Initializers in protocols in Swift must be designated. Why is that so?

Question

If you try to create a protocol in Swift that includes a convenience initialiser, you will be immediately scolded by the compiler:

protocol Zizo{
    convenience init(a:String, b:Int)
}

Convenience initializer not allowed in non class type.

Is there any reason for this limitation? It just doesn't make sense to me.

What could possibly go wrong?

Answer 1

It's because, it makes no sense. protocol only defines implementers have required methods and properties. It does not define implementation detail. When protocol requires init(a:String, b:Int), you can implement it as either designated or convenience initializer, as you like.

protocol Zizo{
    init(a:String, b:Int)
}

class Foo:Zizo {
    init() {
    }
    convenience required init(a:String, b:Int) {
        self.init()
    }
}
class Bar:Foo {
    override convenience init() {
        self.init(a:"baz", b:42)
    }
    required init(a:String, b:Int) {
        super.init()
    }
}

As you can see, protocol forces init(a:String, b:Int) being required initializer, but does nothing about "convenience" or "designated".

Answer 2

In case you'd still like to do this, you could consider putting a static function in the protocol instead:

protocol Zizo {
     static func make(withA a: String, b: Int) -> Self
}

this does get a little hairy when used in a class inheritance tree.