FAQ 10.09 When should the testInvariant() member function be called?

FAQ 10.09 When should the testInvariant() member function be called?

At the end of constructors to make sure the invariant was established and at the end of mutator member functions to make sure the invariant was maintained.

Rule 1: Every public: constructor must establish the invariant. Every public: constructor must initialize its object so that it passes the invariant test (this means avoiding any technique that allows the object to be initialized to garbage and requiring the user to call an init() member function). Thus every public: constructor should call testInvariant() as the last thing it does. Normally this call should be in an #ifdef or an assert() so that it can be easily removed or reinstalled as desired.

Rule 2: Every public: member function must maintain the invariant. Every public: member function may assume that its object passes the invariant test at the beginning and must restore its object's invariant by the time it returns. Thus every public: member function that mutates the object should call testInvariant() as the last thing it does. Normally this call should also be in an #ifdef or an assert() so that it can be easily removed or reinstalled as desired.