In Swift, How do I use an associatedtype in a Generic Class where the type parameter is constrained by the protocol?

Question

In Swift, I've got a protocol like this:

protocol P {
    associatedtype T
    func f(val:T)
}

I want to define a class like this:

class B<X:P> {
}

And then use the associatedtype T within the class B.

I've tried this:

class B<X:P> {
    var v:T // compiler says "Use of undeclared type"

    init() {
    }
}

I've also tried this:

class B<X:P, Y> {
    typealias T = Y
    var v:T

    init() {
    }

    func g(val:X) {
        val.f(val: v) // compiler says "Cannot invoke 'f' with an argument list of type '(val:Y)'
    }
}

Any suggestions?

Answer 1

T是占位符类型的关联类型X，因此您将其引用为X.T。例子：

class B<X: P> {
    var v: X.T

    init(v: X.T) {
        self.v = v
    }

    func g(x: X) {
        x.f(val: v)
    }
}