Can you explain the mechanism of Copy-on-write and its application?

Copy-on-write (COW) is an optimization technique where copying a resource (such as memory or a file) is deferred until one of the copies attempts to modify the data. Initially, both copies refer to the same resource, saving memory and time.

Application in iOS:

• When working with objects that can be shared between threads or components, such as NSMutableData or NSString.
• When creating copies of large data structures, where copying occurs only upon write.

Example:

var original = NSMutableData(data: Data([1, 2, 3]))
var copy = original // copy-on-write, data is not copied immediately
copy.append(Data([4])) // now actual copying occurs to avoid changing original

Thus, COW allows efficient memory management by avoiding unnecessary copies until modification occurs.