Swift Generics & Upcasting

Question

I've got a quick question regarding generics in Swift. The problem is I'm trying to store a variable that takes a generic as a parameter, but am unable to cast it up to the type it is restricted by. It's best explained in a short example:

class Foo { }

class Thing<T: Foo> {
    func produceBar() -> Bar {
        return Bar(aThing: self as! Thing<Foo>)
    }
}

class Bar {
    var thing: Thing<Foo>

    init(var aThing: Thing<Foo>) {
        self.thing = aThing
    }
}

The code above produces the error: "Cast from Thing<T> to unrelated type Thing<Foo> always fails"

Shouldn't it never fail, since T is restricted to being a subclass of Foo? I must be misunderstanding the way generics work in Swift, any guidance or help would be much appreciated!

Answer 1

Swift generics are not covariant. That is to say, exactly what the error says: you can't automatically say a Basket<Apple> is a kind of Basket<Fruit> even if Apple is a kind of Fruit. There is good reason for this.

Consider the following code:

class Fruit {}
class Apple: Fruit {}
class Orange: Fruit {}

class Basket<T: Fruit> {
    private var items: [T]
    func add(item: T) {
        items.append(item)
    }
    init() {}
}

func addItem<T: Fruit>(var basket: Basket<T>, item: T) {
    basket.add(item)
}

let basket:Basket<Apple> = Basket()

addItem(basket as Basket<Fruit>, Orange())

This would be legal code if Basket<Apple> were considered a Basket<Fruit>, and I'd be allowed to add an orange to a basket of apples.