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The Swift Programming Language (Swift 4)

iBooks

Version Compatibility

This book describes Swift 4.0, which is the default version of Swift included in Xcode 9. You can use Xcode 9 to build targets that are written in either Swift 4 or Swift 3.

When you use Xcode 9 to build Swift 3 code, most of the new Swift 4 functionality is available. That said, the following features are available only to Swift 4 code:

  • Substring operations return an instance of the Substring type, instead of String.

  • The @objc attribute is implicitly added in fewer places.

  • Extensions to a type in the same file can access that type’s private members.

A target written in Swift 4 can depend on a target that’s written using Swift 3, and vice versa. This means, if you have a large project that is divided into multiple frameworks, you can migrate your code from Swift 3 to Swift 4 one framework at a time.