How does a category work?

Question

I'm new to objective-c and need to extend a standard class of a framework with an instance variable plus accessors. I heard that this is done with a so called "category", which sounds pretty confusing to me. How does this basically work?

Answer 1

A category adds methods to the table of methods inside a class. It's very handy for adding application specific methods to existing framework classes.

If you need to add instance variables to a class, a category won't do the job -- categories only add methods, not data. To add instance variables, you must subclass.