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, changing valueA.data does not change valueB.data.

In Swift, arrays, dictionaries, strings, and sets are all value types.

On the other hand, reference...