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What are protocol extensions?

Paul Hudson       @twostraws

This might sound obvious, but protocol extensions are extensions to protocols as opposed to concrete types. For example, the BinaryInteger protocol is adopted by all integer types: Int, Int64, UInt8, and so on. If you wanted to add a method to all of those at once, you’d use a protocol extension to modify BinaryInteger, like this:

extension BinaryInteger {
    func squared() -> Self {
        return self * self
    }
}

That squared() method will now existing on all integer types, so you can write code like this:

let i: Int = 5
let j: UInt8 = 7
print(i.squared())
print(j.squared())

Note: Self with a capital S refers to whatever type conforms to the protocol, e.g. Int or UInt32, whereas self with a lowercase S refers to whatever the current value of the type is, e.g. 5 or 99.

Available from iOS 8.0 – learn more in my book Pro Swift

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