Difference between Class and Structure in Swift


In Swift, both classes (class) and structures (struct) are powerful constructs that allow you to define properties and methods. However, there are key differences between them:

  1. Reference vs. Value Types:
    • Classes (Reference Types): When you assign a class instance to a variable or constant, or pass it to a function, you are actually working with a reference to that instance, not a copy of the instance itself. This means that if you modify it in one place, the change will be reflected wherever that reference is being used.
    • Structures (Value Types): Structures are value types. When you assign a struct to a variable or constant, or pass it to a function, it is actually copied. Changes to one copy don’t affect any other copies.
  2. Inheritance:
    • Classes: Classes support inheritance, meaning one class can inherit properties, methods, and other characteristics from another class.
    • Structures: Structures do not support inheritance.
  3. Deinitializers:
    • Classes: Classes can use deinitializers (deinit) to release any resources they have assigned.
    • Structures: Structs do not have deinitializers.
  4. Reference Counting:
    • Classes: Because they are reference types, classes support reference counting, allowing for multiple references to a single instance.
    • Structures: Being value types, structs do not use reference counting.
  5. Type Casting:
    • Classes: You can use type casting to check the type of a class instance at runtime or to cast it to a different superclass or subclass.
    • Structures: Type casting isn’t applicable in the same way with structs as it is with classes.
  6. Mutability:
    • Classes: If an instance of a class is assigned to a constant, you can still change its properties. The reference is constant, not the content it points to.
    • Structures: If you assign an instance of a struct to a constant, that instance becomes immutable and you cannot change its properties.
  7. Extensions:
    • Both classes and structures can be extended using Swift’s extension mechanism. This allows you to add new properties, methods, computed properties, and more. However, you can’t add stored properties in extensions, regardless of whether you’re extending a class or a structure.
  8. Protocols:
    • Both classes and structures can conform to protocols. This allows them to promise to implement certain properties or methods.

When designing your applications, consider the above differences to determine whether a class or a structure is more appropriate. In general, many Swift developers prefer using structures because they are simpler and safer in a multithreaded environment, given their value semantics. Classes come into play when you need features like inheritance or when you want to work with reference semantics.

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