Do all data types or data structures use the Copy-on-Write mechanism, and in what cases is it applied?

The Copy-on-Write (COW) mechanism is not applied to all data types or structures. In iOS and Swift, it is mainly used to optimize working with collections and certain data structures that are value types (e.g., Array, Dictionary, Set).

The idea of COW is that when copying a data structure, a new copy is not immediately created; instead, a reference to a shared buffer is made. The actual data copying occurs only when an attempt is made to modify the copy (i.e., during a write), which saves memory and improves performance.

For example, if you have an array and assign it to another variable, both variables will refer to the same buffer. When one array is modified, the data is copied so that the other array remains unchanged.

Not all data types use COW. For instance, classes in Swift—reference types—do not use this mechanism because they are passed by reference, and changes are reflected across all references. Primitive types (Int, Double, etc.) are copied directly without COW.

Thus, COW is used for value types with potentially large data, where copying can be expensive, and where optimizing performance and memory usage is important.