characters ), ], and } after an operator, and the characters ,, ;, and : are also considered whitespace.
There is one caveat to the rules above. If the ! or ? operator has no whitespace on the left, it is treated as a postfix operator, regardless of whether it has whitespace on the right. To use the ? operator as syntactic sugar for the Optional type, it must not have whitespace on the left. To use it in the conditional (? :) operator, it must have whitespace around both sides.
In certain constructs, operators with a leading < or > may be split into two or more tokens. The remainder is treated the same way and may be split again. As a result, there is no need to use whitespace to disambiguate between the closing > characters in constructs like Dictionary<String, Array<Int>>. In this example, the closing > characters are not treated as a single token that may then be misinterpreted as a bit shift >> operator.
To learn how to define new, custom operators, see Custom Operators and Operator Declaration. To learn how to overload existing operators, see Operator Functions.
In Swift, there are two kinds of types: named types and compound types. A named type is a type that can be given a particular name when it is defined. Named types include classes, structures, enumerations, and protocols. For example, instances of a user-defined class named MyClass have the type MyClass. In addition to user-defined named types, the Swift standard library defines many commonly used named types, including those that represent arrays, dictionaries, and optional values.
Data types that are normally considered basic or primitive in other languages—such as types that represent numbers, characters, and strings—are actually named types, defined and implemented in the Swift standard library using structures. Because they are named types, you can extend their behavior to suit the needs of your program, using an extension declaration, discussed in Extensions and Extension Declaration.
A compound type is a type without a name, defined in the Swift language itself. There are two compound types: function types and tuple types. A compound type may contain named types and other compound types. For instance, the tuple type (Int, (Int, Int)) contains two elements: The first is the named type Int, and the second is another compound type (Int, Int).
This chapter discusses the types defined in the Swift language itself and describes the type inference behavior of Swift.
A type annotation explicitly specifies the type of a variable or expression. Type annotations begin with a colon (:) and end with a type, as the following examples show:
1
2
let someTuple: (Double, Double) = (3.14159, 2.71828)
func someFunction(a: Int) { /* ... */ }
In the first example, the expression someTuple is specified to have the tuple type (Double, Double). In the second example, the parameter a to the function someFunction is specified to have the type Int.
Type annotations can contain an optional list of type attributes before the type.
G R A M M A R O F A T Y P E A N N O T A T I O N
type-annotation → : attributes opt type
Type Identifier
A type identifier refers to either a named type or a type alias of a named or compound type.
Most of the time, a type identifier directly refers to a named type with the same name as the identifier. For example, Int is a type identifier that directly refers to the named type Int, and the type identifier Dictionary<String, Int> directly refers to the named type
Dictionary<String, Int>.
There are two cases in which a type identifier does not refer to a type with the same name. In the first case, a type identifier refers to a type alias of a named or compound type. For instance, in the example below, the use of Point in the type annotation refers to the tuple type (Int, Int).
1
2
typealias Point = (Int, Int)
let origin: Point = (0, 0)
In the second case, a type identifier uses dot (.) syntax to refer to named types declared in other modules or nested within other types. For example, the type identifier in the following code references the named type MyType that is declared in the
A tuple type is a comma-separated list of zero or more types, enclosed in parentheses.
You can use a tuple type as the return type of a function to enable the function to return a single tuple containing multiple values. You can also name the elements of a tuple type and use those names to refer to the values of the individual elements. An element name consists of an identifier followed immediately by a colon (:). For an
auto_closure
example that demonstrates both of these features, see Functions with Multiple Return Values.
Void is a typealias for the the empty tuple type, (). If there is only one element inside the parentheses, the type is simply the type of that element. For example, the type of (Int) is Int, not (Int). As a result, you can label a tuple element only when the tuple type has two or more elements.
A function type represents the type of a function, method, or closure and consists of a parameter and return type separated by an arrow (->):
parameter type -> return type
Because the parameter type and the return type can be a tuple type, function types support functions and methods that take multiple paramaters and return multiple values.
You can apply the auto_closure attribute to a function type that has a parameter type of () and that returns the type of an expression (see Type Attributes). An autoclosure function captures an implicit closure over the specified expression, instead of the expression itself. The following example uses the attribute in defining a very simple assert function:
simpleAssert(testNumber % 2 == 0, "testNumber isn't an even number.")
8 // prints "testNumber isn't an even number."
A function type can have a variadic parameter as the last parameter in its parameter type. Syntactically, a variadic parameter consists of a base type name followed immediately by three dots (...), as in Int.... A variadic parameter is treated as an array that contains elements of the base type name. For instance, the variadic parameter Int... is treated as Int[]. For an example that uses a variadic parameter, see Variadic Parameters.
To specify an in-out parameter, prefix the parameter type with the inout keyword. You can’t mark a variadic parameter or a return type with the inout keyword. In-out parameters are discussed in In-Out Parameters.
The type of a curried function is equivalent to a nested function type. For example, the type of the curried function addTwoNumbers()() below is Int -> Int -> Int:
1
2
3
4
func addTwoNumbers(a: Int)(b: Int) ->Int {
return a + b
}
addTwoNumbers(4)(5) // Returns 9
The function types of a curried function are grouped from right to left. For instance, the function type Int -> Int -> Int is understood as Int -> (Int ->Int)—that is, a function that takes an Int and returns another function that takes and return an Int. For example, you can rewrite the curried function addTwoNumbers()() as the following nested function:
1
2
3
4
5
6
7
func addTwoNumbers(a: Int) -> (Int -> Int) {
func addTheSecondNumber(b: Int) ->Int {
return a + b
}
return addTheSecondNumber
}
addTwoNumbers(4)(5) // Returns 9
G R A M M A R O F A F U N C T I O N T Y P E
function-type → type ->type
Array Type
The Swift language uses square brackets ([]) immediately after the name of a type as
syntactic sugar for the named type Array<T>, which is defined in the Swift standard library. In other words, the following two declarations are equivalent:
1
2
let someArray: String[] = ["Alex", "Brian", "Dave"]
let someArray: Array<String> = ["Alex", "Brian", "Dave"]
In both cases, the constant someArray is declared as an array of strings. The elements of an array can be accessed using square brackets as well: someArray[0] refers to the element at index 0, "Alex".
As the above example also shows, you can use square brackets to create an array using an array literal. 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.
1 var emptyArray: Double[] = []
You can create multidimensional arrays by chaining multiple sets of square brackets to the name of the base type of the elements. For example, you can create a threedimensional array of integers using three sets of square brackets:
When accessing the elements in a multidimensional array, the left-most subscript index refers to the element at that index in the outermost array. The next subscript index to the right refers to the element at that index in the array that’s nested one level in. And so on. This means that in the example above, array3D[0] refers to [[1, 2], [3, 4]], array3D[0][1] refers to [3, 4], and array3D[0][1][1] refers to the value 4.
For a detailed discussion of the Swift standard library Array type, see Arrays.
G R A M M A R O F A N A R R A Y T Y P E
array-type → type [ ] array-type[ ]
Optional Type
The Swift language defines the postfix ? as syntactic sugar for the named type Optional<T>, which is defined in the Swift standard library. In other words, the following two declarations are equivalent:
1
2
var optionalInteger: Int?
var optionalInteger: Optional<Int>
In both cases, the variable optionalInteger is declared to have the type of an optional
integer. Note that no whitespace may appear between the type and the ?.
The type Optional<T> is an enumeration with two cases, None and Some(T), which are used to represent values that may or may not be present. Any type can be explicitly declared to be (or implicitly converted to) an optional type. When declaring an optional type, be sure to use parentheses to properly scope the ? operator. As an example, to declare an optional array of integers, write the type annotation as (Int[])?; writing Int[]? produces an error.
If you don’t provide an initial value when you declare an optional variable or property, its value automatically defaults to nil.
Optionals conform to the LogicValue protocol and therefore may occur in a Boolean context. In that context, if an instance of an optional type T? contains any value of type T (that is, it’s value is Optional.Some(T)), the optional type evaluates to true. Otherwise, it evaluates to false.
If an instance of an optional type contains a value, you can access that value using the postfix operator !, as shown below:
1
2
optionalInteger = 42
optionalInteger! // 42
Using the ! operator to unwrap an optional that has a value of nil results in a runtime error.
You can also use optional chaining and optional binding to conditionally perform an operation on an optional expression. If the value is nil, no operation is performed and therefore no runtime error is produced.
For more information and to see examples that show how to use optional types, see Optionals.
G R A M M A R O F A N O P T I O N A L T Y P E
optional-type → type ?
Implicitly Unwrapped Optional Type
The Swift language defines the postfix ! as syntactic sugar for the named type ImplicitlyUnwrappedOptional<T>, which is defined in the Swift standard library. In other words, the following two declarations are equivalent:
1
2
var implicitlyUnwrappedString: String!
var implicitlyUnwrappedString: ImplicitlyUnwrappedOptional<String>
In both cases, the variable implicitlyUnwrappedString is declared to have the type of an implicitly unwrapped optional string. Note that no whitespace may appear between the type and the !.
You can use implicitly unwrapped optionals in all the same places in your code that you can use optionals. For instance, you can assign values of implicitly unwrapped optionals to variables, constants, and properties of optionals, and vice versa.
As with optionals, if you don’t provide an initial value when you declare an implicitly unwrapped optional variable or property, it’s value automatically defaults to nil.
Because the value of an implicitly unwrapped optional is automatically unwrapped when you use it, there’s no need to use the ! operator to unwrap it. That said, if you try to use an implicitly unwrapped optional that has a value of nil, you’ll get a runtime error.
Use optional chaining to conditionally perform an operation on an implicitly unwrapped optional expression. If the value is nil, no operation is performed and therefore no runtime error is produced.
For more information about implicitly unwrapped optional types, see Implicitly Unwrapped Optionals.
G R A M M A R O F A N I M P L I C I T LY U N W R A P P E D O P T I O N A L T Y P E
implicitly-unwrapped-optional-type → type !
Protocol Composition Type
A protocol composition type describes a type that conforms to each protocol in a list of specified protocols. Protocol composition types may be used in type annotations and in generic parameters.
Protocol composition types have the following form:
protocol< Protocol 1 , Protocol 2 >
A protocol composition type allows you to specify a value whose type conforms to the requirements of multiple protocols without having to explicitly define a new, named protocol that inherits from each protocol you want the type to conform to. For example, specifying a protocol composition type protocol<ProtocolA, ProtocolB, ProtocolC> is effectively the same as defining a new protocol ProtocolD that inherits from ProtocolA, ProtocolB, and ProtocolC, but without having to introduce a new name.
Each item in a protocol composition list must be either the name of protocol or a type alias of a protocol composition type. If the list is empty, it specifies the empty protocol composition type, which every type conforms to.
G R A M M A R O F A P R O T O C O L C O M P O S I T I O N T Y P E
A metatype type refers to the type of any type, including class types, structure types, enumeration types, and protocol types.
The metatype of a class, structure, or enumeration type is the name of that type followed by .Type. The metatype of a protocol type—not the concrete type that conforms to the protocol at runtime—is the name of that protocol followed by .Protocol. For example, the metatype of the class type SomeClass is SomeClass.Type and the metatype
of the protocol SomeProtocol is SomeProtocol.Protocol.
You can use the postfix self expression to access a type as a value. For example,
SomeClass.self returns SomeClass itself, not an instance of SomeClass. And SomeProtocol.self returns
SomeProtocol itself, not an instance of a type that conforms to SomeProtocol at runtime. You can use a dynamicType expression with an instance of a type to access that instance’s runtime type as a value, as the following example shows:
1 class SomeBaseClass {
2class func printClassName() {
3println("SomeBaseClass")
4}
5 }
6 class SomeSubClass: SomeBaseClass {
7override class func printClassName() {
8println("SomeSubClass")
9}
tsomeInstance: SomeBaseClass = SomeSubClass()
/someInstance is of type SomeBaseClass at compile time, but
/someInstance is of type SomeSubClass at runtime omeInstance.dynamicType.printClassName()
/prints "SomeSubClass"
G R A M M A R O F A M E T A T Y P E T Y P E
metatype-type → type . Type type . Protocol
Type Inheritance Clause
A type inheritance clause is used to specify which class a named type inherits from and which protocols a named type conforms to. A type inheritance clause begins with a colon (:), followed by a comma-separated list of type identifiers.
Class types may inherit from a single superclass and conform to any number of protocols. When defining a class, the name of the superclass must appear first in the list of type identifiers, followed by any number of protocols the class must conform to. If the class does not inherit from another class, the list may begin with a protocol instead. For an extended discussion and several examples of class inheritance, see Inheritance.
Other named types may only inherit from or conform to a list of protocols. Protocol types may inherit from any number of other protocols. When a protocol type inherits from other protocols, the set of requirements from those other protocols are aggregated together, and any type that inherits from the current protocol must conform to all of those requirements.
A type inheritance clause in an enumeration definition may be either a list of protocols, or in the case of an enumeration that assigns raw values to its cases, a single, named type that specifies the type of those raw values. For an example of an enumeration definition that uses a type inheritance clause to specify the type of its raw values, see Raw Values.
G R A M M A R O F A T Y P E I N H E R I T A N C E C L A U S E
Swift uses type inference extensively, allowing you to omit the type or part of the type of many variables and expressions in your code. For example, instead of writing var x: Int = 0, you can omit the type completely and simply write var x = 0—the compiler correctly infers that x names a value of type Int. Similarly, you can omit part of a type when the full type can be inferred from context. For instance, if you write let dict: Dictionary = ["A": 1], the compiler infers that dict has the type Dictionary<String, Int>.
In both of the examples above, the type information is passed up from the leaves of the expression tree to its root. That is, the type of x in var x: Int = 0 is inferred by first
