Objective-C: how to extend the implementation of existing classes without using class inheritance

In objective-C 2.0, there are two ways to extend the implementation of existing classes without using class inheritance. One is classification, and the other is class extensions ).

The former can extend the implementation of classes without the source code of existing classes. You can define new methods, but not instance variables.

The latter must have the source code of the existing class to implement expansion. However, compared to classification, class extension can also define instance variables and hide such information. That is to say, class extension is a means to define private variables and private methods.