Implementing the <NSCopying> Protocol
If you try to use the copy method on one of your own classes (for example, on your address book) as follows
newBook = [myBook mutableCopy];
you’ll get an error message that looks something like this:
*** -[AddressBook copyWithZone:]: selector not recognized*** Uncaught exception:*** -[AddressBook copyWithZone:]: selector not recognized
As noted, to implement copying with your own classes, you have to implement one or two methods according to the <NSCopying> protocol.
We now show how you can add a copy method to your Fraction class, which you used extensively in Part I, “The Objective-C Language.” Note that the techniques we describe here for copying ...
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