NSCopying

NSCopying is a protocol in the Foundation framework that defines a standard interface for creating copies of objects. An object that conforms to NSCopying provides an implementation of the method copyWithZone:, which is called when the object is copied (for example, via the copy message). The protocol is commonly adopted by classes whose instances are intended to be duplicated, such as mutable data containers and model objects.

Copying semantics: The copy operation may perform a shallow or deep copy depending on the class. A shallow copy duplicates the object's immediate values, while a deep copy duplicates the entire object graph. If an object is immutable, the copy may be implemented by returning self to avoid unnecessary allocation. If mutation is possible, the copy should produce a distinct instance with the same initial state so that subsequent mutations on the copy do not affect the original.

Implementation: In Objective-C, a class that conforms to NSCopying implements -copyWithZone:, typically allocating a new instance of the class, copying the relevant state, and returning it. Some immutable objects return self. In Swift, a class adopts NSCopying and implements func copy(with zone: NSZone? = nil) -> Any.

Related protocols: NSMutableCopying declares -mutableCopyWithZone: so a class can provide a separate mutable copy. The zone argument is largely historical and often ignored in modern implementations.

Notes: Copying is distinct from archiving or encoding; it is meant for duplicating an object's in-memory state. Developers should document whether their copies are shallow or deep and ensure thread-safety if copies are accessed concurrently.