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swiftsolid-principlesswift-protocols

"init" in Swift's protocols - what for?

Could you please explain why one may want to put initializer in Swift's protocol? I thought that protocols' responsibility is to define what concrete implementation can do, without specyfing implementation details. Isn't initializer such a thing?

Solution

  • Here is a very contrived example:

    protocol P {
    init(a: Int)
}

class B: P {
    let a: Int
    required init(a: Int) {
        self.a = a
    }
}

func factory<T: P>(a: Int) -> T {
    return T(a: a)
}

let b: B = factory(a: 1)

    By allowing the Protocol (P) to specify a required initializer, I can write the factory function which needs to make new objects.

    The init doesn't say what you do with the Int that is passed in -- just that it is possible to make P concrete values from an Int.