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Self vs self - what's the difference?

Swift version: 5.1

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 returns an Int.

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 5 * 5.

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Available from iOS 8.0

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