Practical C Programming, 3rd Edition

This method is, to define as the number of a class the class of all classes similar to the given class.

”Bertrand Russell, Principles of Mathematics , Part II, Chapter 11, Section iii, 1903

C++ has many bells and whistles that give you a lot of flexibility in designing your classes. For example, the friend keyword lets a class specify ordinary functions that are allowed to access its private data. This section also covers constant members as well as how to constrain specific data using the keyword static .