instance_swap.swift

protocol Animal {}
extension Animal {
    init(_animal: Self) {
        self = _animal
    }
}

class AnimalBase: Animal {

    convenience init(with animal: AnimalBase) {
        self.init(_animal: animal)
    }
}

class Cat: AnimalBase {
    let _name = "Tama"
    var name: String { return _name }

    func bark() {
        print(name)
    }
}

class Dog: AnimalBase {}

let animal = Cat(with: Dog())
animal.bark()