In Swift, How to extend a protocol with generic type?

Question

I want to add a function to all the classes that implement Comparable , such as Int, Float, CGFloat. Here is my code with error reported:

extension Comparable{
    func constraintBetween<T: Comparable>(a:T , b: T) -> T{
        if self < a {
            return a
        }else if self > b{
            return b
        }else{
            return self
        }
    }
}

Anyone can help to make it right? Thanks in advance!

Related (what you're implementing is a _clamp_ method): [Standard way to “clamp” a number between two values in Sw](http://stackoverflow.com/questions/36110620/standard-way-to-clamp-a-number-between-two-values-in-swift). — dfrib, Oct 27 '16 at 16:03
No worries, I'm not even certain its a duplicate, but it should at least be related and possibly helpful for you. — dfrib, Oct 27 '16 at 16:07
related http://stackoverflow.com/questions/40289169/in-swift-how-to-extension-a-protocol-with-generic-type — mfaani, Oct 27 '16 at 18:24
@LeoNatan ,yeah, they are structs in SWIFT. Not much deferent from classes. — jiangjiefs, Oct 28 '16 at 01:45

score 4 · Accepted Answer · answered Oct 27 '16 at 16:01

Self is the stand-in for the type adopting the protocol:

extension Comparable {
    func constraintBetween(a: Self, b: Self) -> Self {
        if self < a {
            return a
        } else if self > b {
            return b
        } else {
            return self
        }
    }
}

In Swift, How to extend a protocol with generic type?

1 Answers1