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Categories of the Root Class

A category can add methods to any class, including the root class. Methods added to NSObject become available to all classes that are linked to your code. While this can be useful at times, it can also be quite dangerous. Although it may seem that the modifications the category makes are well understood and of limited impact, inheritance gives them a wide scope. You may be making unintended changes to unseen classes; you may not know all the consequences of what you’re doing. Moreover, others who are unaware of your changes won’t understand what they’re doing.

In addition, there are two other considerations to keep in mind when implementing methods for the root class:

• Messages to super are invalid (there is no superclass).

• Class objects can perform instance methods defined in the root class.

Normally, class objects can perform only class methods. But instance methods defined in the root class are a special case. They define an interface to the runtime system that all objects inherit. Class objects are full-fledged objects and need to share the same interface.

This feature means that you need to take into account the possibility that an instance method you define in a category of the NSObject class might be performed not only by instances but by class objects as well. For example, within the body of the method, self might mean a class object as well as an instance. See the NSObject class specification in the Foundation framework reference for more information on class access to root instance methods.

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