Swift - Type does not conform to protocol when using concrete implementation

Question

In this code:

protocol TestProtocol {
    var someVar: String { get }
}

protocol WithTestProtocol {
    var someVarWithTestProtocol: TestProtocol { get }
}

struct TestProtocolStruct: TestProtocol {
    var someVar: String {
        return ""
    }
}

struct WithTestProtocolStruct: WithTestProtocol {
    var someVarWithTestProtocol: TestProtocolStruct {
        return TestProtocolStruct()
    }
}

I get the error message:

Type 'WithTestProtocolStruct' does not conform to protocol 'WithTestProtocol'

Why is it not possible to conform to a protocol with a concrete implementation? Are there good workarounds?

I know, that this code works:

struct WithTestProtocolStruct: WithTestProtocol {
    var someVarWithTestProtocol: TestProtocol {
        return TestProtocolStruct()
    }
}

.. but I need to use a concrete implementation there because I want to use other stuff of the concrete implementation. I thought, it is a very common case and I`m wondering, why the compiler does not allow this.

In `WithTestProtocol`, your `someVarWithTestProtocol` is of type `TestProtocol`, but in the implementation ( `WithTestProtocolStruct`) you're defining it as `TestProtocolStruct`. It should be `TestProtocol` too. — Alejandro Iván, Aug 29 '18 at 13:37
Yes, I used a concrete implementation because a need the concrete implementation there. Because of the other stuff this implementation has. I thought, it is a really common case and that the compiler should be able to know that `TestProtocolStruct` conforms to `TestProtocol`. — tanaschita, Aug 29 '18 at 13:53
@Hamish, thanks for the duplicate link, it answers the question. — tanaschita, Aug 29 '18 at 14:01

score 0 · Answer 1 · edited Oct 05 '20 at 13:12

0

Your definition of protocol type is TestProtocol

struct WithTestProtocolStruct: WithTestProtocol {
    var someVarWithTestProtocol: TestProtocol {
        return TestProtocolStruct()
    }
}

edited Oct 05 '20 at 13:12

Paul Peelen

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answered Aug 29 '18 at 13:39

Alex L

309
2
9

score 0 · Answer 2 · answered Aug 29 '18 at 13:40

You have to provide specific type while using protocol properties.

struct WithTestProtocolStruct: WithTestProtocol {
  var someVarWithTestProtocol: TestProtocol {
    return TestProtocolStruct()
  }
}

Practically TestProtocolStruct may or may not confirm the TestProtocol compiler won't allow this.

Swift - Type does not conform to protocol when using concrete implementation

2 Answers2