FAQ 6.01 What is the purpose of this chapter?

To provide a practical technique that reduces the ripple effect during both development and maintenance. The technique provided in this chapter is fundamental to understanding proper inheritance as well as reducing both short-term and long-term costs, especially for large systems.

The basic idea is to unambiguously state the external behavior of each member function in some well-known location, then for other programmers to rely on this specification rather than digging into the code and relying on the implementation. This technique is sometimes called programming by contract, and it is extremely valuable whenever a software system is large enough that most programmers can't remember all the ifs, ands and buts of every member function of every class.