Language Reference - Expressions - 《The Swift Programming Language Swift 5.5》

Expressions

Expressions

In Swift, there are four kinds of expressions: prefix expressions, binary expressions, primary expressions, and postfix expressions. Evaluating an expression returns a value, causes a side effect, or both.

Prefix and binary expressions let you apply operators to smaller expressions. Primary expressions are conceptually the simplest kind of expression, and they provide a way to access values. Postfix expressions, like prefix and binary expressions, let you build up more complex expressions using postfixes such as function calls and member access. Each kind of expression is described in detail in the sections below.

Grammar of an expression

expression → try-operator opt await-operator opt prefix-expression binary-expressions opt

expression-list → expression | expression , expression-list

Prefix Expressions

Prefix expressions combine an optional prefix operator with an expression. Prefix operators take one argument, the expression that follows them.

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Operator Declarations [https://developer.apple.com/documentation/swift/operator\_declarations\].

Grammar of a prefix expression

prefix-expression → prefix-operator opt postfix-expression

prefix-expression → in-out-expression

In-Out Expression

An in-out expression marks a variable that’s being passed as an in-out argument to a function call expression.

&expression

For more information about in-out parameters and to see an example, see In-Out Parameters.

In-out expressions are also used when providing a non-pointer argument in a context where a pointer is needed, as described in Implicit Conversion to a Pointer Type.

Grammar of an in-out expression

in-out-expression → & identifier

Try Operator

A try expression consists of the try operator followed by an expression that can throw an error. It has the following form:

try expression

The value of a try expression is the value of the expression.

An optional-try expression consists of the try? operator followed by an expression that can throw an error. It has the following form:

try? expression

If the expression doesn’t throw an error, the value of the optional-try expression is an optional containing the value of the expression. Otherwise, the value of the optional-try expression is nil.

A forced-try expression consists of the try! operator followed by an expression that can throw an error. It has the following form:

try! expression

The value of a forced-try expression is the value of the expression. If the expression throws an error, a runtime error is produced.

When the expression on the left-hand side of a binary operator is marked with try, try?, or try!, that operator applies to the whole binary expression. That said, you can use parentheses to be explicit about the scope of the operator’s application.

// try applies to both function calls
sum = try someThrowingFunction() + anotherThrowingFunction()
// try applies to both function calls
sum = try (someThrowingFunction() + anotherThrowingFunction())
// Error: try applies only to the first function call
sum = (try someThrowingFunction()) + anotherThrowingFunction()

A try expression can’t appear on the right-hand side of a binary operator, unless the binary operator is the assignment operator or the try expression is enclosed in parentheses.

If an expression includes both the try and await operator, the try operator must appear first.

For more information and to see examples of how to use try, try?, and try!, see Error Handling.

Grammar of a try expression

try-operator → try | try ? | try !

Await Operator

An await expression consists of the await operator followed by an expression that uses the result of an asynchronous operation. It has the following form:

await expression

The value of an await expression is the value of the expression.

An expression marked with await is called a potential suspension point. Execution of an asynchronous function can be suspended at each expression that’s marked with await. In addition, execution of concurrent code is never suspended at any other point. This means code between potential suspension points can safely update state that requires temporarily breaking invariants, provided that it completes the update before the next potential suspension point.

An await expression can appear only within an asynchronous context, such as the trailing closure passed to the async(priority:operation:) function. It can’t appear in the body of a defer statement, or in an autoclosure of synchronous function type.

When the expression on the left-hand side of a binary operator is marked with the await operator, that operator applies to the whole binary expression. That said, you can use parentheses to be explicit about the scope of the operator’s application.

// await applies to both function calls
sum = await someAsyncFunction() + anotherAsyncFunction()
// await applies to both function calls
sum = await (someAsyncFunction() + anotherAsyncFunction())
// Error: await applies only to the first function call
sum = (await someAsyncFunction()) + anotherAsyncFunction()

An await expression can’t appear on the right-hand side of a binary operator, unless the binary operator is the assignment operator or the await expression is enclosed in parentheses.

If an expression includes both the await and try operator, the try operator must appear first.

Grammar of an await expression

await-operator → await

Binary Expressions

Binary expressions combine an infix binary operator with the expression that it takes as its left- and right-hand arguments. It has the following form:

left-hand argument operator right-hand argument

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Operator Declarations [https://developer.apple.com/documentation/swift/operator\_declarations\].

Note

At parse time, an expression made up of binary operators is represented as a flat list. This list is transformed into a tree by applying operator precedence. For example, the expression 2 + 3 * 5 is initially understood as a flat list of five items, 2, +, 3, *, and 5. This process transforms it into the tree (2 + (3 * 5)).

Grammar of a binary expression

binary-expression → binary-operator prefix-expression

binary-expression → assignment-operator try-operator opt prefix-expression

binary-expression → conditional-operator try-operator opt prefix-expression

binary-expression → type-casting-operator

binary-expressions → binary-expression binary-expressions opt

Assignment Operator

The assignment operator sets a new value for a given expression. It has the following form:

expression = value

The value of the expression is set to the value obtained by evaluating the value. If the expression is a tuple, the value must be a tuple with the same number of elements. (Nested tuples are allowed.) Assignment is performed from each part of the value to the corresponding part of the expression. For example:

(a, _, (b, c)) = ("test", 9.45, (12, 3))
// a is "test", b is 12, c is 3, and 9.45 is ignored

The assignment operator doesn’t return any value.

Grammar of an assignment operator

assignment-operator → =

Ternary Conditional Operator

The ternary conditional operator evaluates to one of two given values based on the value of a condition. It has the following form:

condition ? expression used if true : expression used if false

If the condition evaluates to true, the conditional operator evaluates the first expression and returns its value. Otherwise, it evaluates the second expression and returns its value. The unused expression isn’t evaluated.

For an example that uses the ternary conditional operator, see Ternary Conditional Operator.

Grammar of a conditional operator

conditional-operator → ? expression :

Type-Casting Operators

There are four type-casting operators: the is operator, the as operator, the as? operator, and the as! operator.

They have the following form:

expression is type
expression as type
expression as? type
expression as! type

The is operator checks at runtime whether the expression can be cast to the specified type. It returns true if the expression can be cast to the specified type; otherwise, it returns false.

The as operator performs a cast when it’s known at compile time that the cast always succeeds, such as upcasting or bridging. Upcasting lets you use an expression as an instance of its type’s supertype, without using an intermediate variable. The following approaches are equivalent:

func f(_ any: Any) { print("Function for Any") }
func f(_ int: Int) { print("Function for Int") }
let x = 10
f(x)
// Prints "Function for Int"
let y: Any = x
f(y)
// Prints "Function for Any"
f(x as Any)
// Prints "Function for Any"

Bridging lets you use an expression of a Swift standard library type such as String as its corresponding Foundation type such as NSString without needing to create a new instance. For more information on bridging, see Working with Foundation Types [https://developer.apple.com/documentation/swift/imported\_c\_and\_objective\_c\_apis/working\_with\_foundation\_types\].

The as? operator performs a conditional cast of the expression to the specified type. The as? operator returns an optional of the specified type. At runtime, if the cast succeeds, the value of expression is wrapped in an optional and returned; otherwise, the value returned is nil. If casting to the specified type is guaranteed to fail or is guaranteed to succeed, a compile-time error is raised.

The as! operator performs a forced cast of the expression to the specified type. The as! operator returns a value of the specified type, not an optional type. If the cast fails, a runtime error is raised. The behavior of x as! T is the same as the behavior of (x as? T)!.

For more information about type casting and to see examples that use the type-casting operators, see Type Casting.

Grammar of a type-casting operator

type-casting-operator → is type

type-casting-operator → as type

type-casting-operator → as ? type

type-casting-operator → as ! type

Primary Expressions

Primary expressions are the most basic kind of expression. They can be used as expressions on their own, and they can be combined with other tokens to make prefix expressions, binary expressions, and postfix expressions.

Grammar of a primary expression

primary-expression → identifier generic-argument-clause opt

primary-expression → literal-expression

primary-expression → self-expression

primary-expression → superclass-expression

primary-expression → closure-expression

primary-expression → parenthesized-expression

primary-expression → tuple-expression

primary-expression → implicit-member-expression

primary-expression → wildcard-expression

primary-expression → key-path-expression

primary-expression → selector-expression

primary-expression → key-path-string-expression

Literal Expression

A literal expression consists of either an ordinary literal (such as a string or a number), an array or dictionary literal, a playground literal, or one of the following special literals:

Literal	Type	Value
`#file`	`String`	The path to the file in which it appears.
`#fileID`	`String`	The name of the file and module in which it appears.
`#filePath`	`String`	The path to the file in which it appears.
`#line`	`Int`	The line number on which it appears.
`#column`	`Int`	The column number in which it begins.
`#function`	`String`	The name of the declaration in which it appears.
`#dsohandle`	`UnsafeRawPointer`	The dynamic shared object (DSO) handle in use where it appears.

The string value of #file depends on the language version, to enable migration from the old #filePath behavior to the new #fileID behavior. Currently, #file has the same value as #filePath. In a future version of Swift, #file will have the same value as #fileID instead. To adopt the future behavior, replace #file with #fileID or #filePath as appropriate.

The string value of a #fileID expression has the form module/file, where file is the name of the file in which the expression appears and module is the name of the module that this file is part of. The string value of a #filePath expression is the full file-system path to the file in which the expression appears. Both of these values can be changed by #sourceLocation, as described in Line Control Statement. Because #fileID doesn’t embed the full path to the source file, unlike #filePath, it gives you better privacy and reduces the size of the compiled binary. Avoid using #filePath outside of tests, build scripts, or other code that doesn’t become part of the shipping program.

Note

To parse a #fileID expression, read the module name as the text before the first slash (/) and the filename as the text after the last slash. In the future, the string might contain multiple slashes, such as MyModule/some/disambiguation/MyFile.swift.

Inside a function, the value of #function is the name of that function, inside a method it’s the name of that method, inside a property getter or setter it’s the name of that property, inside special members like init or subscript it’s the name of that keyword, and at the top level of a file it’s the name of the current module.

When used as the default value of a function or method parameter, the special literal’s value is determined when the default value expression is evaluated at the call site.

func logFunctionName(string: String = #function) {
    print(string)
}
func myFunction() {
    logFunctionName() // Prints "myFunction()".
}

An array literal is an ordered collection of values. It has the following form:

[value 1, value 2, ...]

The last expression in the array can be followed by an optional comma. The value of an array literal has type [T], where T is the type of the expressions inside it. If there are expressions of multiple types, T is their closest common supertype. Empty array literals are written using an empty pair of square brackets and can be used to create an empty array of a specified type.

var emptyArray: [Double] = []

A dictionary literal is an unordered collection of key-value pairs. It has the following form:

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

The last expression in the dictionary can be followed by an optional comma. The value of a dictionary literal has type [Key: Value], where Key is the type of its key expressions and Value is the type of its value expressions. If there are expressions of multiple types, Key and Value are the closest common supertype for their respective values. An empty dictionary literal is written as a colon inside a pair of brackets ([:]) to distinguish it from an empty array literal. You can use an empty dictionary literal to create an empty dictionary literal of specified key and value types.

var emptyDictionary: [String: Double] = [:]

A playground literal is used by Xcode to create an interactive representation of a color, file, or image within the program editor. Playground literals in plain text outside of Xcode are represented using a special literal syntax.

For information on using playground literals in Xcode, see Add a color, file, or image literal [https://help.apple.com/xcode/mac/current/#/dev4c60242fc\] in Xcode Help.

Grammar of a literal expression

literal-expression → literal

literal-expression → array-literal | dictionary-literal | playground-literal

literal-expression → #file | #fileID | #filePath

literal-expression → #line | #column | #function | #dsohandle

array-literal → [ array-literal-items opt ]

array-literal-items → array-literal-item ,opt | array-literal-item , array-literal-items

array-literal-item → expression

dictionary-literal → [ dictionary-literal-items ] | [ : ]

dictionary-literal-items → dictionary-literal-item ,opt | dictionary-literal-item , dictionary-literal-items

dictionary-literal-item → expression : expression

playground-literal → #colorLiteral ( red : expression , green : expression , blue : expression , alpha : expression )

playground-literal → #fileLiteral ( resourceName : expression )

playground-literal → #imageLiteral ( resourceName : expression )

Self Expression

The self expression is an explicit reference to the current type or instance of the type in which it occurs. It has the following forms:

self
self.member name
self[subscript index]
self(initializer arguments)
self.init(initializer arguments)

In an initializer, subscript, or instance method, self refers to the current instance of the type in which it occurs. In a type method, self refers to the current type in which it occurs.

The self expression is used to specify scope when accessing members, providing disambiguation when there’s another variable of the same name in scope, such as a function parameter. For example:

class SomeClass {
    var greeting: String
    init(greeting: String) {
        self.greeting = greeting
    }
}

In a mutating method of a value type, you can assign a new instance of that value type to self. For example:

struct Point {
    var x = 0.0, y = 0.0
    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        self = Point(x: x + deltaX, y: y + deltaY)
    }
}

Grammar of a self expression

self-expression → self | self-method-expression | self-subscript-expression | self-initializer-expression

self-method-expression → self . identifier

self-subscript-expression → self [ function-call-argument-list ]

self-initializer-expression → self . init

Superclass Expression

A superclass expression lets a class interact with its superclass. It has one of the following forms:

super.member name
super[subscript index]
super.init(initializer arguments)

The first form is used to access a member of the superclass. The second form is used to access the superclass’s subscript implementation. The third form is used to access an initializer of the superclass.

Subclasses can use a superclass expression in their implementation of members, subscripting, and initializers to make use of the implementation in their superclass.

Grammar of a superclass expression

superclass-expression → superclass-method-expression | superclass-subscript-expression | superclass-initializer-expression

superclass-method-expression → super . identifier

superclass-subscript-expression → super [ function-call-argument-list ]

superclass-initializer-expression → super . init

Closure Expression

A closure expression creates a closure, also known as a lambda or an anonymous function in other programming languages. Like a function declaration, a closure contains statements, and it captures constants and variables from its enclosing scope. It has the following form:

{ (parameters) -> return type in
    statements
}

The parameters have the same form as the parameters in a function declaration, as described in Function Declaration.

There are several special forms that allow closures to be written more concisely:

A closure can omit the types of its parameters, its return type, or both. If you omit the parameter names and both types, omit the in keyword before the statements. If the omitted types can’t be inferred, a compile-time error is raised.
A closure may omit names for its parameters. Its parameters are then implicitly named $ followed by their position: $0, $1, $2, and so on.
A closure that consists of only a single expression is understood to return the value of that expression. The contents of this expression are also considered when performing type inference on the surrounding expression.

The following closure expressions are equivalent:

myFunction { (x: Int, y: Int) -> Int in
    return x + y
}
myFunction { x, y in
    return x + y
}
myFunction { return $0 + $1 }
myFunction { $0 + $1 }

For information about passing a closure as an argument to a function, see Function Call Expression.

Closure expressions can be used without being stored in a variable or constant, such as when you immediately use a closure as part of a function call. The closure expressions passed to myFunction in code above are examples of this kind of immediate use. As a result, whether a closure expression is escaping or nonescaping depends on the surrounding context of the expression. A closure expression is nonescaping if it’s called immediately or passed as a nonescaping function argument. Otherwise, the closure expression is escaping.

For more information about escaping closures, see Escaping Closures.

Capture Lists

By default, a closure expression captures constants and variables from its surrounding scope with strong references to those values. You can use a capture list to explicitly control how values are captured in a closure.

A capture list is written as a comma-separated list of expressions surrounded by square brackets, before the list of parameters. If you use a capture list, you must also use the in keyword, even if you omit the parameter names, parameter types, and return type.

The entries in the capture list are initialized when the closure is created. For each entry in the capture list, a constant is initialized to the value of the constant or variable that has the same name in the surrounding scope. For example in the code below, a is included in the capture list but b is not, which gives them different behavior.

var a = 0
var b = 0
let closure = { [a] in
    print(a, b)
}
a = 10
b = 10
closure()
// Prints "0 10"

There are two different things named a, the variable in the surrounding scope and the constant in the closure’s scope, but only one variable named b. The a in the inner scope is initialized with the value of the a in the outer scope when the closure is created, but their values aren’t connected in any special way. This means that a change to the value of a in the outer scope doesn’t affect the value of a in the inner scope, nor does a change to a inside the closure affect the value of a outside the closure. In contrast, there’s only one variable named b—the b in the outer scope—so changes from inside or outside the closure are visible in both places.

This distinction isn’t visible when the captured variable’s type has reference semantics. For example, there are two things named x in the code below, a variable in the outer scope and a constant in the inner scope, but they both refer to the same object because of reference semantics.

class SimpleClass {
    var value: Int = 0
}
var x = SimpleClass()
var y = SimpleClass()
let closure = { [x] in
    print(x.value, y.value)
}
x.value = 10
y.value = 10
closure()
// Prints "10 10"

If the type of the expression’s value is a class, you can mark the expression in a capture list with weak or unowned to capture a weak or unowned reference to the expression’s value.

myFunction { print(self.title) }                    // implicit strong capture
myFunction { [self] in print(self.title) }          // explicit strong capture
myFunction { [weak self] in print(self!.title) }    // weak capture
myFunction { [unowned self] in print(self.title) }  // unowned capture

You can also bind an arbitrary expression to a named value in a capture list. The expression is evaluated when the closure is created, and the value is captured with the specified strength. For example:

// Weak capture of "self.parent" as "parent"
myFunction { [weak parent = self.parent] in print(parent!.title) }

For more information and examples of closure expressions, see Closure Expressions. For more information and examples of capture lists, see Resolving Strong Reference Cycles for Closures.

Grammar of a closure expression

closure-expression → { closure-signature opt statements opt }

closure-signature → capture-list opt closure-parameter-clause throwsopt function-result opt in

closure-signature → capture-list in

closure-parameter-clause → ( ) | ( closure-parameter-list ) | identifier-list

closure-parameter-list → closure-parameter | closure-parameter , closure-parameter-list

closure-parameter → closure-parameter-name type-annotation opt

closure-parameter → closure-parameter-name type-annotation ...

closure-parameter-name → identifier

capture-list → [ capture-list-items ]

capture-list-items → capture-list-item | capture-list-item , capture-list-items

capture-list-item → capture-specifier opt identifier

capture-list-item → capture-specifier opt identifier = expression

capture-list-item → capture-specifier opt self-expression

capture-specifier → weak | unowned | unowned(safe) | unowned(unsafe)

Implicit Member Expression

An implicit member expression is an abbreviated way to access a member of a type, such as an enumeration case or a type method, in a context where type inference can determine the implied type. It has the following form:

.member name

For example:

var x = MyEnumeration.someValue
x = .anotherValue

If the inferred type is an optional, you can also use a member of the non-optional type in an implicit member expression.

var someOptional: MyEnumeration? = .someValue

Implicit member expressions can be followed by a postfix operator or other postfix syntax listed in Postfix Expressions. This is called a chained implicit member expression. Although it’s common for all of the chained postfix expressions to have the same type, the only requirement is that the whole chained implicit member expression needs to be convertible to the type implied by its context. Specifically, if the implied type is an optional you can use a value of the non-optional type, and if the implied type is a class type you can use a value of one of its subclasses. For example:

class SomeClass {
    static var shared = SomeClass()
    static var sharedSubclass = SomeSubclass()
    var a = AnotherClass()
}
class SomeSubclass: SomeClass { }
class AnotherClass {
    static var s = SomeClass()
    func f() -> SomeClass { return AnotherClass.s }
}
let x: SomeClass = .shared.a.f()
let y: SomeClass? = .shared
let z: SomeClass = .sharedSubclass

In the code above, the type of x matches the type implied by its context exactly, the type of y is convertible from SomeClass to SomeClass?, and the type of z is convertible from SomeSubclass to SomeClass.

Grammar of a implicit member expression

implicit-member-expression → . identifier

implicit-member-expression → . identifier . postfix-expression

Parenthesized Expression

A parenthesized expression consists of an expression surrounded by parentheses. You can use parentheses to specify the precedence of operations by explicitly grouping expressions. Grouping parentheses don’t change an expression’s type—for example, the type of (1) is simply Int.

Grammar of a parenthesized expression

parenthesized-expression → ( expression )

Tuple Expression

A tuple expression consists of a comma-separated list of expressions surrounded by parentheses. Each expression can have an optional identifier before it, separated by a colon (:). It has the following form:

(identifier 1: expression 1, identifier 2: expression 2, ...)

Each identifier in a tuple expression must be unique within the scope of the tuple expression. In a nested tuple expression, identifiers at the same level of nesting must be unique. For example, (a: 10, a: 20) is invalid because the label a appears twice at the same level. However, (a: 10, b: (a: 1, x: 2)) is valid—although a appears twice, it appears once in the outer tuple and once in the inner tuple.

A tuple expression can contain zero expressions, or it can contain two or more expressions. A single expression inside parentheses is a parenthesized expression.

Note

Both an empty tuple expression and an empty tuple type are written () in Swift. Because Void is a type alias for (), you can use it to write an empty tuple type. However, like all type aliases, Void is always a type—you can’t use it to write an empty tuple expression.

Grammar of a tuple expression

tuple-expression → ( ) | ( tuple-element , tuple-element-list )

tuple-element-list → tuple-element | tuple-element , tuple-element-list

tuple-element → expression | identifier : expression

Wildcard Expression

A wildcard expression is used to explicitly ignore a value during an assignment. For example, in the following assignment 10 is assigned to x and 20 is ignored:

(x, _) = (10, 20)
// x is 10, and 20 is ignored

Grammar of a wildcard expression

wildcard-expression → _

Key-Path Expression

A key-path expression refers to a property or subscript of a type. You use key-path expressions in dynamic programming tasks, such as key-value observing. They have the following form:

\type name.path

The type name is the name of a concrete type, including any generic parameters, such as String, [Int], or Set<Int>.

The path consists of property names, subscripts, optional-chaining expressions, and forced unwrapping expressions. Each of these key-path components can be repeated as many times as needed, in any order.

At compile time, a key-path expression is replaced by an instance of the KeyPath [https://developer.apple.com/documentation/swift/keypath\] class.

To access a value using a key path, pass the key path to the subscript(keyPath:) subscript, which is available on all types. For example:

struct SomeStructure {
    var someValue: Int
}
let s = SomeStructure(someValue: 12)
let pathToProperty = \SomeStructure.someValue
let value = s[keyPath: pathToProperty]
// value is 12

The type name can be omitted in contexts where type inference can determine the implied type. The following code uses \.someProperty instead of \SomeClass.someProperty:

class SomeClass: NSObject {
    @objc dynamic var someProperty: Int
    init(someProperty: Int) {
        self.someProperty = someProperty
    }
}
let c = SomeClass(someProperty: 10)
c.observe(\.someProperty) { object, change in
    // ...
}

The path can refer to self to create the identity key path (\.self). The identity key path refers to a whole instance, so you can use it to access and change all of the data stored in a variable in a single step. For example:

var compoundValue = (a: 1, b: 2)
// Equivalent to compoundValue = (a: 10, b: 20)
compoundValue[keyPath: \.self] = (a: 10, b: 20)

The path can contain multiple property names, separated by periods, to refer to a property of a property’s value. This code uses the key path expression \OuterStructure.outer.someValue to access the someValue property of the OuterStructure type’s outer property:

struct OuterStructure {
    var outer: SomeStructure
    init(someValue: Int) {
        self.outer = SomeStructure(someValue: someValue)
    }
}
let nested = OuterStructure(someValue: 24)
let nestedKeyPath = \OuterStructure.outer.someValue
let nestedValue = nested[keyPath: nestedKeyPath]
// nestedValue is 24

The path can include subscripts using brackets, as long as the subscript’s parameter type conforms to the Hashable protocol. This example uses a subscript in a key path to access the second element of an array:

let greetings = ["hello", "hola", "bonjour", "안녕"]
let myGreeting = greetings[keyPath: \[String].[1]]
// myGreeting is 'hola'

The value used in a subscript can be a named value or a literal. Values are captured in key paths using value semantics. The following code uses the variable index in both a key-path expression and in a closure to access the third element of the greetings array. When index is modified, the key-path expression still references the third element, while the closure uses the new index.

var index = 2
let path = \[String].[index]
let fn: ([String]) -> String = { strings in strings[index] }
print(greetings[keyPath: path])
// Prints "bonjour"
print(fn(greetings))
// Prints "bonjour"
// Setting 'index' to a new value doesn't affect 'path'
index += 1
print(greetings[keyPath: path])
// Prints "bonjour"
// Because 'fn' closes over 'index', it uses the new value
print(fn(greetings))
// Prints "안녕"

The path can use optional chaining and forced unwrapping. This code uses optional chaining in a key path to access a property of an optional string:

let firstGreeting: String? = greetings.first
print(firstGreeting?.count as Any)
// Prints "Optional(5)"
// Do the same thing using a key path.
let count = greetings[keyPath: \[String].first?.count]
print(count as Any)
// Prints "Optional(5)"

You can mix and match components of key paths to access values that are deeply nested within a type. The following code accesses different values and properties of a dictionary of arrays by using key-path expressions that combine these components.

let interestingNumbers = ["prime": [2, 3, 5, 7, 11, 13, 17],
                          "triangular": [1, 3, 6, 10, 15, 21, 28],
                          "hexagonal": [1, 6, 15, 28, 45, 66, 91]]
print(interestingNumbers[keyPath: \[String: [Int]].["prime"]] as Any)
// Prints "Optional([2, 3, 5, 7, 11, 13, 17])"
print(interestingNumbers[keyPath: \[String: [Int]].["prime"]![0]])
// Prints "2"
print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count])
// Prints "7"
print(interestingNumbers[keyPath: \[String: [Int]].["hexagonal"]!.count.bitWidth])
// Prints "64"

You can use a key path expression in contexts where you would normally provide a function or closure. Specifically, you can use a key path expression whose root type is SomeType and whose path produces a value of type Value, instead of a function or closure of type (SomeType) -> Value.

struct Task {
    var description: String
    var completed: Bool
}
var toDoList = [
    Task(description: "Practice ping-pong.", completed: false),
    Task(description: "Buy a pirate costume.", completed: true),
    Task(description: "Visit Boston in the Fall.", completed: false),
]
// Both approaches below are equivalent.
let descriptions = toDoList.filter(\.completed).map(\.description)
let descriptions2 = toDoList.filter { $0.completed }.map { $0.description }

Any side effects of a key path expression are evaluated only at the point where the expression is evaluated. For example, if you make a function call inside a subscript in a key path expression, the function is called only once as part of evaluating the expression, not every time the key path is used.

func makeIndex() -> Int {
    print("Made an index")
    return 0
}
// The line below calls makeIndex().
let taskKeyPath = \[Task][makeIndex()]
// Prints "Made an index"
// Using taskKeyPath doesn't call makeIndex() again.
let someTask = toDoList[keyPath: taskKeyPath]

For more information about using key paths in code that interacts with Objective-C APIs, see Using Objective-C Runtime Features in Swift [https://developer.apple.com/documentation/swift/using\_objective\_c\_runtime\_features\_in\_swift\]. For information about key-value coding and key-value observing, see Key-Value Coding Programming Guide [https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple\_ref/doc/uid/10000107i\] and Key-Value Observing Programming Guide [https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple\_ref/doc/uid/10000177i\].

Grammar of a key-path expression

key-path-expression → \ type opt . key-path-components

key-path-components → key-path-component | key-path-component . key-path-components

key-path-component → identifier key-path-postfixes opt | key-path-postfixes

key-path-postfixes → key-path-postfix key-path-postfixes opt

key-path-postfix → ? | ! | self | [ function-call-argument-list ]

Selector Expression

A selector expression lets you access the selector used to refer to a method or to a property’s getter or setter in Objective-C. It has the following form:

#selector(method name)
#selector(getter: property name)
#selector(setter: property name)

The method name and property name must be a reference to a method or a property that’s available in the Objective-C runtime. The value of a selector expression is an instance of the Selector type. For example:

class SomeClass: NSObject {
    @objc let property: String
    @objc(doSomethingWithInt:)
    func doSomething(_ x: Int) { }
    init(property: String) {
        self.property = property
    }
}
let selectorForMethod = #selector(SomeClass.doSomething(_:))
let selectorForPropertyGetter = #selector(getter: SomeClass.property)

When creating a selector for a property’s getter, the property name can be a reference to a variable or constant property. In contrast, when creating a selector for a property’s setter, the property name must be a reference to a variable property only.

The method name can contain parentheses for grouping, as well the as operator to disambiguate between methods that share a name but have different type signatures. For example:

extension SomeClass {
    @objc(doSomethingWithString:)
    func doSomething(_ x: String) { }
}
let anotherSelector = #selector(SomeClass.doSomething(_:) as (SomeClass) -> (String) -> Void)

Because a selector is created at compile time, not at runtime, the compiler can check that a method or property exists and that they’re exposed to the Objective-C runtime.

Note

Although the method name and the property name are expressions, they’re never evaluated.

For more information about using selectors in Swift code that interacts with Objective-C APIs, see Using Objective-C Runtime Features in Swift [https://developer.apple.com/documentation/swift/using\_objective\_c\_runtime\_features\_in\_swift\].

Grammar of a selector expression

selector-expression → #selector ( expression )

selector-expression → #selector ( getter: expression )

selector-expression → #selector ( setter: expression )

Key-Path String Expression

A key-path string expression lets you access the string used to refer to a property in Objective-C, for use in key-value coding and key-value observing APIs. It has the following form:

#keyPath(property name)

The property name must be a reference to a property that’s available in the Objective-C runtime. At compile time, the key-path string expression is replaced by a string literal. For example:

class SomeClass: NSObject {
    @objc var someProperty: Int
    init(someProperty: Int) {
        self.someProperty = someProperty
    }
}
let c = SomeClass(someProperty: 12)
let keyPath = #keyPath(SomeClass.someProperty)
if let value = c.value(forKey: keyPath) {
    print(value)
}
// Prints "12"

When you use a key-path string expression within a class, you can refer to a property of that class by writing just the property name, without the class name.

extension SomeClass {
    func getSomeKeyPath() -> String {
        return #keyPath(someProperty)
    }
}
print(keyPath == c.getSomeKeyPath())
// Prints "true"

Because the key path string is created at compile time, not at runtime, the compiler can check that the property exists and that the property is exposed to the Objective-C runtime.

For more information about using key paths in Swift code that interacts with Objective-C APIs, see Using Objective-C Runtime Features in Swift [https://developer.apple.com/documentation/swift/using\_objective\_c\_runtime\_features\_in\_swift\]. For information about key-value coding and key-value observing, see Key-Value Coding Programming Guide [https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple\_ref/doc/uid/10000107i\] and Key-Value Observing Programming Guide [https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html#//apple\_ref/doc/uid/10000177i\].

Note

Although the property name is an expression, it’s never evaluated.

Grammar of a key-path string expression

key-path-string-expression → #keyPath ( expression )

Postfix Expressions

Postfix expressions are formed by applying a postfix operator or other postfix syntax to an expression. Syntactically, every primary expression is also a postfix expression.

For information about the behavior of these operators, see Basic Operators and Advanced Operators.

For information about the operators provided by the Swift standard library, see Operator Declarations [https://developer.apple.com/documentation/swift/operator\_declarations\].

Grammar of a postfix expression

postfix-expression → primary-expression

postfix-expression → postfix-expression postfix-operator

postfix-expression → function-call-expression

postfix-expression → initializer-expression

postfix-expression → explicit-member-expression

postfix-expression → postfix-self-expression

postfix-expression → subscript-expression

postfix-expression → forced-value-expression

postfix-expression → optional-chaining-expression

Function Call Expression

A function call expression consists of a function name followed by a comma-separated list of the function’s arguments in parentheses. Function call expressions have the following form:

function name(argument value 1, argument value 2)

The function name can be any expression whose value is of a function type.

If the function definition includes names for its parameters, the function call must include names before its argument values, separated by a colon (:). This kind of function call expression has the following form:

function name(argument name 1: argument value 1, argument name 2: argument value 2)

A function call expression can include trailing closures in the form of closure expressions immediately after the closing parenthesis. The trailing closures are understood as arguments to the function, added after the last parenthesized argument. The first closure expression is unlabeled; any additional closure expressions are preceded by their argument labels. The example below shows the equivalent version of function calls that do and don’t use trailing closure syntax:

// someFunction takes an integer and a closure as its arguments
someFunction(x: x, f: { $0 == 13 })
someFunction(x: x) { $0 == 13 }
// anotherFunction takes an integer and two closures as its arguments
anotherFunction(x: x, f: { $0 == 13 }, g: { print(99) })
anotherFunction(x: x) { $0 == 13 } g: { print(99) }

If the trailing closure is the function’s only argument, you can omit the parentheses.

// someMethod takes a closure as its only argument
myData.someMethod() { $0 == 13 }
myData.someMethod { $0 == 13 }

To include the trailing closures in the arguments, the compiler examines the function’s parameters from left to right as follows:

Trailing Closure	Parameter	Action
Labeled	Labeled	If the labels are the same, the closure matches the parameter; otherwise, the parameter is skipped.
Labeled	Unlabeled	The parameter is skipped.
Unlabeled	Labeled or unlabeled	If the parameter structurally resembles a function type, as defined below, the closure matches the parameter; otherwise, the parameter is skipped.

The trailing closure is passed as the argument for the parameter that it matches. Parameters that were skipped during the scanning process don’t have an argument passed to them—for example, they can use a default parameter. After finding a match, scanning continues with the next trailing closure and the next parameter. At the end of the matching process, all trailing closures must have a match.

A parameter structurally resembles a function type if the parameter isn’t an in-out parameter, and the parameter is one of the following:

A parameter whose type is a function type, like (Bool) -> Int
An autoclosure parameter whose wrapped expression’s type is a function type, like @autoclosure () -> ((Bool) -> Int)
A variadic parameter whose array element type is a function type, like ((Bool) -> Int)...
A parameter whose type is wrapped in one or more layers of optional, like Optional<(Bool) -> Int>
A parameter whose type combines these allowed types, like (Optional<(Bool) -> Int>)...

When a trailing closure is matched to a parameter whose type structurally resembles a function type, but isn’t a function, the closure is wrapped as needed. For example, if the parameter’s type is an optional type, the closure is wrapped in Optional automatically.

To ease migration of code from versions of Swift prior to 5.3—which performed this matching from right to left—the compiler checks both the left-to-right and right-to-left orderings. If the scan directions produce different results, the old right-to-left ordering is used and the compiler generates a warning. A future version of Swift will always use the left-to-right ordering.

typealias Callback = (Int) -> Int
func someFunction(firstClosure: Callback? = nil,
                  secondClosure: Callback? = nil) {
    let first = firstClosure?(10)
    let second = secondClosure?(20)
    print(first ?? "-", second ?? "-")
}
someFunction()  // Prints "- -"
someFunction { return $0 + 100 }  // Ambiguous
someFunction { return $0 } secondClosure: { return $0 }  // Prints "10 20"

In the example above, the function call marked “Ambiguous” prints “- 120” and produces a compiler warning on Swift 5.3. A future version of Swift will print “110 -”.

A class, structure, or enumeration type can enable syntactic sugar for function call syntax by declaring one of several methods, as described in Methods with Special Names.

Implicit Conversion to a Pointer Type

In a function call expression, if the argument and parameter have a different type, the compiler tries to make their types match by applying one of the implicit conversions in the following list:

inout SomeType can become UnsafePointer<SomeType> or UnsafeMutablePointer<SomeType>
inout Array<SomeType> can become UnsafePointer<SomeType> or UnsafeMutablePointer<SomeType>
Array<SomeType> can become UnsafePointer<SomeType>
String can become UnsafePointer<CChar>

The following two function calls are equivalent:

func unsafeFunction(pointer: UnsafePointer<Int>) {
    // ...
}
var myNumber = 1234
unsafeFunction(pointer: &myNumber)
withUnsafePointer(to: myNumber) { unsafeFunction(pointer: $0) }

A pointer that’s created by these implicit conversions is valid only for the duration of the function call. To avoid undefined behavior, ensure that your code never persists the pointer after the function call ends.

Note

When implicitly converting an array to an unsafe pointer, Swift ensures that the array’s storage is contiguous by converting or copying the array as needed. For example, you can use this syntax with an array that was bridged to Array from an NSArray subclass that makes no API contract about its storage. If you need to guarantee that the array’s storage is already contiguous, so the implicit conversion never needs to do this work, use ContiguousArray instead of Array.

Using & instead of an explicit function like withUnsafePointer(to:) can help make calls to low-level C functions more readable, especially when the function takes several pointer arguments. However, when calling functions from other Swift code, avoid using & instead of using the unsafe APIs explicitly.

Grammar of a function call expression

function-call-expression → postfix-expression function-call-argument-clause

function-call-expression → postfix-expression function-call-argument-clause opt trailing-closures

function-call-argument-clause → ( ) | ( function-call-argument-list )

function-call-argument-list → function-call-argument | function-call-argument , function-call-argument-list

function-call-argument → expression | identifier : expression

function-call-argument → operator | identifier : operator

trailing-closures → closure-expression labeled-trailing-closures opt

labeled-trailing-closures → labeled-trailing-closure labeled-trailing-closures opt

labeled-trailing-closure → identifier : closure-expression

Initializer Expression

An initializer expression provides access to a type’s initializer. It has the following form:

expression.init(initializer arguments)

You use the initializer expression in a function call expression to initialize a new instance of a type. You also use an initializer expression to delegate to the initializer of a superclass.

class SomeSubClass: SomeSuperClass {
    override init() {
        // subclass initialization goes here
        super.init()
    }
}

Like a function, an initializer can be used as a value. For example:

// Type annotation is required because String has multiple initializers.
let initializer: (Int) -> String = String.init
let oneTwoThree = [1, 2, 3].map(initializer).reduce("", +)
print(oneTwoThree)
// Prints "123"

If you specify a type by name, you can access the type’s initializer without using an initializer expression. In all other cases, you must use an initializer expression.

let s1 = SomeType.init(data: 3)  // Valid
let s2 = SomeType(data: 1)       // Also valid
let s3 = type(of: someValue).init(data: 7)  // Valid
let s4 = type(of: someValue)(data: 5)       // Error

Grammar of an initializer expression

initializer-expression → postfix-expression . init

initializer-expression → postfix-expression . init ( argument-names )

Explicit Member Expression

An explicit member expression allows access to the members of a named type, a tuple, or a module. It consists of a period (.) between the item and the identifier of its member.

expression.member name

The members of a named type are named as part of the type’s declaration or extension. For example:

class SomeClass {
    var someProperty = 42
}
let c = SomeClass()
let y = c.someProperty  // Member access

The members of a tuple are implicitly named using integers in the order they appear, starting from zero. For example:

var t = (10, 20, 30)
t.0 = t.1
// Now t is (20, 20, 30)

The members of a module access the top-level declarations of that module.

Types declared with the dynamicMemberLookup attribute include members that are looked up at runtime, as described in Attributes.

To distinguish between methods or initializers whose names differ only by the names of their arguments, include the argument names in parentheses, with each argument name followed by a colon (:). Write an underscore (_) for an argument with no name. To distinguish between overloaded methods, use a type annotation. For example:

class SomeClass {
    func someMethod(x: Int, y: Int) {}
    func someMethod(x: Int, z: Int) {}
    func overloadedMethod(x: Int, y: Int) {}
    func overloadedMethod(x: Int, y: Bool) {}
}
let instance = SomeClass()
let a = instance.someMethod              // Ambiguous
let b = instance.someMethod(x:y:)        // Unambiguous
let d = instance.overloadedMethod        // Ambiguous
let d = instance.overloadedMethod(x:y:)  // Still ambiguous
let d: (Int, Bool) -> Void  = instance.overloadedMethod(x:y:)  // Unambiguous

If a period appears at the beginning of a line, it’s understood as part of an explicit member expression, not as an implicit member expression. For example, the following listing shows chained method calls split over several lines:

let x = [10, 3, 20, 15, 4]
    .sorted()
    .filter { $0 > 5 }
    .map { $0 * 100 }

Grammar of an explicit member expression

explicit-member-expression → postfix-expression . decimal-digits

explicit-member-expression → postfix-expression . identifier generic-argument-clause opt

explicit-member-expression → postfix-expression . identifier ( argument-names )

argument-names → argument-name argument-names opt

argument-name → identifier :

Postfix Self Expression

A postfix self expression consists of an expression or the name of a type, immediately followed by .self. It has the following forms:

expression.self
type.self

The first form evaluates to the value of the expression. For example, x.self evaluates to x.

The second form evaluates to the value of the type. Use this form to access a type as a value. For example, because SomeClass.self evaluates to the SomeClass type itself, you can pass it to a function or method that accepts a type-level argument.

Grammar of a postfix self expression

postfix-self-expression → postfix-expression . self

Subscript Expression

A subscript expression provides subscript access using the getter and setter of the corresponding subscript declaration. It has the following form:

expression[index expressions]

To evaluate the value of a subscript expression, the subscript getter for the expression’s type is called with the index expressions passed as the subscript parameters. To set its value, the subscript setter is called in the same way.

For information about subscript declarations, see Protocol Subscript Declaration.

Grammar of a subscript expression

subscript-expression → postfix-expression [ function-call-argument-list ]

Forced-Value Expression

A forced-value expression unwraps an optional value that you are certain isn’t nil. It has the following form:

expression!

If the value of the expression isn’t nil, the optional value is unwrapped and returned with the corresponding non-optional type. Otherwise, a runtime error is raised.

The unwrapped value of a forced-value expression can be modified, either by mutating the value itself, or by assigning to one of the value’s members. For example:

var x: Int? = 0
x! += 1
// x is now 1
var someDictionary = ["a": [1, 2, 3], "b": [10, 20]]
someDictionary["a"]![0] = 100
// someDictionary is now ["a": [100, 2, 3], "b": [10, 20]]

Grammar of a forced-value expression

forced-value-expression → postfix-expression !

Optional-Chaining Expression

An optional-chaining expression provides a simplified syntax for using optional values in postfix expressions. It has the following form:

expression?

The postfix ? operator makes an optional-chaining expression from an expression without changing the expression’s value.

Optional-chaining expressions must appear within a postfix expression, and they cause the postfix expression to be evaluated in a special way. If the value of the optional-chaining expression is nil, all of the other operations in the postfix expression are ignored and the entire postfix expression evaluates to nil. If the value of the optional-chaining expression isn’t nil, the value of the optional-chaining expression is unwrapped and used to evaluate the rest of the postfix expression. In either case, the value of the postfix expression is still of an optional type.

If a postfix expression that contains an optional-chaining expression is nested inside other postfix expressions, only the outermost expression returns an optional type. In the example below, when c isn’t nil, its value is unwrapped and used to evaluate .property, the value of which is used to evaluate .performAction(). The entire expression c?.property.performAction() has a value of an optional type.

var c: SomeClass?
var result: Bool? = c?.property.performAction()

The following example shows the behavior of the example above without using optional chaining.

var result: Bool?
if let unwrappedC = c {
    result = unwrappedC.property.performAction()
}

The unwrapped value of an optional-chaining expression can be modified, either by mutating the value itself, or by assigning to one of the value’s members. If the value of the optional-chaining expression is nil, the expression on the right-hand side of the assignment operator isn’t evaluated. For example:

func someFunctionWithSideEffects() -> Int {
    return 42  // No actual side effects.
}
var someDictionary = ["a": [1, 2, 3], "b": [10, 20]]
someDictionary["not here"]?[0] = someFunctionWithSideEffects()
// someFunctionWithSideEffects isn't evaluated
// someDictionary is still ["a": [1, 2, 3], "b": [10, 20]]
someDictionary["a"]?[0] = someFunctionWithSideEffects()
// someFunctionWithSideEffects is evaluated and returns 42
// someDictionary is now ["a": [42, 2, 3], "b": [10, 20]]

Grammar of an optional-chaining expression

optional-chaining-expression → postfix-expression ?