NSCopying

The exact meaning of “copy” can vary from class to class, but a copy must be a functionally independent object with values identical to the original at the time the copy was made. A copy produced with NSCopying is implicitly retained by the sender, who is responsible for releasing it.

NSCopying declares one method, copy(with:), but copying is commonly invoked with the convenience method copy(). The copy() method is defined for all objects inheriting from NSObject and simply invokes copy(with:) with the default zone.

Your options for implementing this protocol are as follows:

• Implement NSCopying using alloc and init... in classes that don’t inherit copy(with:).

• Implement NSCopying by invoking the superclass’s copy(with:) when NSCopying behavior is inherited. If the superclass implementation might use the NSCopyObject function, make explicit assignments to pointer instance variables for retained objects.

• Implement NSCopying by retaining the original instead of creating a new copy when the class and its contents are immutable.

If a subclass inherits NSCopying from its superclass and declares additional instance variables, the subclass has to override copy(with:) to properly handle its own instance variables, invoking the superclass’s implementation first.