在Swift中扩展泛型类型的具体实例

Question

Is it possible to provide an extension that only applies to a specific instance of a generic type?

For example, say I want to add a method to Int? , but not to any other Optional .

Is this possible?

Answer 1

Kind of. Since Optional is a protocol, you can create an extension and constrain it. However, the constraint can't be on a type, but needs to be on a protocol.

This works:

extension Optional where Wrapped: SignedIntegerType {
    func test() -> Int {
        return 0
    }
}

and then you can use it:

let a:Int? = nil
a.test()

However, if you try to do:

extension Optional where Wrapped: Int {
    func test() -> Int {
        return 0
    }
}

you'll get an error:

type 'Wrapped' constrained to non-protocol type 'Int'