reference is broken, at which point it is clear that you are no longer using the Person instance:

reference3 = nil

// prints "John Appleseed is being deinitialized"

Strong Reference Cycles Between Class Instances

In the examples above, ARC is able to track the number of references to the new Person instance you create and to deallocate that Person instance when it is no longer needed.

However, it is possible to write code in which an instance of a class never gets to a point where it has zero strong references. This can happen if two class instances hold a strong reference to each other, such that each instance keeps the other alive. This is known as a strong reference cycle.

You resolve strong reference cycles by defining some of the relationships between classes as weak or unowned references instead of as strong references. This process is described in Resolving Strong Reference Cycles Between Class Instances. However, before you learn how to resolve a strong reference cycle, it is useful to understand how such a cycle is caused.

Here锟斤拷s an example of how a strong reference cycle can be created by accident. This example defines two classes called Person and Apartment, which model a block of apartments and its residents:

class Person { let name: String init(name: String) { self.name = name } var apartment: Apartment?

deinit { println("\(name) is being deinitialized") }

}

class Apartment { let number: Int

(number: Int) { self.number = number } r tenant: Person? init { println("Apartment #\(number) is being deinitialized") }

Every Person instance has a name property of type String and an optional apartment property that is initially nil. The apartment property is optional, because a person may not always have an apartment.

Similarly, every Apartment instance has a number property of type Int and has an optional tenant property that is initially nil. The tenant property is optional because an apartment may not always have a tenant.

Both of these classes also define a deinitializer, which prints the fact that an instance of that class is being deinitialized. This enables you to see whether instances of Person and Apartment are being deallocated as expected.

This next code snippet defines two variables of optional type called john and number73, which will be set to a specific Apartment and Person instance below. Both of these variables have an initial value of nil, by virtue of being optional:

var john: Person? var number73: Apartment?

You can now create a specific Person instance and Apartment instance and assign these new instances to the john and number73 variables:

john = Person(name: "John Appleseed") number73 = Apartment(number: 73)

Here锟斤拷s how the strong references look after creating and assigning these two instances. The john variable now has a strong reference to the new Person instance, and the number73 variable has a strong reference to the new Apartment instance:

You can now link the two instances together so that the person has an apartment, and the apartment has a tenant. Note that an exclamation mark (!) is used to unwrap and access the instances stored inside the john and number73 optional variables, so that the properties of those instances can be set:

john!.apartment = number73 number73!.tenant = john

Here锟斤拷s how the strong references look after you link the two instances together:

Unfortunately, linking these two instances creates a strong reference cycle between them. The Person instance now has a strong reference to the Apartment instance, and the Apartment instance has a strong reference to the Person instance. Therefore, when you break the strong references held by the john and number73 variables, the reference counts do not drop to zero, and the instances are not deallocated by ARC:

john = nil number73 = nil

Note that neither deinitializer was called when you set these two variables to nil. The strong reference cycle prevents the Person and Apartment instances from ever being deallocated, causing a memory leak in your app.

Here锟斤拷s how the strong references look after you set the john and number73 variables to nil:

The strong references between the Person instance and the Apartment instance remain and cannot be broken.

Resolving Strong Reference Cycles Between Class Instances

Swift provides two ways to resolve strong reference cycles when you work with properties of class type: weak references and unowned references.

Weak and unowned references enable one instance in a reference cycle to refer to the other instance without keeping a strong hold on it. The instances can then refer to each other without creating a strong reference cycle.

Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.

Weak References

A weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. This behavior prevents the reference from becoming part of a strong reference cycle. You indicate a weak reference by placing the weak keyword before a property or variable declaration.

Use a weak reference to avoid reference cycles whenever it is possible for that reference to have 锟斤拷no value锟斤拷 at some point in its life. If the reference will always have a value, use an unowned reference instead, as described in Unowned References. In the Apartment example above, it is appropriate for an apartment to be able to have 锟斤拷no tenant锟斤拷 at some point in its lifetime, and so a weak reference is an appropriate way to break the reference cycle in this case.

NOTE

Weak references must be declared as variables, to indicate that their value can change at runtime. A weak reference cannot be declared as a constant.

Because weak references are allowed to have 锟斤拷no value锟斤拷, you must declare every weak reference as having an optional type. Optional types are the preferred way to represent the possibility for 锟斤拷no value锟斤拷 in Swift.

Because a weak reference does not keep a strong hold on the instance it refers to, it is possible for that instance to be deallocated while the weak reference is still referring to it. Therefore, ARC automatically sets a weak reference to nil when the instance that it refers to is deallocated. You can check for the existence of a value in the weak reference, just like any other optional value, and you will never end up with a reference to an invalid instance that no longer exists.

The example below is identical to the Person and Apartment example from above, with one important difference. This time around, the Apartment type锟斤拷s tenant property is declared as a weak reference:

class Person { let name: String init(name: String) { self.name = name } var apartment: Apartment?

deinit { println("\(name) is being deinitialized") }

}

class Apartment { let number: Int (number: Int) { self.number = number } ak var tenant: Person? init { println("Apartment #\(number) is being deinitialized") }

The strong references from the two variables (john and number73) and the links between the two instances are created as before:

var john: Person? var number73: Apartment?

john = Person(name: "John Appleseed") number73 = Apartment(number: 73)

john!.apartment = number73 number73!.tenant = john

Here锟斤拷s how the references look now that you锟斤拷ve linked the two instances together:

The Person instance still has a strong reference to the Apartment instance, but the Apartment instance now has a weak reference to the Person instance. This means that when you break the strong reference held by the john variables, there are no more strong references to the Person instance:

Because there are no more strong references to the Person instance, it is deallocated:

john = nil

// prints "John Appleseed is being deinitialized"

The only remaining strong reference to the Apartment instance is from the number73 variable. If you break that strong reference, there are no more strong references to the Apartment instance:

Because there are no more strong references to the Apartment instance, it too is deallocated:

number73 = nil

// prints "Apartment #73 is being deinitialized"

The final two code snippets above show that the deinitializers for the Person instance and Apartment instance print their 锟斤拷deinitialized锟斤拷 messages after the john and number73 variables are set to nil. This proves that the reference cycle has been broken.

Unowned References

Like weak references, an unowned reference does not keep a strong hold on the instance it refers to. Unlike a weak reference, however, an unowned reference is assumed to always have a value. Because of this, an unowned reference is always defined as a nonoptional type. You indicate an unowned reference by placing the unowned keyword before a property or variable declaration.

Because an unowned reference is non-optional, you don锟斤拷t need to unwrap the unowned reference each time it is used. An unowned reference can always be accessed directly. However, ARC cannot set the reference to nil when the instance it refers to is deallocated, because variables of a non-optional type cannot be set to nil.

NOTE

If you try to access an unowned reference after the instance that it references is deallocated, you will trigger a runtime error. Use unowned references only when you are sure that the reference will always refer to an instance.

Note also that Swift guarantees your app will crash if you try to access an unowned reference after the instance it references is deallocated. You will never encounter unexpected behavior in this situation. Your app will always crash reliably, although you should, of course, prevent it from doing so.

The following example defines two classes, Customer and CreditCard, which model a bank customer and a possible credit card for that customer. These two classes each store an instance of the other class as a property. This relationship has the potential to create a strong reference cycle.

The relationship between Customer and CreditCard is slightly different from the relationship between Apartment and Person seen in the weak reference example above. In this data model, a customer may or may not have a credit card, but a credit card will always be associated with a customer. To represent this, the Customer class has an optional card property, but the CreditCard class has a non-optional customer property.

Furthermore, a new CreditCard instance can only be created by passing a number value and a customer instance to a custom CreditCard initializer. This ensures that a CreditCard instance always has a customer instance associated with it when the CreditCard instance is created.

Because a credit card will always have a customer, you define its customer property as an unowned reference, to avoid a strong reference cycle:

class Customer { let name: String var card: CreditCard? init(name: String) { self.name = name } deinit { println("\(name) is being deinitialized") }

}

CreditCard { number: Int owned let customer: Customer

(number: Int, customer: Customer) { self.number = number self.customer = customer

init { println("Card #\(number) is being deinitialized") }

This next code snippet defines an optional Customer variable called john, which will be used to store a reference to a specific customer. This variable has an initial value of nil, by virtue of being optional:

var john: Customer?

You can now create a Customer instance, and use it to initialize and assign a new CreditCard instance as that customer锟斤拷s card property:

john = Customer(name: "John Appleseed") john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!)

Here锟斤拷s how the references look, now that you锟斤拷ve linked the two instances:

The Customer instance now has a strong reference to the CreditCard instance, and the CreditCard instance has an unowned reference to the Customer instance.

Because of the unowned customer reference, when you break the strong reference held by the john variable, there are no more strong references to the Customer instance:

Because there are no more strong references to the Customer instance, it is deallocated. After this happens, there are no more strong references to the CreditCard instance, and it too is deallocated:

john = nil

// prints "John Appleseed is being deinitialized"

// prints "Card #1234567890123456 is being deinitialized"

The final code snippet above shows that the deinitializers for the Customer instance and CreditCard instance both print their 锟斤拷deinitialized锟斤拷 messages after the john variable is set to nil.

Unowned References and Implicitly Unwrapped Optional Properties

The examples for weak and unowned references above cover two of the more common scenarios in which it is necessary to break a strong reference cycle.

The Person and Apartment example shows a situation where two properties, both of which are allowed to be nil, have the potential to cause a strong reference cycle. This scenario is best resolved with a weak reference.

The Customer and CreditCard example shows a situation where one property that is allowed to be nil and another property that cannot be nil have the potential to cause a strong reference cycle. This scenario is best resolved with an unowned reference.

However, there is a third scenario, in which both properties should always have a value, and neither property should ever be nil once initialization is complete. In this scenario, it is useful to combine an unowned property on one class with an implicitly unwrapped optional property on the other class.

This enables both properties to be accessed directly (without optional unwrapping) once initialization is complete, while still avoiding a reference cycle. This section shows you how to set up such a relationship.

The example below defines two classes, Country and City, each of which stores an instance of the other class as a property. In this data model, every country must always have a capital city, and every city must always belong to a country. To represent this, the Country class has a capitalCity property, and the City class has a country property:

class Country { let name: String let capitalCity: City!

init(name: String, capitalName: String) { self.name = name self.capitalCity = City(name: capitalName, country: self) }

}

City { name: String owned let country: Country

(name: String, country: Country) { self.name = name self.country = country

To set up the interdependency between the two classes, the initializer for City takes a Country instance, and stores this instance in its country property.

The initializer for City is called from within the initializer for Country. However, the initializer for Country cannot pass self to the City initializer until a new Country instance is fully initialized, as described in Two-Phase Initialization.

To cope with this requirement, you declare the capitalCity property of Country as an implicitly unwrapped optional property, indicated by the exclamation mark at the end of its type annotation (City!). This means that the capitalCity property has a default value of nil, like any other optional, but can be accessed without the need to unwrap its value as described in Implicitly Unwrapped Optionals.

Because capitalCity has a default nil value, a new Country instance is considered fully initialized as soon as the Country instance sets its name property within its initializer. This means that the Country initializer can start to reference and pass around the implicit self property as soon as the name property is set. The Country initializer can therefore pass self as one of the parameters for the City initializer when the Country initializer is setting its own capitalCity property.

All of this means that you can create the Country and City instances in a single statement, without creating a strong reference cycle, and the capitalCity property can be accessed directly, without needing to use an exclamation mark to unwrap its optional value:

var country = Country(name: "Canada", capitalName: "Ottawa") println("\(country.name)'s capital city is called \(country.capitalCity.name)")

// prints "Canada's capital city is called Ottawa"

In the example above, the use of an implicitly unwrapped optional means that all of the two-phase class initializer requirements are satisfied. The capitalCity property can be used and accessed like a non-optional value once initialization is complete, while still avoiding a strong reference cycle.

Strong Reference Cycles for Closures

You saw above how a strong reference cycle can be created when two class instance properties hold a strong reference to each other. You also saw how to use weak and unowned references to break these strong reference cycles.

A strong reference cycle can also occur if you assign a closure to a property of a class instance, and the body of that closure captures the instance. This capture might occur because the closure锟斤拷s body accesses a property of the instance, such as self.someProperty, or because the closure calls a method on the instance, such as self.someMethod(). In either case, these accesses cause the closure to 锟斤拷capture锟斤拷 self, creating a strong reference cycle.

This strong reference cycle occurs because closures, like classes, are reference types. When you assign a closure to a property, you are assigning a reference to that closure. In essence, it锟斤拷s the same problem as above锟斤拷two strong references are keeping each other alive. However, rather than two class instances, this time it锟斤拷s a class instance and a closure that are keeping each other alive.

Swift provides an elegant solution to this problem, known as a closure capture list. However, before you learn how to break a strong reference cycle with a closure capture list, it is useful to understand how such a cycle can be caused.

The example below shows how you can create a strong reference cycle when using a closure that references self. This example defines a class called HTMLElement, which provides a simple model for an individual element within an HTML document:

class HTMLElement {

let name: String let text: String?

@lazy var asHTML: () -> String = {

if let text = self.text { return "<\(self.name)>\(text)</\(self.name)>"

} else { return "<\(self.name) />"

}

(name: String, text: String? = nil) { self.name = name

self.text = text

init { println("\(name) is being deinitialized")

The HTMLElement class defines a name property, which indicates the name of the element, such as "p" for a paragraph element, or "br" for a line break element. HTMLElement also defines an optional text property, which you can set to a string that represents the text to be rendered within that HTML element.

In addition to these two simple properties, the HTMLElement class defines a lazy property called asHTML. This property references a closure that combines name and text into an HTML string fragment. The asHTML property is of type () -> String, or 锟斤拷a function that takes no parameters, and returns a String value锟斤拷.

By default, the asHTML property is assigned a closure that returns a string representation of an HTML tag. This tag contains the optional text value if it exists, or no text content if text does not exist. For a paragraph element, the closure would return "some text" or "", depending on whether the text property equals "some text" or nil.

The asHTML property is named and used somewhat like an instance method. However, because asHTML is a closure property rather than an instance method, you can replace the default value of the asHTML property with a custom closure, if you want to change the HTML rendering for a particular HTML element.

NOTE

The asHTML property is declared as a lazy property, because it is only needed if and when the element actually needs to be rendered as a string value for some HTML output target. The fact that asHTML is a lazy property means that you can refer to self within the default closure, because the lazy property will not be accessed until after initialization has been completed and self is known to exist.

The HTMLElement class provides a single initializer, which takes a name argument and (if desired) a text argument to initialize a new element. The class also defines a deinitializer, which prints a message to show when an HTMLElement instance is deallocated.

Here锟斤拷s how you use the HTMLElement class to create and print a new instance:

var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") println(paragraph!.asHTML())

// prints "hello, world"

NOTE

The paragraph variable above is defined as an optional HTMLElement, so that it can be set to nil below to demonstrate the presence of a strong reference cycle.

Unfortunately, the HTMLElement class, as written above, creates a strong reference cycle between an HTMLElement instance and the closure used for its default asHTML value. Here锟斤拷s how the cycle looks:

The instance锟斤拷s asHTML property holds a strong reference to its closure. However, because the closure refers to self within its body (as a way to reference self.name and self.text), the closure captures self, which means that it holds a strong reference back to the HTMLElement instance. A strong reference cycle is created between the two. (For more information about capturing values in a closure, see Capturing Values.)

NOTE

Even though the closure refers to self multiple times, it only captures one strong reference to the HTMLElement instance.

If you set the paragraph variable to nil and break its strong reference to the HTMLElement instance, neither the HTMLElement instance nor its closure are deallocated, because of the strong reference cycle:

paragraph = nil

Note that the message in the HTMLElement deinitializer is not printed, which shows that the HTMLElement instance is not deallocated.

Resolving Strong Reference Cycles for Closures

You resolve a strong reference cycle between a closure and a class instance by defining a capture list as part of the closure锟斤拷s definition. A capture list defines the rules to use when capturing one or more reference types within the closure锟斤拷s body. As with strong reference cycles between two class instances, you declare each captured reference to be a weak or unowned reference rather than a strong reference. The appropriate choice of weak or unowned depends on the relationships between the different parts of your code.

NOTE

Swift requires you to write self.someProperty or self.someMethod (rather than just someProperty or someMethod) whenever you refer to a member of self within a closure. This helps you remember that it锟斤拷s possible to capture self by accident.

Defining a Capture List

Each item in a capture list is a pairing of the weak or unowned keyword with a reference to a class instance (such as self or someInstance). These pairings are written within a pair of square braces, separated by commas.

Place the capture list before a closure锟斤拷s parameter list and return type if they are provided:

@lazy var someClosure: (Int, String) -> String = {

[unowned self] (index: Int, stringToProcess: String) -> String in

// closure body goes here

}

If a closure does not specify a parameter list or return type because they can be inferred from context, place the capture list at the very start of the closure, followed by the in keyword:

@lazy var someClosure: () -> String = {

[unowned self] in

// closure body goes here

}

Weak and Unowned References

Define a capture in a closure as an unowned reference when the closure and the instance it captures will always refer to each other, and will always be deallocated at the same time.

Conversely, define a capture as a weak reference when the captured reference may become nil at some point in the future. Weak references are always of an optional type, and automatically become nil when the instance they reference is deallocated. This enables you to check for their existence within the closure锟斤拷s body.

NOTE

If the captured reference will never become nil, it should always be captured as an unowned reference, rather than a weak reference.

An unowned reference is the appropriate capture method to use to resolve the strong reference cycle in the HTMLElement example from earlier. Here锟斤拷s how you write the HTMLElement class to avoid the cycle:

class HTMLElement {

let name: String let text: String?

@lazy var asHTML: () -> String = {

[unowned self] in if let text = self.text { return "<\(self.name)>\(text)</\(self.name)>"

} else { return "<\(self.name) />"

}

(name: String, text: String? = nil) { self.name = name

self.text = text

init { println("\(name) is being deinitialized")

This implementation of HTMLElement is identical to the previous implementation, apart from the addition of a capture list within the asHTML closure. In this case, the capture list is [unowned self], which means 锟斤拷capture self as an unowned reference rather than a strong reference锟斤拷.

You can create and print an HTMLElement instance as before:

var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world") println(paragraph!.asHTML())

// prints "hello, world"

Here锟斤拷s how the references look with the capture list in place:

This time, the capture of self by the closure is an unowned reference, and does not keep a strong hold on the HTMLElement instance it has captured. If you set the strong reference from the paragraph variable to nil, the HTMLElement instance is deallocated, as can be seen from the printing of its deinitializer message in the example below:

paragraph = nil

// prints "p is being deinitialized"

Optional Chaining

Optional chaining is a process for querying and calling properties, methods, and subscripts on an optional that might currently be nil. If the optional contains a value, the property, method, or subscript call succeeds; if the optional is nil, the property, method, or subscript call returns nil. Multiple queries can be chained together, and the entire chain fails gracefully if any link in the chain is nil.

NOTE

Optional chaining in Swift is similar to messaging nil in Objective-C, but in a way that works for any type, and that can be checked for success or failure.

Optional Chaining as an Alternative to Forced Unwrapping

You specify optional chaining by placing a question mark (?) after the optional value on which you wish to call a property, method or subscript if the optional is non-nil. This is very similar to placing an exclamation mark (!) after an optional value to force the unwrapping of its value. The main difference is that optional chaining fails gracefully when the optional is nil, whereas forced unwrapping triggers a runtime error when the optional is nil.

To reflect the fact that optional chaining can be called on a nil value, the result of an optional chaining call is always an optional value, even if the property, method, or subscript you are querying returns a non-optional value. You can use this optional return value to check whether the optional chaining call was successful (the returned optional contains a value), or did not succeed due to a nil value in the chain (the returned optional value is nil).

Specifically, the result of an optional chaining call is of the same type as the expected return value, but wrapped in an optional. A property that normally returns an Int will return an Int? when accessed through optional chaining.

The next several code snippets demonstrate how optional chaining differs from forced unwrapping and enables you to check for success.

First, two classes called Person and Residence are defined:

class Person {

println("John's street name is \(johnsStreet).")

} else { println("Unable to retrieve the address.")

}

// prints "Unable to retrieve the address."

The value of john.residence currently contains a valid Residence instance. However, the value of john.residence.address is currently nil. Because of this, the call to john.residence?.address?.street fails.

Note that in the example above, you are trying to retrieve the value of the street property. The type of this property is String?. The return value of john.residence?.address?.street is therefore also String?, even though two levels of optional chaining are applied in addition to the underlying optional type of the property.

If you set an actual Address instance as the value for john.street.address, and set an an actual value for the address锟斤拷s street property, you can access the value of property through the multi-level optional chaining:

let johnsAddress = Address() johnsAddress.buildingName = "The Larches" johnsAddress.street = "Laurel Street" john.residence!.address = johnsAddress

if let johnsStreet = john.residence?.address?.street { println("John's street name is \(johnsStreet).")

} else { println("Unable to retrieve the address.")

nts "John's street name is Laurel Street."

Note the use of an exclamation mark during the assignment of an address instance to john.residence.address. The john.residence property has an optional type, and so you need to unwrap its actual value with an exclamation mark before accessing the residence锟斤拷s address property.

Chaining on Methods With Optional Return Values

The previous example shows how to retrieve the value of a property of optional type through optional chaining. You can also use optional chaining to call a method that returns a value of optional type, and to chain on that method锟斤拷s return value if needed.

The example below calls the Address class锟斤拷s buildingIdentifier method through optional chaining. This method returns a value of type String?. As described above, the ultimate return type of this method call after optional chaining is also String?:

if let buildingIdentifier = john.residence?.address?.buildingIdentifier() { println("John's building identifier is \(buildingIdentifier).")

}

// prints "John's building identifier is The Larches."

If you want to perform further optional chaining on this method锟斤拷s return value, place the optional chaining question mark after the method锟斤拷s parentheses:

if let upper = john.residence?.address?.buildingIdentifier()?.uppercaseString { println("John's uppercase building identifier is \(upper).")

}

// prints "John's uppercase building identifier is THE LARCHES."

NOTE

In the example above, you place the optional chaining question mark after the parentheses, because the optional value you are chaining on is the buildingIdentifier method锟斤拷s return value, and not the buildingIdentifier method itself.

Type Casting

Type casting is a way to check the type of an instance, and/or to treat that instance as if it is a different superclass or subclass from somewhere else in its own class hierarchy.

Type casting in Swift is implemented with the is and as operators. These two operators provide a simple and expressive way to check the type of a value or cast a value to a different type.

You can also use type casting to check whether a type conforms to a protocol, as described in Checking for Protocol Conformance.

Defining a Class Hierarchy for Type Casting

You can use type casting with a hierarchy of classes and subclasses to check the type of a particular class instance and to cast that instance to another class within the same hierarchy. The three code snippets below define a hierarchy of classes and an array containing instances of those classes, for use in an example of type casting.

The first snippet defines a new base class called MediaItem. This class provides basic functionality for any kind of item that appears in a digital media library. Specifically, it declares a name property of type String, and an init name initializer. (It is assumed that all media items, including all movies and songs, will have a name.)

class MediaItem { var name: String init(name: String) { self.name = name

}

The next snippet defines two subclasses of MediaItem. The first subclass, Movie, encapsulates additional information about a movie or film. It adds a director property on top of the base

MediaItem class, with a corresponding initializer. The second subclass, Song, adds an artist property and initializer on top of the base class:

class Movie: MediaItem {

var director: String init(name: String, director: String) { self.director = director super.init(name: name)

}

class Song: MediaItem {

r artist: String

(name: String, artist: String) { self.artist = artist super.init(name: name)

The final snippet creates a constant array called library, which contains two Movie instances and three Song instances. The type of the library array is inferred by initializing it with the contents of an array literal. Swift锟斤拷s type checker is able to deduce that Movie and Song have a common superclass of MediaItem, and so it infers a type of MediaItem[] for the library array:

let library = [

Movie(name: "Casablanca", director: "Michael Curtiz"),

Song(name: "Blue Suede Shoes", artist: "Elvis Presley"),

Movie(name: "Citizen Kane", director: "Orson Welles"),

Song(name: "The One And Only", artist: "Chesney Hawkes"),

Song(name: "Never Gonna Give You Up", artist: "Rick Astley") ]

// the type of "library" is inferred to be MediaItem[]

The items stored in library are still Movie and Song instances behind the scenes. However, if you iterate over the contents of this array, the items you receive back are typed as MediaItem, and not as Movie or Song. In order to work with them as their native type, you need to check their type, or downcast them to a different type, as described below.

Checking Type

Use the type check operator (is) to check whether an instance is of a certain subclass type. The type check operator returns true if the instance is of that subclass type and false if it is not.

The example below defines two variables, movieCount and songCount, which count the number of Movie and Song instances in the library array:

var movieCount = 0 var songCount = 0

for item in library { if item is Movie {

++movieCount

} else if item is Song {

++songCount

}

("Media library contains \(movieCount) movies and \(songCount) songs") nts "Media library contains 2 movies and 3 songs"

This example iterates through all items in the library array. On each pass, the for-in loop sets the item constant to the next MediaItem in the array.

item is Movie returns true if the current MediaItem is a Movie instance and false if it is not. Similarly, item is Song checks whether the item is a Song instance. At the end of the for-in loop, the values of movieCount and songCount contain a count of how many MediaItem instances were found of each type.

Downcasting

A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with the type cast operator (as).

Because downcasting can fail, the type cast operator comes in two different forms. The optional form, as?, returns an optional value of the type you are trying to downcast to. The forced form, as, attempts the downcast and force-unwraps the result as a single compound action.

Use the optional form of the type cast operator (as?) when you are not sure if the downcast will succeed. This form of the operator will always return an optional value, and the value will be nil if the downcast was not possible. This enables you to check for a successful downcast.

Use the forced form of the type cast operator (as) only when you are sure that the downcast will always succeed. This form of the operator will trigger a runtime error if you try to downcast to an incorrect class type.

The example below iterates over each MediaItem in library, and prints an appropriate description for each item. To do this, it needs to access each item as a true Movie or Song, and not just as a MediaItem. This is necessary in order for it to be able to access the director or artist property of a Movie or Song for use in the description.

In this example, each item in the array might be a Movie, or it might be a Song. You don锟斤拷t know in advance which actual class to use for each item, and so it is appropriate to use the optional form of the type cast operator (as?) to check the downcast each time through the loop:

for item in library { if let movie = item as? Movie { println("Movie: '\(movie.name)', dir. \(movie.director)")

} else if let song = item as? Song { println("Song: '\(song.name)', by \(song.artist)") }

}

// Movie: 'Casablanca', dir. Michael Curtiz ng: 'Blue Suede Shoes', by Elvis Presley vie: 'Citizen Kane', dir. Orson Welles ng: 'The One And Only', by Chesney Hawkes ng: 'Never Gonna Give You Up', by Rick Astley

The example starts by trying to downcast the current item as a Movie. Because item is a MediaItem instance, it锟斤拷s possible that it might be a Movie; equally, it锟斤拷s also possible that it might a Song, or even just a base MediaItem. Because of this uncertainty, the as? form of the type cast operator returns an optional value when attempting to downcast to a subclass type. The result of item as Movie is of type Movie?, or 锟斤拷optional Movie锟斤拷.

Downcasting to Movie fails when applied to the two Song instances in the library array. To cope with this, the example above uses optional binding to check whether the optional Movie actually contains a value (that is, to find out whether the downcast succeeded.) This optional binding is written 锟斤拷if let movie = item as? Movie锟斤拷, which can be read as:

锟斤拷Try to access item as a Movie. If this is successful, set a new temporary constant called movie to the value stored in the returned optional Movie.锟斤拷

If the downcasting succeeds, the properties of movie are then used to print a description for that Movie instance, including the name of its director. A similar principle is used to check for Song instances, and to print an appropriate description (including artist name) whenever a Song is found in the library.

NOTE

Casting does not actually modify the instance or change its values. The underlying instance remains the same; it is simply treated and accessed as an instance of the type to which it has been cast.

Type Casting for Any and AnyObject

Swift provides two special type aliases for working with non-specific types:

NOTE

Use Any and AnyObject only when you explicitly need the behavior and capabilities they provide. It is always better to be specific about the types you expect to work with in your code.

AnyObject

When working with Cocoa APIs, it is common to receive an array with a type of AnyObject[], or 锟斤拷an array of values of any object type锟斤拷. This is because Objective-C does not have explicitly typed arrays. However, you can often be confident about the type of objects contained in such an array just from the information you know about the API that provided the array.

In these situations, you can use the forced version of the type cast operator (as) to downcast each item in the array to a more specific class type than AnyObject, without the need for optional unwrapping.

The example below defines an array of type AnyObject[] and populates this array with three instances of the Movie class:

let someObjects: AnyObject[] = [

Movie(name: "2001: A Space Odyssey", director: "Stanley Kubrick"),

Movie(name: "Moon", director: "Duncan Jones"),

Movie(name: "Alien", director: "Ridley Scott")

]

Because this array is known to contain only Movie instances, you can downcast and unwrap directly to a non-optional Movie with the forced version of the type cast operator (as):

for object in someObjects { let movie = object as Movie println("Movie: '\(movie.name)', dir. \(movie.director)")

}

// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick

// Movie: 'Moon', dir. Duncan Jones

// Movie: 'Alien', dir. Ridley Scott

For an even shorter form of this loop, downcast the someObjects array to a type of Movie[] instead of downcasting each item:

for movie in someObjects as Movie[] { println("Movie: '\(movie.name)', dir. \(movie.director)")

}

// Movie: '2001: A Space Odyssey', dir. Stanley Kubrick

// Movie: 'Moon', dir. Duncan Jones

// Movie: 'Alien', dir. Ridley Scott

Any

Here锟斤拷s an example of using Any to work with a mix of different types, including non-class types. The example creates an array called things, which can store values of type Any:

var things = Any[]()

things.append(0) things.append(0.0) things.append(42) things.append(3.14159) things.append("hello") things.append((3.0, 5.0)) things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))

The things array contains two Int values, two Double values, a String value, a tuple of type (Double, Double), and the movie 锟斤拷Ghostbusters锟斤拷, directed by Ivan Reitman.

You can use the is and as operators in a switch statement锟斤拷s cases to discover the specific type of a constant or variable that is known only to be of type Any or AnyObject. The example below iterates over the items in the things array and queries the type of each item with a switch statement. Several of the switch statement锟斤拷s cases bind their matched value to a constant of the specified type to enable its value to be printed:

for thing in things { switch thing { case 0 as Int:

println("zero as an Int") case 0 as Double:

println("zero as a Double") case let someInt as Int:

println("an integer value of \(someInt)")

case let someDouble as Double where someDouble > 0:

println("a positive double value of \(someDouble)") se is Double:

println("some other double value that I don't want to print") se let someString as String:

println("a string value of \"\(someString)\"") se let (x, y) as (Double, Double):

println("an (x, y) point at \(x), \(y)") se let movie as Movie:

println("a movie called '\(movie.name)', dir. \(movie.director)") fault: println("something else") o as an Int o as a Double integer value of 42 positive double value of 3.14159 tring value of "hello"

(x, y) point at 3.0, 5.0 movie called 'Ghostbusters', dir. Ivan Reitman

NOTE

The cases of a switch statement use the forced version of the type cast operator (as, not as?) to check and cast to a specific type. This check is always safe within the context of a switch case statement.

Nested Types

Enumerations are often created to support a specific class or structure锟斤拷s functionality. Similarly, it can be convenient to define utility classes and structures purely for use within the context of a more complex type. To accomplish this, Swift enables you to define nested types, whereby you nest supporting enumerations, classes, and structures within the definition of the type they support.

To nest a type within another type, write its definition within the outer braces of the type it supports. Types can be nested to as many levels as are required.

Nested Types in Action

The example below defines a structure called BlackjackCard, which models a playing card as used in the game of Blackjack. The BlackJack structure contains two nested enumeration types called Suit and Rank.

In Blackjack, the Ace cards have a value of either one or eleven. This feature is represented by a structure called Values, which is nested within the Rank enumeration:

struct BlackjackCard {

// nested Suit enumeration enum Suit: Character { case Spades = "♠", Hearts = "♡", Diamonds = "♢", Clubs = "♣"

}

// nested Rank enumeration enum Rank: Int { case Two = 2, Three, Four, Five, Six, Seven, Eight, Nine, Ten case Jack, Queen, King, Ace struct Values { let first: Int, second: Int?

}

var values: Values { switch self { case .Ace:

return Values(first: 1, second: 11) case .Jack, .Queen, .King:

return Values(first: 10, second: nil) default:

return Values(first: self.toRaw(), second: nil) }

}

BlackjackCard properties and methods rank: Rank, suit: Suit r description: String { r output = "suit is \(suit.toRaw())," output += " value is \(rank.values.first)" if let second = rank.values.second { output += " or \(second)"

} return output

The Suit enumeration describes the four common playing card suits, together with a raw Character value to represent their symbol.

The Rank enumeration describes the thirteen possible playing card ranks, together with a raw Int value to represent their face value. (This raw Int value is not used for the Jack, Queen, King, and Ace cards.)

As mentioned above, the Rank enumeration defines a further nested structure of its own, called Values. This structure encapsulates the fact that most cards have one value, but the Ace card has two values. The Values structure defines two properties to represent this:

Rank also defines a computed property, values, which returns an instance of the Values structure. This computed property considers the rank of the card and initializes a new Values instance with appropriate values based on its rank. It uses special values for Jack, Queen, King, and Ace. For the numeric cards, it uses the rank锟斤拷s raw Int value.

The BlackjackCard structure itself has two properties锟斤拷rank and suit. It also defines a computed property called description, which uses the values stored in rank and suit to build a description of the name and value of the card. The description property uses optional binding to check whether there is a second value to display, and if so, inserts additional description detail for that second value.

Because BlackjackCard is a structure with no custom initializers, it has an implicit memberwise initializer, as described in Memberwise Initializers for Structure Types. You can use this initializer to initialize a new constant called theAceOfSpades:

let theAceOfSpades = BlackjackCard(rank: .Ace, suit: .Spades) println("theAceOfSpades: \(theAceOfSpades.description)")

// prints "theAceOfSpades: suit is ♠, value is 1 or 11"

Even though Rank and Suit are nested within BlackjackCard, their type can be inferred from context, and so the initialization of this instance is able to refer to the enumeration members by their member names (.Ace and .Spades) alone. In the example above, the description property correctly reports that the Ace of Spades has a value of 1 or 11.

Referring to Nested Types

To use a nested type outside of its definition context, prefix its name with the name of the type it is nested within:

let heartsSymbol = BlackjackCard.Suit.Hearts.toRaw()

// heartsSymbol is "♡"

For the example above, this enables the names of Suit, Rank, and Values to be kept deliberately short, because their names are naturally qualified by the context in which they are defined.

Extensions

Extensions add new functionality to an existing class, structure, or enumeration type. This includes the ability to extend types for which you do not have access to the original source code (known as retroactive modeling). Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions do not have names.) Extensions in Swift can:

Add computed properties and computed static properties

Define instance methods and type methods

Provide new initializers

Define subscripts

Define and use new nested types

Make an existing type conform to a protocol

NOTE

If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined.

Extension Syntax

Declare extensions with the extension keyword:

extension SomeType {

// new functionality to add to SomeType goes here

}

An extension can extend an existing type to make it adopt one or more protocols. Where this is the case, the protocol names are written in exactly the same way as for a class or structure:

extension SomeType: SomeProtocol, AnotherProtocol {

// implementation of protocol requirements goes here

}

Adding protocol conformance in this way is described in Adding Protocol Conformance with an Extension.

Computed Properties

Extensions can add computed instance properties and computed type properties to existing types. This example adds five computed instance properties to Swift锟斤拷s built-in Double type, to provide basic support for working with distance units:

extension Double { var km: Double { return self * 1_000.0 } var m: Double { return self } var cm: Double { return self / 100.0 } var mm: Double { return self / 1_000.0 } var ft: Double { return self / 3.28084 }

} let oneInch = 25.4.mm println("One inch is \(oneInch) meters") nts "One inch is 0.0254 meters" reeFeet = 3.ft

("Three feet is \(threeFeet) meters") nts "Three feet is 0.914399970739201 meters"

These computed properties express that a Double value should be considered as a certain unit of length. Although they are implemented as computed properties, the names of these properties can be appended to a floating-point literal value with dot syntax, as a way to use that literal value to perform distance conversions.

In this example, a Double value of 1.0 is considered to represent 锟斤拷one meter锟斤拷. This is why the m computed property returns self锟斤拷the expression 1.m is considered to calculate a Double value of 1.0.

Other units require some conversion to be expressed as a value measured in meters. One kilometer is the same as 1,000 meters, so the km computed property multiplies the value by 1_000.00 to convert into a number expressed in meters. Similarly, there are 3.28024 feet in a meter, and so the ft computed property divides the underlying Double value by 3.28024, to convert it from feet to meters.

These properties are read-only computed properties, and so they are expressed without the get keyword, for brevity. Their return value is of type Double, and can be used within mathematical calculations wherever a Double is accepted:

let aMarathon = 42.km + 195.m println("A marathon is \(aMarathon) meters long")

// prints "A marathon is 42195.0 meters long"

NOTE

Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.

Initializers

Extensions can add new initializers to existing types. This enables you to extend other types to accept your own custom types as initializer parameters, or to provide additional initialization options that were not included as part of the type锟斤拷s original implementation.

Extensions can add new convenience initializers to a class, but they cannot add new designated initializers or deinitializers to a class. Designated initializers and deinitializers must always be provided by the original class implementation.

NOTE

If you use an extension to add an initializer to a value type that provides default values for all of its stored properties and does not define any custom initializers, you can call the default initializer and memberwise initializer for that value type from within your extension锟斤拷s initializer.

This would not be the case if you had written the initializer as part of the value type锟斤拷s original implementation, as described in Initializer Delegation for Value Types.

The example below defines a custom Rect structure to represent a geometric rectangle. The example also defines two supporting structures called Size and Point, both of which provide default values of 0.0 for all of their properties:

struct Size { var width = 0.0, height = 0.0

} struct Point { var x = 0.0, y = 0.0

} struct Rect { var origin = Point() var size = Size()

Because the Rect structure provides default values for all of its properties, it receives a default initializer and a memberwise initializer automatically, as described in Default Initializers. These initializers can be used to create new Rect instances:

let defaultRect = Rect() let memberwiseRect = Rect(origin: Point(x: 2.0, y: 2.0), size: Size(width: 5.0, height: 5.0))

You can extend the Rect structure to provide an additional initializer that takes a specific center point and size:

extension Rect { init(center: Point, size: Size) { let originX = center.x - (size.width / 2) let originY = center.y - (size.height / 2) self.init(origin: Point(x: originX, y: originY), size: size) }

}

This new initializer starts by calculating an appropriate origin point based on the provided center point and size value. The initializer then calls the structure锟斤拷s automatic memberwise initializer init(origin:size:), which stores the new origin and size values in the appropriate properties:

let centerRect = Rect(center: Point(x: 4.0, y: 4.0),

size: Size(width: 3.0, height: 3.0))

// centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0)

NOTE

If you provide a new initializer with an extension, you are still responsible for making sure that each instance is fully initialized once the initializer completes.

Methods

Extensions can add new instance methods and type methods to existing types. The following example adds a new instance method called repetitions to the Int type:

extension Int { func repetitions(task: () -> ()) { for i in 0..self { task()

}

The repetitions method takes a single argument of type () -> (), which indicates a function that has no parameters and does not return a value.

After defining this extension, you can call the repetitions method on any integer number to perform a task that many number of times:

3.repetitions({ println("Hello!")

})

// Hello!

Use trailing closure syntax to make the call more succinct:

3.repetitions { println("Goodbye!")

}

// Goodbye!

Mutating Instance Methods

Instance methods added with an extension can also modify (or mutate) the instance itself. Structure and enumeration methods that modify self or its properties must mark the instance method as mutating, just like mutating methods from an original implementation.

The example below adds a new mutating method called square to Swift锟斤拷s Int type, which squares the original value:

extension Int { mutating func square() { self = self * self

}

} var someInt = 3 someInt.square()

// someInt is now 9

Subscripts

Extensions can add new subscripts to an existing type. This example adds an integer subscript to Swift锟斤拷s built-in Int type. This subscript [n] returns the decimal digit n places in from the right of the number:

锟斤拷and so on:

extension Int { subscript(digitIndex: Int) -> Int { var decimalBase = 1 for _ in 1...digitIndex { decimalBase *= 10

} return (self / decimalBase) % 10

}

81295[0] urns 5

81295[1] urns 9

81295[2] urns 2

81295[8] urns 7

If the Int value does not have enough digits for the requested index, the subscript implementation returns 0, as if the number had been padded with zeroes to the left:

746381295[9]

// returns 0, as if you had requested:

0746381295[9]

Nested Types

Extensions can add new nested types to existing classes, structures and enumerations:

extension Character { enum Kind { case Vowel, Consonant, Other

} var kind: Kind { switch String(self).lowercaseString { case "a", "e", "i", "o", "u":

return .Vowel case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",

", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":

return .Consonant

fault:

return .Other

}

This example adds a new nested enumeration to Character. This enumeration, called Kind, expresses the kind of letter that a particular character represents. Specifically, it expresses whether the character is a vowel or a consonant in a standard Latin script (without taking into account accents or regional variations), or whether it is another kind of character.

This example also adds a new computed instance property to Character, called kind, which returns the appropriate Kind enumeration member for that character.

The nested enumeration can now be used with Character values:

func printLetterKinds(word: String) { println("'\(word)' is made up of the following kinds of letters:") for character in word { switch character.kind { case .Vowel:

print("vowel ") case .Consonant: print("consonant ") case .Other: print("other ")

}

nt("\n")

etterKinds("Hello") llo' is made up of the following kinds of letters: nsonant vowel consonant consonant vowel

This function, printLetterKinds, takes an input String value and iterates over its characters. For each character, it considers the kind computed property for that character, and prints an appropriate description of that kind. The printLetterKinds function can then be called to print the kinds of letters in an entire word, as shown here for the word "Hello".

NOTE

character.kind is already known to be of type Character.Kind. Because of this, all of the Character.Kind member values can be written in shorthand form inside the switch statement, such as .Vowel rather than Character.Kind.Vowel.

Protocols

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol doesn锟斤拷t actually provide an implementation for any of these requirements锟斤拷it only describes what an implementation will look like. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

Protocols can require that conforming types have specific instance properties, instance methods, type methods, operators, and subscripts.

Protocol Syntax

You define protocols in a very similar way to classes, structures, and enumerations:

protocol SomeProtocol {

// protocol definition goes here

}

Custom types state that they adopt a particular protocol by placing the protocol锟斤拷s name after the type锟斤拷s name, separated by a colon, as part of their definition. Multiple protocols can be listed, and are separated by commas:

struct SomeStructure: FirstProtocol, AnotherProtocol { // structure definition goes here

}

If a class has a superclass, list the superclass name before any protocols it adopts, followed by a comma:

class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol { // class definition goes here

}

Property Requirements

A protocol can require any conforming type to provide an instance property or type property with a particular name and type. The protocol doesn锟斤拷t specify whether the property should be a stored property or a computed property锟斤拷it only specifies the required property name and type. The protocol also specifies whether each property must be gettable or gettable and settable.

If a protocol requires a property to be gettable and settable, that property requirement cannot be fulfilled by a constant stored property or a read-only computed property. If the protocol only requires a property to be gettable, the requirement can be satisfied by any kind of property, and it is valid for it also to be settable if this is useful for your own code.

Property requirements are always declared as variable properties, prefixed with the var keyword. Gettable and settable properties are indicated by writing { get set } after their type declaration, and gettable properties are indicated by writing { get }.

protocol SomeProtocol { var mustBeSettable: Int { get set } var doesNotNeedToBeSettable: Int { get }

}

Always prefix type property requirements with the class keyword when you define them in a protocol. This is true even though type property requirements are prefixed with the static keyword when implemented by a structure or enumeration:

protocol AnotherProtocol { class var someTypeProperty: Int { get set }

}

Here锟斤拷s an example of a protocol with a single instance property requirement:

protocol FullyNamed { var fullName: String { get }

}

The FullyNamed protocol defines any kind of thing that has a fully-qualified name. It doesn锟斤拷t specify what kind of thing it must be锟斤拷it only specifies that the thing must be able to provide a full name for itself. It specifies this requirement by stating that any FullyNamed type must have a gettable instance property called fullName, which is of type String.

Here锟斤拷s an example of a simple structure that adopts and conforms to the FullyNamed protocol:

struct Person: FullyNamed { var fullName: String

} let john = Person(fullName: "John Appleseed") // john.fullName is "John Appleseed"

This example defines a structure called Person, which represents a specific named person. It states that it adopts the FullyNamed protocol as part of the first line of its definition.

Each instance of Person has a single stored property called fullName, which is of type String. This matches the single requirement of the FullyNamed protocol, and means that Person has correctly conformed to the protocol. (Swift reports an error at compile-time if a protocol requirement is not fulfilled.)

Here锟斤拷s a more complex class, which also adopts and conforms to the FullyNamed protocol:

class Starship: FullyNamed { var prefix: String? var name: String init(name: String, prefix: String? = nil) { self.name = name self.prefix = prefix

} var fullName: String { return (prefix ? prefix! + " " : "") + name cc1701 = Starship(name: "Enterprise", prefix: "USS") c1701.fullName is "USS Enterprise"

This class implements the fullName property requirement as a computed read-only property for a starship. Each Starship class instance stores a mandatory name and an optional prefix. The fullName property uses the prefix value if it exists, and prepends it to the beginning of name to create a full name for the starship.

Method Requirements

Protocols can require specific instance methods and type methods to be implemented by conforming types. These methods are written as part of the protocol锟斤拷s definition in exactly the same way as for normal instance and type methods, but without curly braces or a method body. Variadic parameters are allowed, subject to the same rules as for normal methods.

NOTE

Protocols use the same syntax as normal methods, but are not allowed to specify default values for method parameters.

As with type property requirements, you always prefix type method requirements with the class keyword when they are defined in a protocol. This is true even though type method requirements are prefixed with the static keyword when implemented by a structure or enumeration:

protocol SomeProtocol { class func someTypeMethod()

}

The following example defines a protocol with a single instance method requirement:

protocol RandomNumberGenerator { func random() -> Double

}

This protocol, RandomNumberGenerator, requires any conforming type to have an instance method called random, which returns a Double value whenever it is called. (Although it is not specified as part of the protocol, it is assumed that this value will be a number between 0.0 and 1.0 inclusive.)

The RandomNumberGenerator protocol does not make any assumptions about how each random number will be generated锟斤拷it simply requires the generator to provide a standard way to generate a new random number.

Here锟斤拷s an implementation of a class that adopts and conforms to the RandomNumberGenerator protocol. This class implements a pseudorandom number generator algorithm known as a linear congruential generator:

class LinearCongruentialGenerator: RandomNumberGenerator { var lastRandom = 42.0 let m = 139968.0 let a = 3877.0 let c = 29573.0 func random() -> Double { lastRandom = ((lastRandom * a + c) % m) return lastRandom / m

}

nerator = LinearCongruentialGenerator()

("Here's a random number: \(generator.random())") nts "Here's a random number: 0.37464991998171"

("And another one: \(generator.random())") nts "And another one: 0.729023776863283"

Mutating Method Requirements

It is sometimes necessary for a method to modify (or mutate) the instance it belongs to. For instance methods on value types (that is, structures and enumerations) you place the mutating keyword before a method锟斤拷s func keyword to indicate that the method is allowed to modify the instance it belongs to and/or any properties of that instance. This process is described in Modifying Value Types from Within Instance Methods.

If you define a protocol instance method requirement that is intended to mutate instances of any type that adopts the protocol, mark the method with the mutating keyword as part of the protocol锟斤拷s definition. This enables structures and enumerations to adopt the protocol and satisfy that method requirement.

NOTE

If you mark a protocol instance method requirement as mutating, you do not need to write the mutating keyword when writing an implementation of that method for a class. The mutating keyword is only used by structures and enumerations.

The example below defines a protocol called Togglable, which defines a single instance method requirement called toggle. As its name suggests, the toggle method is intended to toggle or invert the state of any conforming type, typically by modifying a property of that type.

The toggle method is marked with the mutating keyword as part of the Togglable protocol definition, to indicate that the method is expected to mutate the state of a conforming instance when it is called:

protocol Togglable { mutating func toggle()

}

If you implement the Togglable protocol for a structure or enumeration, that structure or enumeration can conform to the protocol by providing an implementation of the toggle method that is also marked as mutating.

The example below defines an enumeration called OnOffSwitch. This enumeration toggles between two states, indicated by the enumeration cases On and Off. The enumeration锟斤拷s toggle implementation is marked as mutating, to match the Togglable protocol锟斤拷s requirements:

enum OnOffSwitch: Togglable { case Off, On mutating func toggle() { switch self { case Off:

self = On case On:

self = Off

}

ghtSwitch = OnOffSwitch.Off witch.toggle() tSwitch is now equal to .On

Protocols as Types

Protocols do not actually implement any functionality themselves. Nonetheless, any protocol you create will become a fully-fledged type for use in your code.

Because it is a type, you can use a protocol in many places where other types are allowed, including:

NOTE

Because protocols are types, begin their names with a capital letter (such as FullyNamed and

RandomNumberGenerator) to match the names of other types in Swift (such as Int, String, and Double).

Here锟斤拷s an example of a protocol used as a type:

class Dice { let sides: Int

let generator: RandomNumberGenerator init(sides: Int, generator: RandomNumberGenerator) { self.sides = sides self.generator = generator

} func roll() -> Int { return Int(generator.random() * Double(sides)) + 1

This example defines a new class called Dice, which represents an n-sided dice for use in a board game. Dice instances have an integer property called sides, which represents how many sides they have, and a property called generator, which provides a random number generator from which to create dice roll values.

The generator property is of type RandomNumberGenerator. Therefore, you can set it to an instance of any type that adopts the RandomNumberGenerator protocol. Nothing else is required of the instance you assign to this property, except that the instance must adopt the RandomNumberGenerator protocol.

Dice also has an initializer, to set up its initial state. This initializer has a parameter called generator, which is also of type RandomNumberGenerator. You can pass a value of any conforming type in to this parameter when initializing a new Dice instance.

Dice provides one instance method, roll, which returns an integer value between 1 and the number of sides on the dice. This method calls the generator锟斤拷s random method to create a new random number between 0.0 and 1.0, and uses this random number to create a dice roll value within the correct range. Because generator is known to adopt RandomNumberGenerator, it is guaranteed to have a random method to call.

Here锟斤拷s how the Dice class can be used to create a six-sided dice with a LinearCongruentialGenerator instance as its random number generator:

var d6 = Dice(sides: 6, generator: LinearCongruentialGenerator()) for _ in 1...5 { println("Random dice roll is \(d6.roll())")

}

// Random dice roll is 3

// Random dice roll is 5

// Random dice roll is 4

// Random dice roll is 5

// Random dice roll is 4

Delegation

Delegation is a design pattern that enables a class or structure to hand off (or delegate) some of its responsibilities to an instance of another type. This design pattern is implemented by defining a protocol that encapsulates the delegated responsibilities, such that a conforming type (known as a delegate) is guaranteed to provide the functionality that has been delegated. Delegation can be used to respond to a particular action, or to retrieve data from an external source without needing to know the underlying type of that source.

The example below defines two protocols for use with dice-based board games:

protocol DiceGame { var dice: Dice { get } func play()

} protocol DiceGameDelegate { func gameDidStart(game: DiceGame) func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) func gameDidEnd(game: DiceGame)

}

The DiceGame protocol is a protocol that can be adopted by any game that involves dice.

The DiceGameDelegate protocol can be adopted by any type to track the progress of a DiceGame.

Here锟斤拷s a version of the Snakes and Ladders game originally introduced in Control Flow. This version is adapted to use a Dice instance for its dice-rolls; to adopt the DiceGame protocol; and to notify a DiceGameDelegate about its progress:

class SnakesAndLadders: DiceGame { let finalSquare = 25 let dice = Dice(sides: 6, generator: LinearCongruentialGenerator()) var square = 0 var board: Int[]

init() {

board = Int[](count: finalSquare + 1, repeatedValue: 0) board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02 board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08

r delegate: DiceGameDelegate?

nc play() { square = 0 delegate?.gameDidStart(self) gameLoop: while square != finalSquare {

let diceRoll = dice.roll()

delegate?.game(self, didStartNewTurnWithDiceRoll: diceRoll) switch square + diceRoll { case finalSquare: break gameLoop case let newSquare where newSquare > finalSquare:

continue gameLoop default:

square += diceRoll square += board[square]

} } delegate?.gameDidEnd(self)

For a description of the Snakes and Ladders gameplay, see the Break section of the Control Flow chapter.

This version of the game is wrapped up as a class called SnakesAndLadders, which adopts the DiceGame protocol. It provides a gettable dice property and a play method in order to conform to the protocol. (The dice property is declared as a constant property because it does not need to change after initialization, and the protocol only requires that it is gettable.)

The Snakes and Ladders game board setup takes place within the class锟斤拷s init() initializer. All game logic is moved into the protocol锟斤拷s play method, which uses the protocol锟斤拷s required dice property to provide its dice roll values.

Note that the delegate property is defined as an optional DiceGameDelegate, because a delegate isn锟斤拷t required in order to play the game. Because it is of an optional type, the delegate property is automatically set to an initial value of nil. Thereafter, the game instantiator has the option to set the property to a suitable delegate.

DiceGameDelegate provides three methods for tracking the progress of a game. These three methods have been incorporated into the game logic within the play method above, and are called when a new game starts, a new turn begins, or the game ends.

Because the delegate property is an optional DiceGameDelegate, the play method uses optional chaining each time it calls a method on the delegate. If the delegate property is nil, these delegate calls fail gracefully and without error. If the delegate property is non-nil, the delegate methods are called, and are passed the SnakesAndLadders instance as a parameter.

This next example shows a class called DiceGameTracker, which adopts the DiceGameDelegate protocol:

class DiceGameTracker: DiceGameDelegate { var numberOfTurns = 0 func gameDidStart(game: DiceGame) { numberOfTurns = 0 if game is SnakesAndLadders { println("Started a new game of Snakes and Ladders")

} println("The game is using a \(game.dice.sides)-sided dice")

}

nc game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) {

++numberOfTurns

println("Rolled a \(diceRoll)")

nc gameDidEnd(game: DiceGame) { println("The game lasted for \(numberOfTurns) turns")

DiceGameTracker implements all three methods required by DiceGameDelegate. It uses these methods to keep track of the number of turns a game has taken. It resets a numberOfTurns property to zero when the game starts; increments it each time a new turn begins; and prints out the total number of turns once the game has ended.

The implementation of gameDidStart shown above uses the game parameter to print some introductory information about the game that is about to be played. The game parameter has a type of DiceGame, not SnakesAndLadders, and so gameDidStart can access and use only methods and properties that are implemented as part of the DiceGame protocol. However, the method is still able to use type casting to query the type of the underlying instance.

In this example, it checks whether game is actually an instance of SnakesAndLadders behind the scenes, and prints an appropriate message if so.

gameDidStart also accesses the dice property of the passed game parameter. Because game is known to conform to the DiceGame protocol, it is guaranteed to have a dice property, and so the gameDidStart method is able to access and print the dice锟斤拷s sides property, regardless of what kind of game is being played.

Here锟斤拷s how DiceGameTracker looks in action:

let tracker = DiceGameTracker() let game = SnakesAndLadders() game.delegate = tracker game.play()

// Started a new game of Snakes and Ladders

// The game is using a 6-sided dice

// Rolled a 3

// Rolled a 5

// Rolled a 4 led a 5 e game lasted for 4 turns

Adding Protocol Conformance with an Extension

You can extend an existing type to adopt and conform to a new protocol, even if you do not have access to the source code for the existing type. Extensions can add new properties, methods, and subscripts to an existing type, and are therefore able to add any requirements that a protocol may demand. For more about extensions, see Extensions.

NOTE

Existing instances of a type automatically adopt and conform to a protocol when that conformance is added to the instance锟斤拷s type in an extension.

For example, this protocol, called TextRepresentable, can be implemented by any type that has a way to be represented as text. This might be a description of itself, or a text version of its current state:

protocol TextRepresentable { func asText() -> String

}

The Dice class from earlier can be extended to adopt and conform to TextRepresentable:

extension Dice: TextRepresentable { func asText() -> String { return "A \(sides)-sided dice"

}

This extension adopts the new protocol in exactly the same way as if Dice had provided it in its original implementation. The protocol name is provided after the type name, separated by a colon, and an implementation of all requirements of the protocol is provided within the extension锟斤拷s curly braces.

Any Dice instance can now be treated as TextRepresentable:

let d12 = Dice(sides: 12, generator: LinearCongruentialGenerator()) println(d12.asText())

// prints "A 12-sided dice"

Similarly, the SnakesAndLadders game class can be extended to adopt and conform to the TextRepresentable protocol:

extension SnakesAndLadders: TextRepresentable { func asText() -> String { return "A game of Snakes and Ladders with \(finalSquare) squares" }

} println(game.asText())

// prints "A game of Snakes and Ladders with 25 squares"

Declaring Protocol Adoption with an Extension

If a type already conforms to all of the requirements of a protocol, but has not yet stated that it adopts that protocol, you can make it adopt the protocol with an empty extension:

struct Hamster { var name: String func asText() -> String { return "A hamster named \(name)"

}

} extension Hamster: TextRepresentable {}

Instances of Hamster can now be used wherever TextRepresentable is the required type:

let simonTheHamster = Hamster(name: "Simon") let somethingTextRepresentable: TextRepresentable = simonTheHamster println(somethingTextRepresentable.asText())

// prints "A hamster named Simon"

NOTE

Types do not automatically adopt a protocol just by satisfying its requirements. They must always explicitly declare their adoption of the protocol.

Collections of Protocol Types

A protocol can be used as the type to be stored in a collection such as an array or a dictionary, as mentioned in Protocols as Types. This example creates an array of TextRepresentable things: let things: TextRepresentable[] = [game, d12, simonTheHamster]

It is now possible to iterate over the items in the array, and print each item锟斤拷s textual representation:

for thing in things { println(thing.asText())

}

// A game of Snakes and Ladders with 25 squares

// A 12-sided dice

// A hamster named Simon

Note that the thing constant is of type TextRepresentable. It is not of type Dice, or DiceGame, or Hamster, even if the actual instance behind the scenes is of one of those types.

Nonetheless, because it is of type TextRepresentable, and anything that is TextRepresentable is known to have an asText method, it is safe to call thing.asText each time through the loop.

Protocol Inheritance

A protocol can inherit one or more other protocols and can add further requirements on top of the requirements it inherits. The syntax for protocol inheritance is similar to the syntax for class inheritance, but with the option to list multiple inherited protocols, separated by commas:

protocol InheritingProtocol: SomeProtocol, AnotherProtocol { // protocol definition goes here }

Here锟斤拷s an example of a protocol that inherits the TextRepresentable protocol from above:

protocol PrettyTextRepresentable: TextRepresentable { func asPrettyText() -> String

}

This example defines a new protocol, PrettyTextRepresentable, which inherits from

TextRepresentable. Anything that adopts PrettyTextRepresentable must satisfy all of the requirements enforced by TextRepresentable, plus the additional requirements enforced by PrettyTextRepresentable. In this example, PrettyTextRepresentable adds a single requirement to provide an instance method called asPrettyText that returns a String.

The SnakesAndLadders class can be extended to adopt and conform to PrettyTextRepresentable:

extension SnakesAndLadders: PrettyTextRepresentable { func asPrettyText() -> String { var output = asText() + ":\n" for index in 1...finalSquare {

switch board[index] { case let ladder where ladder > 0:

output += "锟斤拷 " case let snake where snake < 0:

output += "锟斤拷 " default:

output += "锟斤拷 "

} } return output

This extension states that it adopts the PrettyTextRepresentable protocol and provides an implementation of the asPrettyText method for the SnakesAndLadders type. Anything that is PrettyTextRepresentable must also be TextRepresentable, and so the asPrettyText implementation starts by calling the asText method from the TextRepresentable protocol to begin an output string. It appends a colon and a line break, and uses this as the start of its pretty text representation. It then iterates through the array of board squares, and appends an emoji representation for each square:

The method implementation can now be used to print a pretty text description of any SnakesAndLadders instance:

println(game.asPrettyText())

// A game of Snakes and Ladders with 25 squares:

// 锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷锟斤拷

Protocol Composition

It can be useful to require a type to conform to multiple protocols at once. You can combine multiple protocols into a single requirement with a protocol composition.

Protocol compositions have the form protocol<SomeProtocol, AnotherProtocol>. You can list as many protocols within the pair of angle brackets (<>) as you need, separated by commas.

Here锟斤拷s an example that combines two protocols called Named and Aged into a single protocol composition requirement on a function parameter:

protocol Named { var name: String { get }

} protocol Aged { var age: Int { get }

} struct Person: Named, Aged { var name: String var age: Int

wishHappyBirthday(celebrator: protocol<Named, Aged>) { ntln("Happy birthday \(celebrator.name) - you're \(celebrator.age)!")

thdayPerson = Person(name: "Malcolm", age: 21) appyBirthday(birthdayPerson) nts "Happy birthday Malcolm - you're 21!"

This example defines a protocol called Named, with a single requirement for a gettable String property called name. It also defines a protocol called Aged, with a single requirement for a gettable Int property called age. Both of these protocols are adopted by a structure called Person.

The example also defines a function called wishHappyBirthday, which takes a single parameter called celebrator. The type of this parameter is protocol<Named, Aged>, which means 锟斤拷any type that conforms to both the Named and Aged protocols.锟斤拷 It doesn锟斤拷t matter what specific type is passed to the function, as long as it conforms to both of the required protocols.

The example then creates a new Person instance called birthdayPerson and passes this new instance to the wishHappyBirthday function. Because Person conforms to both protocols, this is a valid call, and the wishHappyBirthday function is able to print its birthday greeting.

NOTE

Protocol compositions do not define a new, permanent protocol type. Rather, they define a temporary local protocol that has the combined requirements of all protocols in the composition.

Checking for Protocol Conformance

You can use the is and as operators described in Type Casting to check for protocol conformance, and to cast to a specific protocol. Checking for and casting to a protocol follows exactly the same syntax as checking for and casting to a type:

This example defines a protocol called HasArea, with a single property requirement of a gettable Double property called area:

@objc protocol HasArea { var area: Double { get }

}

NOTE

You can check for protocol conformance only if your protocol is marked with the @objc attribute, as seen for the HasArea protocol above. This attribute indicates that the protocol should be exposed to Objective-C code and is described in Using Swift with Cocoa and Objective-C. Even if you are not interoperating with Objective-C, you need to mark your protocols with the @objc attribute if you want to be able to check for protocol conformance.

Note also that @objc protocols can be adopted only by classes, and not by structures or enumerations. If you mark your protocol as @objc in order to check for conformance, you will be able to apply that protocol only to class types.

Here are two classes, Circle and Country, both of which conform to the HasArea protocol:

class Circle: HasArea { let pi = 3.1415927 var radius: Double var area: Double { return pi * radius * radius } init(radius: Double) { self.radius = radius }

} class Country: HasArea { var area: Double init(area: Double) { self.area = area }

The Circle class implements the area property requirement as a computed property, based on a stored radius property. The Country class implements the area requirement directly as a stored property. Both classes correctly conform to the HasArea protocol.

Here锟斤拷s a class called Animal, which does not conform to the HasArea protocol:

class Animal { var legs: Int init(legs: Int) { self.legs = legs }

}

The Circle, Country and Animal classes do not have a shared base class. Nonetheless, they are all classes, and so instances of all three types can be used to initialize an array that stores values of type AnyObject:

let objects: AnyObject[] = [ Circle(radius: 2.0),

Country(area: 243_610),

Animal(legs: 4)

]

The objects array is initialized with an array literal containing a Circle instance with a radius of 2 units; a Country instance initialized with the surface area of the United Kingdom in square kilometers; and an Animal instance with four legs.

The objects array can now be iterated, and each object in the array can be checked to see if it conforms to the HasArea protocol:

for object in objects { if let objectWithArea = object as? HasArea { println("Area is \(objectWithArea.area)")

} else { println("Something that doesn't have an area") }

}

// Area is 12.5663708

// Area is 243610.0 mething that doesn't have an area

Whenever an object in the array conforms to the HasArea protocol, the optional value returned by the as? operator is unwrapped with optional binding into a constant called objectWithArea. The objectWithArea constant is known to be of type HasArea, and so its area property can be accessed and printed in a type-safe way.

Note that the underlying objects are not changed by the casting process. They continue to be a Circle, a Country and an Animal. However, at the point that they are stored in the objectWithArea constant, they are only known to be of type HasArea, and so only their area property can be accessed.

Optional Protocol Requirements

You can define optional requirements for protocols, These requirements do not have to be implemented by types that conform to the protocol. Optional requirements are prefixed by the @optional keyword as part of the protocol锟斤拷s definition.

An optional protocol requirement can be called with optional chaining, to account for the possibility that the requirement was not implemented by a type that conforms to the protocol. For information on optional chaining, see Optional Chaining.

You check for an implementation of an optional requirement by writing a question mark after the name of the requirement when it is called, such as someOptionalMethod?(someArgument).

Optional property requirements, and optional method requirements that return a value, will always return an optional value of the appropriate type when they are accessed or called, to reflect the fact that the optional requirement may not have been implemented.

NOTE

Optional protocol requirements can only be specified if your protocol is marked with the @objc attribute. Even if you are not interoperating with Objective-C, you need to mark your protocols with the @objc attribute if you want to specify optional requirements.

Note also that @objc protocols can be adopted only by classes, and not by structures or enumerations. If you mark your protocol as @objc in order to specify optional requirements, you will only be able to apply that protocol to class types.

The following example defines an integer-counting class called Counter, which uses an external data source to provide its increment amount. This data source is defined by the CounterDataSource protocol, which has two optional requirements:

@objc protocol CounterDataSource {

@optional func incrementForCount(count: Int) -> Int

@optional var fixedIncrement: Int { get }

}

The CounterDataSource protocol defines an optional method requirement called incrementForCount and an optional property requirement called fixedIncrement. These requirements define two different ways for data sources to provide an appropriate increment amount for a Counter instance.

NOTE

Strictly speaking, you can write a custom class that conforms to CounterDataSource without implementing either protocol requirement. They are both optional, after all. Although technically allowed, this wouldn锟斤拷t make for a very good data source.

The Counter class, defined below, has an optional dataSource property of type CounterDataSource?:

@objc class Counter { var count = 0 var dataSource: CounterDataSource?

func increment() {

if let amount = dataSource?.incrementForCount?(count) { count += amount

} else if let amount = dataSource?.fixedIncrement? { count += amount

}

The Counter class stores its current value in a variable property called count. The Counter class also defines a method called increment, which increments the count property every time the method is called.

The increment method first tries to retrieve an increment amount by looking for an implementation of the incrementForCount method on its data source. The increment method uses optional chaining to try to call incrementForCount, and passes the current count value as the method锟斤拷s single argument.

Note two levels of optional chaining at play here. Firstly, it is possible that dataSource may be nil, and so dataSource has a question mark after its name to indicate that incrementForCount should only be called if dataSource is non-nil. Secondly, even if dataSource does exist, there is no guarantee that it implements incrementForCount, because it is an optional requirement. This is why incrementForCount is also written with a question mark after its name.

Because the call to incrementForCount can fail for either of these two reasons, the call returns an optional Int value. This is true even though incrementForCount is defined as returning a nonoptional Int value in the definition of CounterDataSource.

After calling incrementForCount, the optional Int that it returns is unwrapped into a constant called amount, using optional binding. If the optional Int does contain a value锟斤拷that is, if the delegate and method both exist, and the method returned a value锟斤拷the unwrapped amount is added onto the stored count property, and incrementation is complete.

If it is not possible to retrieve a value from the incrementForCount method锟斤拷either because dataSource is nil, or because the data source does not implement incrementForCount锟斤拷then the increment method tries to retrieve a value from the data source锟斤拷s fixedIncrement property instead. The fixedIncrement property is also an optional requirement, and so its name is also written using optional chaining with a question mark on the end, to indicate that the attempt to access the property锟斤拷s value can fail. As before, the returned value is an optional Int value, even though fixedIncrement is defined as a non-optional Int property as part of the CounterDataSource protocol definition.

Here锟斤拷s a simple CounterDataSource implementation where the data source returns a constant value of 3 every time it is queried. It does this by implementing the optional fixedIncrement property requirement:

class ThreeSource: CounterDataSource { let fixedIncrement = 3 }

You can use an instance of ThreeSource as the data source for a new Counter instance:

var counter = Counter() counter.dataSource = ThreeSource()

for _ in 1...4 { counter.increment() println(counter.count)

}

// 3

// 6

// 9

The code above creates a new Counter instance; sets its data source to be a new ThreeSource instance; and calls the counter锟斤拷s increment method four times. As expected, the counter锟斤拷s count property increases by three each time increment is called.

Here锟斤拷s a more complex data source called TowardsZeroSource, which makes a Counter instance count up or down towards zero from its current count value:

class TowardsZeroSource: CounterDataSource { func incrementForCount(count: Int) -> Int { if count == 0 { return 0

} else if count < 0 { return 1 } else { return -1

}

The TowardsZeroSource class implements the optional incrementForCount method from the CounterDataSource protocol and uses the count argument value to work out which direction to count in. If count is already zero, the method returns 0 to indicate that no further counting should take place.

You can use an instance of TowardsZeroSource with the existing Counter instance to count from 4 to zero. Once the counter reaches zero, no more counting takes place:

counter.count = -4 counter.dataSource = TowardsZeroSource()

for _ in 1...5 { counter.increment() println(counter.count)

}

// -3

// -2

// -1

Generics

Generic code enables you to write flexible, reusable functions and types that can work with any type, subject to requirements that you define. You can write code that avoids duplication and expresses its intent in a clear, abstracted manner.

Generics are one of the most powerful features of Swift, and much of the Swift standard library is built with generic code. In fact, you锟斤拷ve been using generics throughout this Language Guide, even if you didn锟斤拷t realize it. For example, Swift锟斤拷s Array and Dictionary types are both generic collections. You can create an array that holds Int values, or an array that holds String values, or indeed an array for any other type that can be created in Swift. Similarly, you can create a dictionary to store values of any specified type, and there are no limitations on what that type can be.

The Problem That Generics Solve

Here锟斤拷s a standard, non-generic function called swapTwoInts, which swaps two Int values:

func swapTwoInts(inout a: Int, inout b: Int) { let temporaryA = a a = b b = temporaryA

}

This function makes use of in-out parameters to swap the values of a and b, as described in In-Out Parameters.

The swapTwoInts function swaps the original value of b into a, and the original value of a into b. You can call this function to swap the values in two Int variables:

var someInt = 3 var anotherInt = 107 swapTwoInts(&someInt, &anotherInt) println("someInt is now \(someInt), and anotherInt is now \(anotherInt)")

// prints "someInt is now 107, and anotherInt is now 3"

The swapTwoInts function is useful, but it can only be used with Int values. If you want to swap two String values, or two Double values, you have to write more functions, such as the swapTwoStrings and swapTwoDoubles functions shown below:

func swapTwoStrings(inout a: String, inout b: String) { let temporaryA = a a = b b = temporaryA

}

func swapTwoDoubles(inout a: Double, inout b: Double) { let temporaryA = a a = b

= temporaryA

You may have noticed that the bodies of the swapTwoInts, swapTwoStrings, and swapTwoDoubles functions are identical. The only difference is the type of the values that they accept (Int, String, and Double).

It would be much more useful, and considerably more flexible, to write a single function that could swap two values of any type. This is the kind of problem that generic code can solve. (A generic version of these functions is defined below.)

NOTE

In all three functions, it is important that the types of a and b are defined to be the same as each other. If a and b were not of the same type, it would not be possible to swap their values. Swift is a type-safe language, and does not allow (for example) a variable of type String and a variable of type Double to swap values with each other. Attempting to do so would be reported as a compile-time error.

Generic Functions

Generic functions can work with any type. Here锟斤拷s a generic version of the swapTwoInts function from above, called swapTwoValues:

func swapTwoValues<T>(inout a: T, inout b: T) {

let temporaryA = a a = b b = temporaryA

}

The body of the swapTwoValues function is identical to the body of the swapTwoInts function. However, the first line of swapTwoValues is slightly different from swapTwoInts. Here锟斤拷s how the first lines compare:

func swapTwoInts(inout a: Int, inout b: Int) func swapTwoValues<T>(inout a: T, inout b: T)

The generic version of the function uses a placeholder type name (called T, in this case) instead of an actual type name (such as Int, String, or Double). The placeholder type name doesn锟斤拷t say anything about what T must be, but it does say that both a and b must be of the same type T, whatever T represents. The actual type to use in place of T will be determined each time the swapTwoValues function is called.

The other difference is that the generic function锟斤拷s name (swapTwoValues) is followed by the placeholder type name (T) inside angle brackets (<T>). The brackets tell Swift that T is a placeholder type name within the swapTwoValues function definition. Because T is a placeholder, Swift does not look for an actual type called T.

The swapTwoValues function can now be called in the same way as swapTwoInts, except that it can be passed two values of any type, as long as both of those values are of the same type as each other. Each time swapTwoValues is called, the type to use for T is inferred from the types of values passed to the function.

In the two examples below, T is inferred to be Int and String respectively:

var someInt = 3 var anotherInt = 107 swapTwoValues(&someInt, &anotherInt)

// someInt is now 107, and anotherInt is now 3

var someString = "hello" var anotherString = "world" swapTwoValues(&someString, &anotherString)

// someString is now "world", and anotherString is now "hello"

NOTE

The swapTwoValues function defined above is inspired by a generic function called swap, which is part of the Swift standard library, and is automatically made available for you to use in your apps. If you need the behavior of the swapTwoValues function in your own code, you can use Swift锟斤拷s existing swap function rather than providing your own implementation.

Type Parameters

In the swapTwoValues example above, the placeholder type T is an example of a type parameter. Type parameters specify and name a placeholder type, and are written immediately after the function锟斤拷s name, between a pair of matching angle brackets (such as <T>).

Once specified, a type parameter can be used to define the type of a function锟斤拷s parameters (such as the a and b parameters of the swapTwoValues function); or as the function锟斤拷s return type; or as a type annotation within the body of the function. In each case, the placeholder type represented by the type parameter is replaced with an actual type whenever the function is called. (In the swapTwoValues example above, T was replaced with Int the first time the function was called, and was replaced with String the second time it was called.)

You can provide more than one type parameter by writing multiple type parameter names within the angle brackets, separated by commas.

Naming Type Parameters

In simple cases where a generic function or generic type needs to refer to a single placeholder type (such as the swapTwoValues generic function above, or a generic collection that stores a single type, such as Array), it is traditional to use the single-character name T for the type parameter. However, you are can use any valid identifier as the type parameter name.

If you are defining more complex generic functions, or generic types with multiple parameters, it can be useful to provide more descriptive type parameter names. For example, Swift锟斤拷s Dictionary type has two type parameters锟斤拷one for its keys and one for its values. If you were writing Dictionary yourself, you might name these two type parameters KeyType and ValueType to remind you of their purpose as you use them within your generic code.

NOTE

Always give type parameters UpperCamelCase names (such as T and KeyType) to indicate that they are a placeholder for a type, not a value.

Generic Types

In addition to generic functions, Swift enables you to define your own generic types. These are custom classes, structures, and enumerations that can work with any type, in a similar way to Array and Dictionary.

This section shows you how to write a generic collection type called Stack. A stack is an ordered set of values, similar to an array, but with a more restricted set of operations than Swift锟斤拷s Array type. An array allows new items to be inserted and removed at any location in the array. A stack, however, allows new items to be appended only to the end of the collection (known as pushing a new value on to the stack). Similarly, a stack allows items to be removed only from the end of the collection (known as popping a value off the stack).

NOTE

The concept of a stack is used by the UINavigationController class to model the view controllers in its navigation hierarchy. You call the UINavigationController class pushViewController:animated: method to add (or push) a view controller on to the navigation stack, and its popViewControllerAnimated: method to remove (or pop) a view controller from the navigation stack. A stack is a useful collection model whenever you need a strict 锟斤拷last in, first out锟斤拷 approach to managing a collection.

The illustration below shows the push / pop behavior for a stack:

1. There are currently three values on the stack.

2. A fourth value is 锟斤拷pushed锟斤拷 on to the top of the stack.

3. The stack now holds four values, with the most recent one at the top.

4. The top item in the stack is removed, or 锟斤拷popped锟斤拷.

5. After popping a value, the stack once again holds three values.

Here锟斤拷s how to write a non-generic version of a stack, in this case for a stack of Int values:

struct IntStack { var items = Int[]() mutating func push(item: Int) { items.append(item)

} mutating func pop() -> Int { return items.removeLast()

}

This structure uses an Array property called items to store the values in the stack. Stack provides two methods, push and pop, to push and pop values on and off the stack. These methods are marked as mutating, because they need to modify (or mutate) the structure锟斤拷s items array.

The IntStack type shown above can only be used with Int values, however. It would be much more useful to define a generic Stack class, that can manage a stack of any type of value.

Here锟斤拷s a generic version of the same code:

struct Stack<T> { var items = T[]() mutating func push(item: T) { items.append(item)

} mutating func pop() -> T { return items.removeLast() }

}

Note how the generic version of Stack is essentially the same as the non-generic version, but with a placeholder type parameter called T instead of an actual type of Int. This type parameter is written within a pair of angle brackets (<T>) immediately after the structure锟斤拷s name.

T defines a placeholder name for 锟斤拷some type T锟斤拷 to be provided later on. This future type can be referred to as 锟斤拷T锟斤拷 anywhere within the structure锟斤拷s definition. In this case, T is used as a placeholder in three places:

You create instances of Stack in a similar way to Array and Dictionary, by writing the actual type to be used for this specific stack within angle brackets after the type name when creating a new instance with initializer syntax:

var stackOfStrings = Stack<String>() stackOfStrings.push("uno") stackOfStrings.push("dos") stackOfStrings.push("tres") stackOfStrings.push("cuatro")

// the stack now contains 4 strings

Here锟斤拷s how stackOfStrings looks after pushing these four values on to the stack:

Popping a value from the stack returns and removes the top value, "cuatro":

let fromTheTop = stackOfStrings.pop()

// fromTheTop is equal to "cuatro", and the stack now contains 3 strings

Here锟斤拷s how the stack looks after popping its top value:

Because it is a generic type, Stack can be used to create a stack of any valid type in Swift, in a similar manner to Array and Dictionary.

Type Constraints

The swapTwoValues function and the Stack type can work with any type. However, it is sometimes useful to enforce certain type constraints on the types that can be used with generic functions and generic types. Type constraints specify that a type parameter must inherit from a specific class, or conform to a particular protocol or protocol composition.

For example, Swift锟斤拷s Dictionary type places a limitation on the types that can be used as keys for a dictionary. As described in Dictionaries, the type of a dictionary锟斤拷s keys must be hashable. That is, it must provide a way to make itself uniquely representable. Dictionary needs its keys to be hashable so that it can check whether it already contains a value for a particular key. Without this requirement, Dictionary could not tell whether it should insert or replace a value for a particular key, nor would it be able to find a value for a given key that is already in the dictionary.

This requirement is enforced by a type constraint on the key type for Dictionary, which specifies that the key type must conform to the Hashable protocol, a special protocol defined in the Swift standard library. All of Swift锟斤拷s basic types (such as String, Int, Double, and Bool) are hashable by default.

You can define your own type constraints when creating custom generic types, and these constraints provide much of the power of generic programming. Abstract concepts like Hashable characterize types in terms of their conceptual characteristics, rather than their explicit type.

Type Constraint Syntax

You write type constraints by placing a single class or protocol constraint after a type parameter锟斤拷s name, separated by a colon, as part of the type parameter list. The basic syntax for type constraints on a generic function is shown below (although the syntax is the same for generic types):

func someFunction<T: SomeClass, U: SomeProtocol>(someT: T, someU: U) { // function body goes here

}

The hypothetical function above has two type parameters. The first type parameter, T, has a type constraint that requires T to be a subclass of SomeClass. The second type parameter, U, has a type constraint that requires U to conform to the protocol SomeProtocol.

Type Constraints in Action

Here锟斤拷s a non-generic function called findStringIndex, which is given a String value to find and an array of String values within which to find it. The findStringIndex function returns an optional Int value, which will be the index of the first matching string in the array if it is found, or nil if the string cannot be found:

func findStringIndex(array: String[], valueToFind: String) -> Int? { for (index, value) in enumerate(array) { if value == valueToFind {

return index

} } return nil }

The findStringIndex function can be used to find a string value in an array of strings:

let strings = ["cat", "dog", "llama", "parakeet", "terrapin"] if let foundIndex = findStringIndex(strings, "llama") { println("The index of llama is \(foundIndex)")

}

// prints "The index of llama is 2"

The principle of finding the index of a value in an array isn锟斤拷t useful only for strings, however. You can write the same functionality as a generic function called findIndex, by replacing any mention of strings with values of some type T instead.

Here锟斤拷s how you might expect a generic version of findStringIndex, called findIndex, to be written. Note that the return type of this function is still Int?, because the function returns an optional index number, not an optional value from the array. Be warned, though锟斤拷this function does not compile, for reasons explained after the example:

func findIndex<T>(array: T[], valueToFind: T) -> Int? { for (index, value) in enumerate(array) { if value == valueToFind { return index

} } return nil

}

This function does not compile as written above. The problem lies with the equality check, 锟斤拷if value == valueToFind锟斤拷. Not every type in Swift can be compared with the equal to operator (==). If you create your own class or structure to represent a complex data model, for example, then the meaning of 锟斤拷equal to锟斤拷 for that class or structure is not something that Swift can guess for you. Because of this, it is not possible to guarantee that this code will work for every possible type T, and an appropriate error is reported when you try to compile the code.

All is not lost, however. The Swift standard library defines a protocol called Equatable, which requires any conforming type to implement the equal to operator (==) and the not equal to operator (!=) to compare any two values of that type. All of Swift锟斤拷s standard types automatically support the Equatable protocol.

Any type that is Equatable can be used safely with the findIndex function, because it is guaranteed to support the equal to operator. To express this fact, you write a type constraint of Equatable as part of the type parameter锟斤拷s definition when you define the function:

func findIndex<T: Equatable>(array: T[], valueToFind: T) -> Int? { for (index, value) in enumerate(array) { if value == valueToFind { return index

} } return nil

}

The single type parameter for findIndex is written as T: Equatable, which means 锟斤拷any type T that conforms to the Equatable protocol.锟斤拷

The findIndex function now compiles successfully and can be used with any type that is Equatable, such as Double or String:

let doubleIndex = findIndex([3.14159, 0.1, 0.25], 9.3)

// doubleIndex is an optional Int with no value, because 9.3 is not in the array let stringIndex = findIndex(["Mike", "Malcolm", "Andrea"], "Andrea")

// stringIndex is an optional Int containing a value of 2

Associated Types

When defining a protocol, it is sometimes useful to declare one or more associated types as part of the protocol锟斤拷s definition. An associated type gives a placeholder name (or alias) to a type that is used as part of the protocol. The actual type to use for that associated type is not specified until the protocol is adopted. Associated types are specified with the typealias keyword.

Associated Types in Action

Here锟斤拷s an example of a protocol called Container, which declares an associated type called ItemType:

protocol Container { typealias ItemType mutating func append(item: ItemType) var count: Int { get } subscript(i: Int) -> ItemType { get }

}

The Container protocol defines three required capabilities that any container must provide:

This protocol doesn锟斤拷t specify how the items in the container should be stored or what type they are allowed to be. The protocol only specifies the three bits of functionality that any type must provide in order to be considered a Container. A conforming type can provide additional functionality, as long as it satisfies these three requirements.

Any type that conforms to the Container protocol must be able to specify the type of values it stores. Specifically, it must ensure that only items of the right type are added to the container, and it must be clear about the type of the items returned by its subscript.

To define these requirements, the Container protocol needs a way to refer to the type of the elements that a container will hold, without knowing what that type is for a specific container. The Container protocol needs to specify that any value passed to the append method must have the same type as the container锟斤拷s element type, and that the value returned by the container锟斤拷s subscript will be of the same type as the container锟斤拷s element type.

To achieve this, the Container protocol declares an associated type called ItemType, written as typealias ItemType. The protocol does not define what ItemType is an alias for锟斤拷that information is left for any conforming type to provide. Nonetheless, the ItemType alias provides a way to refer to the type of the items in a Container, and to define a type for use with the append method and subscript, to ensure that the expected behavior of any Container is enforced.

Here锟斤拷s a version of the non-generic IntStack type from earlier, adapted to conform to the Container protocol:

struct IntStack: Container {

// original IntStack implementation var items = Int[]() mutating func push(item: Int) { items.append(item)

} mutating func pop() -> Int { return items.removeLast()

} conformance to the Container protocol pealias ItemType = Int utating func append(item: Int) { self.push(item)

r count: Int { turn items.count

bscript(i: Int) -> Int { return items[i]

The IntStack type implements all three of the Container protocol锟斤拷s requirements, and in each case wraps part of the IntStack type锟斤拷s existing functionality to satisfy these requirements.

Moreover, IntStack specifies that for this implementation of Container, the appropriate ItemType to use is a type of Int. The definition of typealias ItemType = Int turns the abstract type of ItemType into a concrete type of Int for this implementation of the Container protocol.

Thanks to Swift锟斤拷s type inference, you don锟斤拷t actually need to declare a concrete ItemType of Int as part of the definition of IntStack. Because IntStack conforms to all of the requirements of the Container protocol, Swift can infer the appropriate ItemType to use, simply by looking at the type of the append method锟斤拷s item parameter and the return type of the subscript. Indeed, if you delete the typealias ItemType = Int line from the code above, everything still works, because it is clear what type should be used for ItemType.

You can also make the generic Stack type conform to the Container protocol:

struct Stack<T>: Container {

// original Stack<T> implementation var items = T[]() mutating func push(item: T) { items.append(item)

} mutating func pop() -> T { return items.removeLast()

} conformance to the Container protocol utating func append(item: T) { self.push(item)

r count: Int { turn items.count

bscript(i: Int) -> T { return items[i]

This time, the placeholder type parameter T is used as the type of the append method锟斤拷s item parameter and the return type of the subscript. Swift can therefore infer that T is the appropriate type to use as the ItemType for this particular container.

Extending an Existing Type to Specify an Associated Type

You can extend an existing type to add conformance to a protocol, as described in Adding Protocol Conformance with an Extension. This includes a protocol with an associated type.

Swift锟斤拷s Array type already provides an append method, a count property, and a subscript with an Int index to retrieve its elements. These three capabilities match the requirements of the Container protocol. This means that you can extend Array to conform to the Container protocol simply by declaring that Array adopts the protocol. You do this with an empty extension, as described in Declaring Protocol Adoption with an Extension:

extension Array: Container {}

Array锟斤拷s existing append method and subscript enable Swift to infer the appropriate type to use for ItemType, just as for the generic Stack type above. After defining this extension, you can use any Array as a Container.

Where Clauses

Type constraints, as described in Type Constraints, enable you to define requirements on the type parameters associated with a generic function or type.

It can also be useful to define requirements for associated types. You do this by defining where clauses as part of a type parameter list. A where clause enables you to require that an associated type conforms to a certain protocol, and/or that certain type parameters and associated types be the same. You write a where clause by placing the where keyword immediately after the list of type parameters, followed by one or more constraints for associated types, and/or one or more equality relationships between types and associated types.

The example below defines a generic function called allItemsMatch, which checks to see if two Container instances contain the same items in the same order. The function returns a Boolean value of true if all items match and a value of false if they do not.

The two containers to be checked do not have to be the same type of container (although they can be), but they do have to hold the same type of items. This requirement is expressed through a combination of type constraints and where clauses:

func allItemsMatch<

C1: Container, C2: Container where C1.ItemType == C2.ItemType, C1.ItemType: Equatable> (someContainer: C1, anotherContainer: C2) -> Bool {

// check that both containers contain the same number of items if someContainer.count != anotherContainer.count { return false

}

// check each pair of items to see if they are equivalent for i in 0..someContainer.count { if someContainer[i] != anotherContainer[i] { return false

}

// all items match, so return true return true

This function takes two arguments called someContainer and anotherContainer. The someContainer argument is of type C1, and the anotherContainer argument is of type C2. Both C1 and C2 are placeholder type parameters for two container types to be determined when the function is called.

The function锟斤拷s type parameter list places the following requirements on the two type parameters:

The third and fourth requirements are defined as part of a where clause, and are written after the where keyword as part of the function锟斤拷s type parameter list.

These requirements mean:

The third and fourth requirements combine to mean that the items in anotherContainer can also be checked with the != operator, because they are exactly the same type as the items in someContainer.

These requirements enable the allItemsMatch function to compare the two containers, even if they are of a different container type.

The allItemsMatch function starts by checking that both containers contain the same number of items. If they contain a different number of items, there is no way that they can match, and the function returns false.

After making this check, the function iterates over all of the items in someContainer with a for-in loop and the half-closed range operator (..). For each item, the function checks whether the item from someContainer is not equal to the corresponding item in anotherContainer. If the two items are not equal, then the two containers do not match, and the function returns false.

If the loop finishes without finding a mismatch, the two containers match, and the function returns true.

Here锟斤拷s how the allItemsMatch function looks in action:

var stackOfStrings = Stack<String>() stackOfStrings.push("uno") stackOfStrings.push("dos") stackOfStrings.push("tres")

var arrayOfStrings = ["uno", "dos", "tres"]

if allItemsMatch(stackOfStrings, arrayOfStrings) { println("All items match.")

{ ntln("Not all items match.")

nts "All items match."

The example above creates a Stack instance to store String values, and pushes three strings onto the stack. The example also creates an Array instance initialized with an array literal containing the same three strings as the stack. Even though the stack and the array are of a different type, they both conform to the Container protocol, and both contain the same type of values. You can therefore call the allItemsMatch function with these two containers as its arguments. In the example above, the allItemsMatch function correctly reports that all of the items in the two containers match.

Advanced Operators

In addition to the operators described in Basic Operators, Swift provides several advanced operators that perform more complex value manipulation. These include all of the bitwise and bit shifting operators you will be familiar with from C and Objective-C.

Unlike arithmetic operators in C, arithmetic operators in Swift do not overflow by default. Overflow behavior is trapped and reported as an error. To opt in to overflow behavior, use Swift锟斤拷s second set of arithmetic operators that overflow by default, such as the overflow addition operator (&+). All of these overflow operators begin with an ampersand (&).

When you define your own structures, classes, and enumerations, it can be useful to provide your own implementations of the standard Swift operators for these custom types. Swift makes it easy to provide tailored implementations of these operators and to determine exactly what their behavior should be for each type you create.

You锟斤拷re not just limited to the predefined operators. Swift gives you the freedom to define your own custom infix, prefix, postfix, and assignment operators, with custom precedence and associativity values. These operators can be used and adopted in your code just like any of the predefined operators, and you can even extend existing types to support the custom operators you define.

Bitwise Operators

Bitwise operators enable you to manipulate the individual raw data bits within a data structure. They are often used in low-level programming, such as graphics programming and device driver creation. Bitwise operators can also be useful when you work with raw data from external sources, such as encoding and decoding data for communication over a custom protocol.

Swift supports all of the bitwise operators found in C, as described below.

Bitwise NOT Operator

The bitwise NOT operator (~) inverts all bits in a number:

The bitwise NOT operator is a prefix operator, and appears immediately before the value it operates on, without any white space:

let initialBits: UInt8 = 0b00001111 let invertedBits = ~initialBits // equals 11110000

UInt8 integers have eight bits and can store any value between 0 and 255. This example initializes a UInt8 integer with the binary value 00001111, which has its first four bits set to 0, and its second four bits set to 1. This is equivalent to a decimal value of 15.

The bitwise NOT operator is then used to create a new constant called invertedBits, which is equal to initialBits, but with all of the bits inverted. Zeroes become ones, and ones become zeroes. The value of invertedBits is 11110000, which is equal to an unsigned decimal value of 240.

Bitwise AND Operator

The bitwise AND operator (&) combines the bits of two numbers. It returns a new number whose bits are set to 1 only if the bits were equal to 1 in both input numbers:

In the example below, the values of firstSixBits and lastSixBits both have four middle bits equal to 1. The bitwise AND operator combines them to make the number 00111100, which is equal to an unsigned decimal value of 60:

let firstSixBits: UInt8 = 0b11111100

let lastSixBits: UInt8 = 0b00111111 let middleFourBits = firstSixBits & lastSixBits // equals 00111100

Bitwise OR Operator

The bitwise OR operator (|) compares the bits of two numbers. The operator returns a new number whose bits are set to 1 if the bits are equal to 1 in either input number:

In the example below, the values of someBits and moreBits have different bits set to 1. The bitwise OR operator combines them to make the number 11111110, which equals an unsigned decimal of 254:

let someBits: UInt8 = 0b10110010 let moreBits: UInt8 = 0b01011110 let combinedbits = someBits | moreBits // equals 11111110

Bitwise XOR Operator

The bitwise XOR operator, or 锟斤拷exclusive OR operator锟斤拷 (^), compares the bits of two numbers. The operator returns a new number whose bits are set to 1 where the input bits are different and are set to 0 where the input bits are the same:

In the example below, the values of firstBits and otherBits each have a bit set to 1 in a location that the other does not. The bitwise XOR operator sets both of these bits to 1 in its output value. All of the other bits in firstBits and otherBits match and are set to 0 in the output value:

let firstBits: UInt8 = 0b00010100 let otherBits: UInt8 = 0b00000101 let outputBits = firstBits ^ otherBits // equals 00010001

Bitwise Left and Right Shift Operators

The bitwise left shift operator (<<) and bitwise right shift operator (>>) move all bits in a number to the left or the right by a certain number of places, according to the rules defined below.

Bitwise left and right shifts have the effect of multiplying or dividing an integer number by a factor of two. Shifting an integer锟斤拷s bits to the left by one position doubles its value, whereas shifting it to the right by one position halves its value.

Shifting Behavior for Unsigned Integers

The bit-shifting behavior for unsigned integers is as follows:

1. Existing bits are moved to the left or right by the requested number of places.

2. Any bits that are moved beyond the bounds of the integer锟斤拷s storage are discarded.

3. Zeroes are inserted in the spaces left behind after the original bits are moved to the left or right.

This approach is known as a logical shift.

The illustration below shows the results of 11111111 << 1 (which is 11111111 shifted to the left by 1 place), and 11111111 >> 1 (which is 11111111 shifted to the right by 1 place). Blue numbers are shifted, gray numbers are discarded, and orange zeroes are inserted:

Here锟斤拷s how bit shifting looks in Swift code:

let shiftBits: UInt8 = 4 // 00000100 in binary shiftBits << 1 // 00001000 shiftBits << 2 // 00010000 shiftBits << 5 // 10000000 shiftBits << 6 // 00000000 shiftBits >> 2 // 00000001

You can use bit shifting to encode and decode values within other data types:

let pink: UInt32 = 0xCC6699 let redComponent = (pink & 0xFF0000) >> 16 // redComponent is 0xCC, or 204 let greenComponent = (pink & 0x00FF00) >> 8 // greenComponent is 0x66, or 102 let blueComponent = pink & 0x0000FF // blueComponent is 0x99, or 153

This example uses a UInt32 constant called pink to store a Cascading Style Sheets color value for the color pink. The CSS color value #CC6699 is written as 0xCC6699 in Swift锟斤拷s hexadecimal number representation. This color is then decomposed into its red (CC), green (66), and blue (99) components by the bitwise AND operator (&) and the bitwise right shift operator (>>).

The red component is obtained by performing a bitwise AND between the numbers

0xCC6699 and 0xFF0000. The zeroes in 0xFF0000 effectively 锟斤拷mask锟斤拷 the second and third bytes of 0xCC6699, causing the 6699 to be ignored and leaving 0xCC0000 as the result.

This number is then shifted 16 places to the right (>> 16). Each pair of characters in a hexadecimal number uses 8 bits, so a move 16 places to the right will convert 0xCC0000 into 0x0000CC. This is the same as 0xCC, which has a decimal value of 204.

Similarly, the green component is obtained by performing a bitwise AND between the numbers 0xCC6699 and 0x00FF00, which gives an output value of 0x006600. This output value is then shifted eight places to the right, giving a a value of 0x66, which has a decimal value of 102.

Finally, the blue component is obtained by performing a bitwise AND between the numbers 0xCC6699 and 0x0000FF, which gives an output value of 0x000099. There锟斤拷s no need to shift this to the right, as 0x000099 already equals 0x99, which has a decimal value of 153.

Shifting Behavior for Signed Integers

The shifting behavior is more complex for signed integers than for unsigned integers, because of the way signed integers are represented in binary. (The examples below are based on 8-bit signed integers for simplicity, but the same principles apply for signed integers of any size.)

Signed integers use their first bit (known as the sign bit) to indicate whether the integer is positive or negative. A sign bit of 0 means positive, and a sign bit of 1 means negative.

The remaining bits (known as the value bits) store the actual value. Positive numbers are stored in exactly the same way as for unsigned integers, counting upwards from 0. Here锟斤拷s how the bits inside an Int8 look for the number 4:

The sign bit is 0 (meaning 锟斤拷positive锟斤拷), and the seven value bits are just the number 4, written in binary notation.

Negative numbers, however, are stored differently. They are stored by subtracting their absolute value from 2 to the power of n, where n is the number of value bits. An eight-bit number has seven value bits, so this means 2 to the power of 7, or 128.

Here锟斤拷s how the bits inside an Int8 look for the number -4:

This time, the sign bit is 1 (meaning 锟斤拷negative锟斤拷), and the seven value bits have a binary value of 124 (which is 128 - 4):

The encoding for negative numbers is known as a two锟斤拷s complement representation. It may seem an unusual way to represent negative numbers, but it has several advantages.

First, you can add -1 to -4, simply by performing a standard binary addition of all eight bits (including the sign bit), and discarding anything that doesn锟斤拷t fit in the eight bits once you锟斤拷re done:

Second, the two锟斤拷s complement representation also lets you shift the bits of negative numbers to the left and right like positive numbers, and still end up doubling them for every shift you make to the left, or halving them for every shift you make to the right. To achieve this, an extra rule is used when signed integers are shifted to the right:

This action ensures that signed integers have the same sign after they are shifted to the right, and is known as an arithmetic shift.

Because of the special way that positive and negative numbers are stored, shifting either of them to the right moves them closer to zero. Keeping the sign bit the same during this shift means that negative integers remain negative as their value moves closer to zero.

Overflow Operators

If you try to insert a number into an integer constant or variable that cannot hold that value, by default Swift reports an error rather than allowing an invalid value to be created. This behavior gives extra safety when you work with numbers that are too large or too small.

For example, the Int16 integer type can hold any signed integer number between -32768 and 32767. Trying to set a UInt16 constant or variable to a number outside of this range causes an error:

var potentialOverflow = Int16.max

// potentialOverflow equals 32767, which is the largest value an Int16 can hold potentialOverflow += 1

// this causes an error

Providing error handling when values get too large or too small gives you much more flexibility when coding for boundary value conditions.

However, when you specifically want an overflow condition to truncate the number of available bits, you can opt in to this behavior rather than triggering an error. Swift provides five arithmetic overflow operators that opt in to the overflow behavior for integer calculations. These operators all begin with an ampersand (&):

Value Overflow

Here锟斤拷s an example of what happens when an unsigned value is allowed to overflow, using the overflow addition operator (&+):

var willOverflow = UInt8.max

// willOverflow equals 255, which is the largest value a UInt8 can hold willOverflow = willOverflow &+ 1

// willOverflow is now equal to 0

The variable willOverflow is initialized with the largest value a UInt8 can hold (255, or 11111111 in binary). It is then incremented by 1 using the overflow addition operator (&+). This pushes its binary representation just over the size that a UInt8 can hold, causing it to overflow beyond its bounds, as shown in the diagram below. The value that remains within the bounds of the UInt8 after the overflow addition is 00000000, or zero:

Value Underflow

Numbers can also become too small to fit in their type锟斤拷s maximum bounds. Here锟斤拷s an example.

The smallest value that a UInt8 can hold is 0 (which is 00000000 in eight-bit binary form). If you subtract 1 from 00000000 using the overflow subtraction operator, the number will overflow back round to 11111111, or 255 in decimal:

Here锟斤拷s how that looks in Swift code:

var willUnderflow = UInt8.min

// willUnderflow equals 0, which is the smallest value a UInt8 can hold

willUnderflow = willUnderflow &- 1

// willUnderflow is now equal to 255

A similar underflow occurs for signed integers. All subtraction for signed integers is performed as straight binary subtraction, with the sign bit included as part of the numbers being subtracted, as described in Bitwise Left and Right Shift Operators. The smallest number that an Int8 can hold is -128, which is 10000000 in binary. Subtracting 1 from this binary number with the overflow operator gives a binary value of 01111111, which toggles the sign bit and gives positive 127, the largest positive value that an Int8 can hold:

Here锟斤拷s the same thing in Swift code:

var signedUnderflow = Int8.min

// signedUnderflow equals -128, which is the smallest value an Int8 can hold signedUnderflow = signedUnderflow &- 1

// signedUnderflow is now equal to 127

The end result of the overflow and underflow behavior described above is that for both signed and unsigned integers, overflow always wraps around from the largest valid integer value back to the smallest, and underflow always wraps around from the smallest value to the largest.

Division by Zero

Dividing a number by zero (i / 0), or trying to calculate remainder by zero (i % 0), causes an error:

let x = 1 let y = x / 0

However, the overflow versions of these operators (&/ and &%) return a value of zero if you divide by zero:

let x = 1 let y = x &/ 0

// y is equal to 0

Precedence and Associativity

Operator precedence gives some operators higher priority than others; these operators are calculated first.

Operator associativity defines how operators of the same precedence are grouped together (or associated)锟斤拷either grouped from the left, or grouped from the right. Think of it as meaning 锟斤拷they associate with the expression to their left,锟斤拷 or 锟斤拷they associate with the expression to their right.锟斤拷

It is important to consider each operator锟斤拷s precedence and associativity when working out the order in which a compound expression will be calculated. Here锟斤拷s an example. Why does the following expression equal 4?

2 + 3 * 4 % 5

// this equals 4

Taken strictly from left to right, you might expect this to read as follows:

However, the actual answer is 4, not 0. Higher-precedence operators are evaluated before lower-precedence ones. In Swift, as in C, the multiplication operator (*) and the remainder operator (%) have a higher precedence than the addition operator (+). As a result, they are both evaluated before the addition is considered.

However, multiplication and remainder have the same precedence as each other. To work out the exact evaluation order to use, you also need to consider their associativity. Multiplication and remainder both associate with the expression to their left. Think of this as adding implicit parentheses around these parts of the expression, starting from their left:

2 + ((3 * 4) % 5)

(3 * 4) is 12, so this is equivalent to:

2 + (12 % 5)

(12 % 5) is 2, so this is equivalent to:

2 + 2

This calculation yields the final answer of 4.

For a complete list of Swift operator precedences and associativity rules, see Expressions.

NOTE

Swift锟斤拷s operator precedences and associativity rules are simpler and more predictable than those found in C and Objective-C. However, this means that they are not the same as in C-based languages. Be careful to ensure that operator interactions still behave in the way you intend when porting existing code to Swift.

Operator Functions

Classes and structures can provide their own implementations of existing operators. This is known as overloading the existing operators.

The example below shows how to implement the arithmetic addition operator (+) for a custom structure. The arithmetic addition operator is a binary operator because it operates on two targets and is said to be infix because it appears in between those two targets.

The example defines a Vector2D structure for a two-dimensional position vector (x, y), followed by a definition of an operator function to add together instances of the Vector2D structure:

struct Vector2D { var x = 0.0, y = 0.0

}

@infix func + (left: Vector2D, right: Vector2D) -> Vector2D { return Vector2D(x: left.x + right.x, y: left.y + right.y)

}

The operator function is defined as a global function called +, which takes two input parameters of type Vector2D and returns a single output value, also of type Vector2D. You implement an infix operator by writing the @infix attribute before the func keyword when declaring the operator function.

In this implementation, the input parameters are named left and right to represent the Vector2D instances that will be on the left side and right side of the + operator. The function returns a new Vector2D instance, whose x and y properties are initialized with the sum of the x and y properties from the two Vector2D instances that are added together.

The function is defined globally, rather than as a method on the Vector2D structure, so that it can be used as an infix operator between existing Vector2D instances:

let vector = Vector2D(x: 3.0, y: 1.0) let anotherVector = Vector2D(x: 2.0, y: 4.0) let combinedVector = vector + anotherVector

// combinedVector is a Vector2D instance with values of (5.0, 5.0)

This example adds together the vectors (3.0, 1.0) and (2.0, 4.0) to make the vector (5.0, 5.0), as illustrated below.

Prefix and Postfix Operators

The example shown above demonstrates a custom implementation of a binary infix operator. Classes and structures can also provide implementations of the standard unary operators. Unary operators operate on a single target. They are prefix if they precede their target (such as -a) and postfix operators if they follow their target (such as i++).

You implement a prefix or postfix unary operator by writing the @prefix or @postfix attribute before the func keyword when declaring the operator function:

@prefix func - (vector: Vector2D) -> Vector2D { return Vector2D(x: -vector.x, y: -vector.y)

}

The example above implements the unary minus operator (-a) for Vector2D instances. The unary minus operator is a prefix operator, and so this function has to be qualified with the @prefix attribute.

For simple numeric values, the unary minus operator converts positive numbers into their negative equivalent and vice versa. The corresponding implementation for Vector2D instances performs this operation on both the x and y properties:

let positive = Vector2D(x: 3.0, y: 4.0) let negative = -positive

// negative is a Vector2D instance with values of (-3.0, -4.0) let alsoPositive = -negative

// alsoPositive is a Vector2D instance with values of (3.0, 4.0)

Compound Assignment Operators

Compound assignment operators combine assignment (=) with another operation. For example, the addition assignment operator (+=) combines addition and assignment into a single operation. Operator functions that implement compound assignment must be qualified with the @assignment attribute. You must also mark a compound assignment operator锟斤拷s left input parameter as inout, because the parameter锟斤拷s value will be modified directly from within the operator function.

The example below implements an addition assignment operator function for Vector2D instances:

@assignment func += (inout left: Vector2D, right: Vector2D) { left = left + right

}

Because an addition operator was defined earlier, you don锟斤拷t need to reimplement the addition process here. Instead, the addition assignment operator function takes advantage of the existing addition operator function, and uses it to set the left value to be the left value plus the right value:

var original = Vector2D(x: 1.0, y: 2.0) let vectorToAdd = Vector2D(x: 3.0, y: 4.0) original += vectorToAdd

// original now has values of (4.0, 6.0)

You can combine the @assignment attribute with either the @prefix or @postfix attribute, as in this implementation of the prefix increment operator (++a) for Vector2D instances:

@prefix @assignment func ++ (inout vector: Vector2D) -> Vector2D { vector += Vector2D(x: 1.0, y: 1.0) return vector

}

The prefix increment operator function above takes advantage of the addition assignment operator defined earlier. It adds a Vector2D with x and y values of 1.0 to the Vector2D on which it is called, and returns the result:

var toIncrement = Vector2D(x: 3.0, y: 4.0) let afterIncrement = ++toIncrement

// toIncrement now has values of (4.0, 5.0)

// afterIncrement also has values of (4.0, 5.0)

NOTE

It is not possible to overload the default assignment operator (=). Only the compound assignment operators can be overloaded. Similarly, the ternary conditional operator (a ? b : c) cannot be overloaded.

Equivalence Operators

Custom classes and structures do not receive a default implementation of the equivalence operators, known as the 锟斤拷equal to锟斤拷 operator (==) and 锟斤拷not equal to锟斤拷 operator (!=). It is not possible for Swift to guess what would qualify as 锟斤拷equal锟斤拷 for your own custom types, because the meaning of 锟斤拷equal锟斤拷 depends on the roles that those types play in your code.

To use the equivalence operators to check for equivalence of your own custom type, provide an implementation of the operators in the same way as for other infix operators:

@infix func == (left: Vector2D, right: Vector2D) -> Bool { return (left.x == right.x) && (left.y == right.y)

}

@infix func != (left: Vector2D, right: Vector2D) -> Bool { return !(left == right)

}

The above example implements an 锟斤拷equal to锟斤拷 operator (==) to check if two Vector2D instances have equivalent values. In the context of Vector2D, it makes sense to consider 锟斤拷equal锟斤拷 as meaning 锟斤拷both instances have the same x values and y values锟斤拷, and so this is the logic used by the operator implementation. The example also implements the 锟斤拷not equal to锟斤拷 operator (!=), which simply returns the inverse of the result of the 锟斤拷equal to锟斤拷 operator.

You can now use these operators to check whether two Vector2D instances are equivalent:

let twoThree = Vector2D(x: 2.0, y: 3.0) let anotherTwoThree = Vector2D(x: 2.0, y: 3.0) if twoThree == anotherTwoThree { println("These two vectors are equivalent.")

}

// prints "These two vectors are equivalent."

Custom Operators

You can declare and implement your own custom operators in addition to the standard operators provided by Swift. Custom operators can be defined only with the characters / =

- + * % < > ! & | ^ . ~.

New operators are declared at a global level using the operator keyword, and can be declared as prefix, infix or postfix: operator prefix +++ {}

The example above defines a new prefix operator called +++. This operator does not have an existing meaning in Swift, and so it is given its own custom meaning below in the specific context of working with Vector2D instances. For the purposes of this example, +++ is treated as a new 锟斤拷prefix doubling incrementer锟斤拷 operator. It doubles the x and y values of a Vector2D instance, by adding the vector to itself with the addition assignment operator defined earlier:

@prefix @assignment func +++ (inout vector: Vector2D) -> Vector2D { vector += vector return vector

}

This implementation of +++ is very similar to the implementation of ++ for Vector2D, except that this operator function adds the vector to itself, rather than adding Vector2D(1.0, 1.0):

var toBeDoubled = Vector2D(x: 1.0, y: 4.0) let afterDoubling = +++toBeDoubled

// toBeDoubled now has values of (2.0, 8.0)

// afterDoubling also has values of (2.0, 8.0)

Precedence and Associativity for Custom Infix Operators

Custom infix operators can also specify a precedence and an associativity. See Precedence and Associativity for an explanation of how these two characteristics affect an infix operator锟斤拷s interaction with other infix operators.

The possible values for associativity are left, right, and none. Left-associative operators associate to the left if written next to other left-associative operators of the same precedence. Similarly, right-associative operators associate to the right if written next to other rightassociative operators of the same precedence. Non-associative operators cannot be written next to other operators with the same precedence.

The associativity value defaults to none if it is not specified. The precedence value defaults to 100 if it is not specified.

The following example defines a new custom infix operator called +-, with left associativity and a precedence of 140:

operator infix +- { associativity left precedence 140 } func +- (left: Vector2D, right: Vector2D) -> Vector2D { return Vector2D(x: left.x + right.x, y: left.y - right.y)

} let firstVector = Vector2D(x: 1.0, y: 2.0) let secondVector = Vector2D(x: 3.0, y: 4.0) let plusMinusVector = firstVector +- secondVector

// plusMinusVector is a Vector2D instance with values of (4.0, -2.0)

This operator adds together the x values of two vectors, and subtracts the y value of the second vector from the first. Because it is in essence an 锟斤拷additive锟斤拷 operator, it has been given the same associativity and precedence values (left and 140) as default additive infix operators such as + and -. For a complete list of the default Swift operator precedence and associativity settings, see Expressions.

Language Reference

About the Language Reference

This part of the book describes the formal grammar of the Swift programming language. The grammar described here is intended to help you understand the language in more detail, rather than to allow you to directly implement a parser or compiler.

The Swift language is relatively small, because many common types, functions, and operators that appear virtually everywhere in Swift code are actually defined in the Swift standard library. Although these types, functions, and operators are not part of the Swift language itself, they are used extensively in the discussions and code examples in this part of the book.

How to Read the Grammar

The notation used to describe the formal grammar of the Swift programming language follows a few conventions:

An arrow (锟斤拷) is used to mark grammar productions and can be read as 锟斤拷can consist of.锟斤拷

Syntactic categories are indicated by italic text and appear on both sides of a grammar production rule.

Literal words and punctuation are indicated by boldface constant width text and appear only on the right-hand side of a grammar production rule.

Alternative grammar productions are separated by vertical bars (|). When alternative productions are too long to read easily, they are broken into multiple grammar production rules on new lines.

In a few cases, regular font text is used to describe the right-hand side of a grammar production rule.

Optional syntactic categories and literals are marked by a trailing subscript, opt.

As an example, the grammar of a getter-setter block is defined as follows:

GRA MMA R OF A GETT ER-SETT ER BLOCK

getter-setter-block 锟斤拷 { getter-clause setter-clause_opt} { setter-clause getterclause }

Welcome to Swift

About Swift

A Swift Tour

NOTE

Simple Values

EXPERIMENT

EXPERIMENT

EXPERIMENT

Control Flow

EXPERIMENT

EXPERIMENT

EXPERIMENT

Functions and Closures

EXPERIMENT

EXPERIMENT

EXPERIMENT

Objects and Classes

EXPERIMENT

EXPERIMENT

Enumerations and Structures

EXPERIMENT

EXPERIMENT

EXPERIMENT

EXPERIMENT

Protocols and Extensions

EXPERIMENT

EXPERIMENT

Generics

EXPERIMENT

Language Guide

The Basics

Constants and Variables

Declaring Constants and Variables

NOTE

Type Annotations

NOTE

Naming Constants and Variables

NOTE

Printing Constants and Variables

Comments

Semicolons

Integers

Integer Bounds

Int

UInt

NOTE

Floating-Point Numbers

NOTE

Type Safety and Type Inference

Numeric Literals

Numeric Type Conversion

Integer Conversion

Integer and Floating-Point Conversion

NOTE

Type Aliases

Booleans

Tuples

NOTE

Optionals

NOTE

If Statements and Forced Unwrapping

NOTE

Optional Binding

nil

NOTE

NOTE

Implicitly Unwrapped Optionals

NOTE

NOTE

Assertions

Debugging with Assertions

When to Use Assertions

NOTE

Basic Operators

Terminology

Assignment Operator

Arithmetic Operators

Remainder Operator

NOTE

Floating-Point Remainder Calculations