For which entities is the copy on write mechanism used?

Copy on Write (CoW) is used to optimize memory management and improve performance, especially when working with large mutable structures. Entities in Swift that use CoW:

1. Arrays (Array)
2. Dictionaries (Dictionary)
3. Sets (Set)
4. Strings (String) (with some implementation nuances)

Example of working with an array:

// Initial array
var array1 = [1, 2, 3] 

// Assignment. Currently, the second array references the same data.
var array2 = array1 

// Modifying array2. At this moment, data from array1 is copied.
array2.append(4) 

// array1 remains unchanged
print(array1) // Output: [1, 2, 3] 

// array2 contains the new value
print(array2) // Output: [1, 2, 3, 4] 

When executing var array2 = array1, Swift does not create a full copy of the array. Instead, array1 and array2 refer to the same memory area containing the array data. Only when array2 or array1 is modified (mutated), Swift performs an actual data copy, and changes are applied to the new copy, leaving the original unchanged. This significantly saves memory and time, as copying occurs only when mutation is necessary.