Swift 2 add protocol conformance to protocols

Question

Can I add protocol conformance to a protocol via a swift extension?

//Plain old protocol here
protocol MyData {
    var myDataID: Int { get }
}

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Answer 1

As the error message says: an extension of a protocol cannot have an inheritance clause. Instead, you could make MyData protocol inherit from Equatable in the original declaration.

protocol MyData: Equatable {
    var myDataID: Int { get }
}

You could then extend add an implementation of == for types that conform to MyData:

func == (lhs: T, rhs: T) -> Bool {
    return lhs.myDataID == rhs.myDataID
}

However, I would highly not recommend this! If you add more properties to conforming types, their properties won't be checked for equality. Take the example below:

struct SomeData: MyData {
    var myDataID: Int
    var myOtherData: String
}

let b1 = SomeData(myDataID: 1, myOtherData: "String1")
let b2 = SomeData(myDataID: 1, myOtherData: "String2")

b1 == b2 // true, although `myOtherData` properties aren't equal.

In the case above you'd need to override == for SomeData for the correct result, thus making the == that accepts MyData redundant.