Generic Parameters and Arguments

This chapter describes parameters and arguments for generic types, functions, and initializers. When you declare a generic type, function, subscript, or initializer, you specify the type parameters that the generic type, function, or initializer can work with. These type parameters act as placeholders that are replaced by actual concrete type arguments when an instance of a generic type is created or a generic function or initializer is called.

For an overview of generics in Swift, see Generics.

Generic Parameter Clause

A generic parameter clause specifies the type parameters of a generic type or function, along with any associated constraints and requirements on those parameters. A generic parameter clause is enclosed in angle brackets (<>) and has the following form:

<<#generic parameter list#>>

The generic parameter list is a comma-separated list of generic parameters, each of which has the following form:

<#type parameter#>: <#constraint#>

A generic parameter consists of a type parameter followed by an optional constraint. A type parameter is simply the name of a placeholder type (for example, T, U, V, Key, Value, and so on). You have access to the type parameters (and any of their associated types) in the rest of the type, function, or initializer declaration, including in the signature of the function or initializer.

The constraint specifies that a type parameter inherits from a specific class or conforms to a protocol or protocol composition. For example, in the generic function below, the generic parameter T: Comparable indicates that any type argument substituted for the type parameter T must conform to the Comparable protocol.

func simpleMax<T: Comparable>(_ x: T, _ y: T) -> T {
 
    if x < y {
        return y
    }
 
    return x
}

Because Int and Double, for example, both conform to the Comparable protocol, this function accepts arguments of either type. In contrast with generic types, you don’t specify a generic argument clause when you use a generic function or initializer. The type arguments are instead inferred from the type of the arguments passed to the function or initializer.

simpleMax(17, 42) // T is inferred to be Int
simpleMax(3.14159, 2.71828) // T is inferred to be Double

Generic Where Clauses

You can specify additional requirements on type parameters and their associated types by including a generic where clause right before the opening curly brace of a type or function’s body. A generic where clause consists of the where keyword, followed by a comma-separated list of one or more requirements.

where <#requirements#>

The requirements in a generic where clause specify that a type parameter inherits from a class or conforms to a protocol or protocol composition. Although the generic where clause provides syntactic sugar for expressing simple constraints on type parameters (for example, <T: Comparable> is equivalent to <T> where T: Comparable and so on), you can use it to provide more complex constraints on type parameters and their associated types. For example, you can constrain the associated types of type parameters to conform to protocols. For example, <S: Sequence> where S.Iterator.Element: Equatable specifies that S conforms to the Sequence protocol and that the associated type S.Iterator.Element conforms to the Equatable protocol. This constraint ensures that each element of the sequence is equatable.

You can also specify the requirement that two types be identical, using the == operator. For example, <S1: Sequence, S2: Sequence> where S1.Iterator.Element == S2.Iterator.Element expresses the constraints that S1 and S2 conform to the Sequence protocol and that the elements of both sequences must be of the same type.

Any type argument substituted for a type parameter must meet all the constraints and requirements placed on the type parameter.

A generic where clause can appear as part of a declaration that includes type parameters, or as part of a declaration that’s nested inside of a declaration that includes type parameters. The generic where clause for a nested declaration can still refer to the type parameters of the enclosing declaration; however, the requirements from that where clause apply only to the declaration where it’s written.

If the enclosing declaration also has a where clause, the requirements from both clauses are combined. In the example below, startsWithZero() is available only if Element conforms to both SomeProtocol and Numeric.

extension Collection where Element: SomeProtocol {
    func startsWithZero() -> Bool where Element: Numeric {
        return first == .zero
    }
}

You can overload a generic function or initializer by providing different constraints, requirements, or both on the type parameters. When you call an overloaded generic function or initializer, the compiler uses these constraints to resolve which overloaded function or initializer to invoke.

For more information about generic where clauses and to see an example of one in a generic function declaration, see Generic Where Clauses.

Generic Argument Clause

A generic argument clause specifies the type arguments of a generic type. A generic argument clause is enclosed in angle brackets (<>) and has the following form:

<<#generic argument list#>>

The generic argument list is a comma-separated list of type arguments. A type argument is the name of an actual concrete type that replaces a corresponding type parameter in the generic parameter clause of a generic type. The result is a specialized version of that generic type. The example below shows a simplified version of the Swift standard library’s generic dictionary type.

struct Dictionary<Key: Hashable, Value>: Collection, ExpressibleByDictionaryLiteral {
    /* ... */
}

The specialized version of the generic Dictionary type, Dictionary<String, Int> is formed by replacing the generic parameters Key: Hashable and Value with the concrete type arguments String and Int. Each type argument must satisfy all the constraints of the generic parameter it replaces, including any additional requirements specified in a generic where clause. In the example above, the Key type parameter is constrained to conform to the Hashable protocol and therefore String must also conform to the Hashable protocol.

You can also replace a type parameter with a type argument that’s itself a specialized version of a generic type (provided it satisfies the appropriate constraints and requirements). For example, you can replace the type parameter Element in Array<Element> with a specialized version of an array, Array<Int>, to form an array whose elements are themselves arrays of integers.

let arrayOfArrays: Array<Array<Int>> = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

As mentioned in Generic Parameter Clause, you don’t use a generic argument clause to specify the type arguments of a generic function or initializer.