FREE: Watch my new YouTube videos, Xcode in 20 Seconds! >>

Self vs self - what's the difference?

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
}

Sponsored You’re already busy updating your app for Swift 4.2 and iOS 12, so why not let Instabug help you find and fix bugs? Add just two lines of code to your project and receive comprehensive reports with all the feedback you need to ship a world-class app – click here to learn more!

Available from iOS 8.0

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Build for watchOS

Take your existing Swift skills to Apple's tiniest platform – check out Hacking with watchOS!

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>