What are protocol extensions?

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

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 cubed() -> Self {
        return self * self * self
    }
}

That cubed() 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.cubed())
print(j.cubed())

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.