Programming in Objective-C, Fifth Edition

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Sometimes you might be working with a class definition and want to add some new methods to it. For example, you might decide for your Fraction class that, in addition to the add: method for adding two fractions, you want to have methods to subtract, multiply, and divide two fractions.

As another example, suppose that you are working on a large programming project and, as part of that project, your group is defining a new class that contains many different methods. You have been assigned the task of writing methods for the class that work with files. Other project members have been assigned methods responsible for creating and initializing instances of the class, performing operations on objects in the class, and drawing representations ...