Create generic object with subclass as parameter

Question

Greetings.

I have run up against another generic brick wall, so to speak.

I have a generic class:

class Thing<ownerT>
{
  private var owner: ownerT

  init(owner: ownerT)
  {
    self.owner = owner
  }
}

Now I would like to use that type inside a generic protocol:

protocol Owner
{
  var thing: Thing<Self> { get set }
}

Next, I would like to implement that protocol in a hierarchy of classes, such that the "owner" passed to the generic class is the current subclass:

class Base : Owner
{
  var thing: Thing<Self> // meant to allow use of Derived but obviously this won't compile
}

class Derived : Base
{

}

Problem 1: the use of Self in the protocol makes its compulsory to use a final class to implement the protocol.

Problem 2: derived classes need to use Thing<…> as if it belonged to their concrete type, not Base.

Any bright ideas how I can get Derived to see Thing ?

If this were C#, I could bind the generic type to something akin to

type(of: self)

But this is Swift and I am struggling with the lack of dynamicism

Swift

1k

Posted by

Joanna Carter

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Answer 1

Your code fragments have lost their "< T >" annotations, so it's hard to see exactly what you're trying to do. Can you edit your post to fix this?

Posted by

QuinceyMorris

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Answer 2

Oops! Must have happened when copy/pasting. I've fixed it in my post.

Many thanks for looking at this Quincey

Posted by

Joanna Carter

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Answer 3

There's no good solution here. Owner isn't really a "generic protocol", and protocols don't really compose well with base classes, and abstract base classes don't really exist in Swift. I think the best you can do is something like this:

protocol Owner {
    var thing: Owned { get set }
}

protocol Owned {
    var owner: Owner { get set }
}

class Thing {
    private var owner: OwnerT
    init(owner: OwnerT) {
        self.owner = owner
    }
}

class Base: Owner {
    var thing: Owned
    init(thing: Owned) {
        self.thing = thing
    }
}

class Derived: Base {
    var derivedThing: Thing {
        return thing as! Thing
    }
}

In code that uses this, if you have a Base reference, you can get to the Thing, but you don't know what class it really is without casting. If you have a Derived reference, this gives it a convenience method that does the casting for you. (Alternatively, you could make "derivedThing" a stored property.)

Posted by

QuinceyMorris

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Create generic object with subclass as parameter

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