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Why does Swift have both classes and structs?

Paul Hudson    @twostraws   

Updated for Xcode 14.0 beta 1

Classes and structs give Swift developers the ability to create custom, complex types with properties and methods, but they have five important differences:

  • Classes do not come with synthesized memberwise initializers.
  • One class can be built upon (“inherit from”) another class, gaining its properties and methods.
  • Copies of structs are always unique, whereas copies of classes actually point to the same shared data.
  • Classes have deinitializers, which are methods that are called when an instance of the class is destroyed, but structs do not.
  • Variable properties in constant classes can be modified freely, but variable properties in constant structs cannot.

I’ll explain these differences in more detail soon, but the point is that they exist and that they matter. Most Swift developers prefer to use structs rather than classes when possible, which means when you choose a class over a struct you’re doing so because you want one of the above behaviors.

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