What is the principle of copy-on-write and why is it used in programming?

The copy-on-write (COW) principle means that when an object is copied, the actual data duplication does not happen immediately. Instead, copies share the same memory space until one of the copies attempts to modify the data. At that moment, a real copy ("write") occurs to ensure that changes do not affect other copies.

COW is used to optimize memory and improve performance, especially when working with large data structures where copying can be expensive.

Example: in iOS, structures like String or Array use COW to avoid full copying when passing by value until a modification is needed.