Protocol+Class+Erasure.swift

gistfile1.txt

class Animal: AnimalConvertible {
    func bark() -> String { fatalError() }
    func asAnimal() -> Animal {
        return self
    }
}

protocol AnimalConvertible {
    func bark() -> String
    func asAnimal() -> Animal
}

protocol AnimalProxy: AnimalConvertible {
    associatedtype Animal: AnimalConvertible
    var animal: Animal { get }
}

extension AnimalProxy {
    @inline(__always)
    func bark() -> String {
        return animal.bark()
    }
}

private class Box<A: AnimalConvertible>: Animal {
    let a: A
    init(a: A) {
        self.a = a
    }
    override func bark() -> String {
        return a.bark()
    }
}

extension AnimalConvertible {
    func asAnimal() -> Animal {
        return Box(a: self)
    }
}