FAQ 10.05 When is a class correct?

When it meets or exceeds its external agreements and abides by its internal constraints.

A class's external agreements include requirements imposed on users of the class and promises made to the users. This behavior is observable in the sense that it is expressed in terms of the class's public: member functions. This behavior can and should be tested. For example, just before a member function returns, the member function can check that it actually fulfilled its promises (a.k.a. postconditions; see FAQ 6.04). This is called behavioral self-testing, and it often involves adding checks at the bottom of the member function that check the member function's promises. These checks are normally put in an assert() statement or perhaps in an #ifdef; that way the checks can be removed if they cause too much performance degradation (see FAQ 10.06).

A class's internal constraints define the allowed states of data structures associated with objects of the class. Every object of the class must abide by these restrictions at all times. The class can and should test these class invariants (see FAQ 10.07).