How to replace self on its implementation

Question

I have this protocol

protocol BinaryTreeProtocol {
    associatedtype T
    var info: T { get set }
    var left: Self? {get set}
    var right: Self? {get set}
    func addItem(item: T)
}

And I can't complete its implementation.

struct BinaryTreeImplementation: BinaryTreeProtocol {
    typealias T = Int
    var info: Int

    var left: BinaryTreeImplementation?  // There is an error here.
    var right: BinaryTreeImplementation? // how can I sort it?

    func addItem(item: Int) {

    }
}

well you can not have a property that contains itself – Leo Dabus Mar 09 '19 at 23:54 — Leo Dabus, Mar 09 '19 at 23:54
@LeoDabus Even if it is optional? – Hola Soy Edu Feliz Navidad Mar 10 '19 at 00:05 — Hola Soy Edu Feliz Navidad, Mar 10 '19 at 00:05
it doesn't make any difference – Leo Dabus Mar 10 '19 at 00:06 — Leo Dabus, Mar 10 '19 at 00:06
Use a class instead of a struct. – matt Mar 14 '19 at 01:43 — matt, Mar 14 '19 at 01:43

Ivan S Ivanov · Accepted Answer · 2019-03-11T09:29:03.663

I think the problem you are facing has two elements:

the recursive reference to the same type as hinted by the compiler. Basically, you are using a struct,whose size should be known at compile time; adding a recursive reference to its type prevents this
you are using 'Self' in the protocol declaration, which may lead to cases where non-fully initialised instances are created (see: Protocol func returning Self); i.e you need to use final for implementation to make sure the Self instances are of specific class and thus fully initialised;

I've modified your code reflecting my comments:

protocol BinaryTreeProtocol {
   associatedtype T
   var info: T { get set }
   var left: Self? {get set}
   var right: Self? {get set}
   func addItem(item: T)
}


final class BinaryTreeImplementation: BinaryTreeProtocol {
   typealias T = Int
   var info: Int = 0

   var left: BinaryTreeImplementation?
   var right: BinaryTreeImplementation?

   func addItem(item: Int) {

   }
}

Hope this helps and enjoy Swift :)

How to replace self on its implementation

1 Answers1