protocol Factoryable { init(factory: () -> Self) } extension Factoryable { init(factory: () -> Self) { self = factory() } } class MyAnimal: Factoryable { convenience init(name: String) { if name.contains("Cat") { self.init { MyCat() } } else { self.init { MyDog() } } } } class MyCat: MyAnimal { var meou: String { return "meou" } } class MyDog: MyAnimal { } let x = MyCat(name: "Dog") x.meou // Crash!!