Swift Functional Programming - Second Edition

Value versus reference types

In Swift, there are two kinds of types in terms of memory allocation: value and reference.

Value type instances keep a copy of their data. Each type has its own data and is not referenced by another variable. Structures, enums, and tuples are value types; therefore, they do not share data between their instances. Assignments copy the data of an instance to the other and there is no reference counting involved. The following example presents a struct with copying:

struct OurStruct {     var data: Int = 3 } var valueA = OurStruct() var valueB = valueA // valueA is copied to valueB valueA.data = 5 // Changes valueA, not valueB print("\(valueA.data), \(valueB.data)") // prints "5, 3"

As seen from the preceding example, ...