Comparing Structures and Classes
- Structures and classes in Swift have many things in common. Both can:
- Define properties to store values
- Define methods to provide functionality
- Define subscripts to provide access to their values using subscript syntax
- Define initializers to set up their initial state
- Be extended to expand their functionality beyond a default implementation
- Conform to protocols to provide standard functionality of a certain kind
Classes have additional capabilities that structures don’t have:
- Inheritance enables one class to inherit the characteristics of another.
- Type casting enables you to check and interpret the type of a class instance at runtime.
- Deinitializers enable an instance of a class to free up any resources it has assigned.
- Reference counting allows more than one reference to a class instance.
All structures and enumerations are value types in Swift. This means that any structure and enumeration instances you create—and any value types they have as properties—are always copied when they’re passed around in your code.
Unlike value types, reference types are not copied when they’re assigned to a variable or constant, or when they’re passed to a function. Rather than a copy, a reference to the same existing instance is used.