Swift generic reference type

Question

I'm having issues trying to constrain generic type requirements to just reference types. Here's some example code:

class WeakHolder<Element: AnyObject> {
    weak var element: Element?

    init(element: Element) {
        self.element = element
    }
}

protocol Animal: class { }

class Dog: Animal { }

let dog: Animal = Dog()
let holder = WeakHolder<Animal>(element: dog) // Error: Using "Animal" as a concrete type conforming to protocol 'AnyObject' is not supported.

If I change the generic requirement to <Element: class>, I get the error class constraint can only appear on protocol declarations.

Is this a limitation of generics? Marking a protocol as class is enough to have a weak reference to that protocol, is there no equivalent in generics?

Based on my small experience this may be a limitation for generics as of swift 2.3. You may be able to fix this though by using classes instead of protocols — Zonily Jame, Nov 30 '16 at 21:20
You're trying to create a class that's reusable for a huge part of your code aren't you? — Zonily Jame, Nov 30 '16 at 21:22
is doing this not acceptable? `class WeakHolder { weak var element: Element? init(element: Element) { self.element = element } }` — Zonily Jame, Nov 30 '16 at 21:25
I'm using dependency injection in my project, so I work entirely with protocols. Yeah, I'm more looking for a way to constrain a generic to a reference type. — Mark, Nov 30 '16 at 21:35
The problem is that [protocols don't conform to themselves](http://stackoverflow.com/questions/33112559/protocol-doesnt-conform-to-itself) – you can nearly always solve this problem with a type erasure, such as Rob shows in his answer to the Q&A. In your case, you'd want to make an `AnyAnimal`. — Hamish, Nov 30 '16 at 21:46

score 0 · Accepted Answer · answered Dec 01 '16 at 00:56

The simple answer is that you cannot have a generic type that is a protocol.

Writing out the syntax makes it clear how this works: class/struct GenericType<TypeName: TypeConstraints> {}

let thing = GenericType<Type>() where Type is a class or struct that adheres to any constraints

A Protocol Requiring adopting Types to be a class means any adopters are classes, but the Protocol itself is still not a Type.

It's possible generics could at some point support protocols, but it would require changing the general approach to either protocols or generics. Though your specific example may be possible with a smaller amount of work behind the scenes, so it's possible this may be implemented at some point.

You can take a look at The Generics Manifesto if you want to see the direction they're going. Skimming it I didn't find anything directly related to your use case, but it's fairly specific so it may not be included in the parameters of the document.

Ah, the generalized class constraint is exactly what I'm looking for. Good to know that it's being considered for a future version of Swift. — Mark, Dec 01 '16 at 02:32

score 0 · Answer 2 · answered Dec 01 '16 at 15:37

Another solution that worked in my particular case is the following:

class WeakHolder<Element: AnyObject> {
    weak var element: Element?

    init(element: Element) {
        self.element = element
    }
}

protocol Animal: class { }

class Dog: Animal { }

let dog: Animal = Dog()
let holder = WeakHolder<AnyObject>(element: dog as AnyObject)

When accessing element, I simply need to perform a downcast back to my protocol. Of course, I'll lose compile time safety when using this class with value types, but that's a non-issue in my situation.

Swift generic reference type

2 Answers2