The NSMutableCopying protocol declares a method for providing mutable copies of an object. Only classes that define an “immutable vs. mutable” distinction should adopt this protocol. Classes that don’t define such a distinction should adopt NSCopying instead.It is available in iOS 2.0 and later.
This is declared in Apple's Foundation framework. A class that defines an immutable vs. mutable distinction adopts this protocol to allow mutable copies of its instances to be made. A mutable copy of an object is usually a shallow copy (as opposed to the deep copy defined in the NSCopying protocol specification). The original and its copy share references to the same instance variables, so that if a component of the copy is changed, for example, that change is reflected in the original.
From Apple's official document:
NSMutableCopying declares one method, mutableCopyWithZone:, but mutable copying is commonly invoked with the convenience method mutableCopy. The mutableCopy method is defined for all NSObjects and simply invokes mutableCopyWithZone: with the default zone.
If a subclass inherits NSMutableCopying from its superclass and declares additional instance variables, the subclass has to override mutableCopyWithZone: to properly handle its own instance variables, invoking the superclass’s implementation first.
Source: NSMutableCopying Protocol Reference
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