6.8 Accidental DAG Multiple Inheritance via Poor Implementation of Optional Containment

Poor object-oriented designers who accidentally implement classes with more than one optional component using inheritance will find themselves with a bad case of DAG multiple inheritance. The House class from the previous chapter (Section 5.18) was defined with four optional data members, a heating system, a cooling system, a plumbing system, and an electrical system. If a designer tries to capture these optional data members in an inheritance hierarchy, then he or she will find themselves in trouble (see Figure 6.10).

Of course, all of this is unnecessary. The actual solution is to use containment by reference to implement optional data members. (See Chapter 5, Section 5.18, for a description of this solution.)