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Self vs self - what's the difference?

Written by Paul Hudson    @twostraws

When you’re writing protocols and protocol extensions, there’s a difference between Self (capital S) and self (lowercase S). When used with a capital S, Self refers to the type that conform to the protocol, e.g. String or Int. When used with a lowercase S, self refers to the value inside that type, e.g. “hello” or 556.

As an example, consider this extension on BinaryInteger:

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

Remember, Self with a capital S refers to whatever type is conforming to the protocol. In the example above, Int conforms to BinaryInteger, so when called on Int the method effectively reads this:

func squared() -> Int {
    return self * self
}

On the other hand, self with a lowercase S refers to whatever value the type holds. If the example above were called on an Int storing the value 5 it would effectively be this:

func squared() -> Int {
    return 5 * 5
}

Available from iOS 8.0

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