Swift protocol defining class method returning self

Question

I had code that was working in XCode 6 beta but stopped working recently after updating to xcode 6.1.

This is my protocol:

protocol CanDeserialiseFromJson {
    class func FromJson(json : JSONValue) -> Self
}

This is implementation:

extension Invoice : CanDeserialiseFromJson {
    class func FromJson(json : JSONValue) -> Self {
        return Invoice()
    }
}

This fails giving error:

'Invoice' is not convertable to 'Self'

As I said, this used to work and I can't work out why it doesn't anymore

See: http://stackoverflow.com/questions/25645090/protocol-func-returning-self/25645689#25645689 — Rob Napier, Oct 07 '14 at 20:43

score 4 · Answer 1 · answered Oct 08 '14 at 07:24

It's right. Your method is declared to return Self, whereas you are returning Invoice. Class methods are inherited, and in subclasses, Self will be that subclass type, and Invoice is not a subtype of that type.

To actually return Self, assuming Invoice has a required init() constructor, you can do something like this:

extension Invoice : CanDeserialiseFromJson {
    class func FromJson(json : JSONValue) -> Self {
        return self()
    }
}

rickster · Accepted Answer · 2014-10-07T23:51:47.860

2

Self in a protocol is a requirement that implementations of the protocol use their own type. Since Invoice is the type you're adopting the protocol in, your implementation of FromJson should have a return type of Invoice.

edited Oct 07 '14 at 23:51

answered Oct 07 '14 at 20:42

rickster

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1

Which it does: return Invoice() – Stanislaw Wozniak Oct 07 '14 at 20:43
He means the return type should be `Invoice` :) – fluidsonic Oct 07 '14 at 20:48
1

Then it would not conform to protocol: Method 'FromJson' in non-final class 'Invoice' must return `Self` to conform to protocol 'CanDeserialiseFromJson' – Stanislaw Wozniak Oct 07 '14 at 20:57
6

I marked Invoice class as final and it seems to work – Stanislaw Wozniak Oct 07 '14 at 20:59
3

What should I do if I don't want to mark it as `final`? – Klein Mioke Jan 04 '17 at 08:18
@KleinMioke see my answer bellow. I had the same issue. Also discussed at more length here: https://stackoverflow.com/a/24181556/343155 – Морт Dec 05 '18 at 12:57

score 1 · Answer 3 · answered Dec 05 '18 at 12:56

In case you really need to return Self (in my case I have an Objective-C protocol that translates a method into swift function returning Self) and not mark your class as final, you need to create a required initializer and use that:

class Foo {
    static func bar() -> Self {
        return self.init()
    }

    required init() {

    }
}

The final/required requirement comes from the fact that you might subclass this class and have a different initialiser. Final removes the option for a subclass, while the required init makes sure that any subclass will implement the required initialiser used in the static method.

Swift protocol defining class method returning self

3 Answers3

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