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What’s the difference between a class and a struct?

Swift version: 5.1

Paul Hudson    @twostraws   

Classes and structures (structs) are so similar in Swift that it's easy to get them confused at first, but actually there are some important underlying differences:

  • A struct cannot inherit from another kind of struct, whereas classes can build on other classes.
  • You can change the type of an object at runtime using typecasting. Structs cannot have inheritance, so have only one type.
  • If you point two variables at the same struct, they have their own independent copy of the data. With objects, they both point at the same variable.

That last point is particularly important: with a struct you know your data is fixed in place, like an integer or other value. This means if you pass your struct into a function, you know it's not going to get modified.

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