6.5 Frameworks That Incorporate Multiple Inheritance

Consider a modification to our notion of WoodenDoor, where we take into consideration the fact that doors have a body of some material (e.g., wood, steel, plastic), hinges of some material (e.g., brass, steel, gold), an optional window of some material (glass, plastic, Plexiglass), and a door knob of some material (e.g., steel, brass, wood). This implies that there is only one door class, but it is implemented as a framework (see Figure 6.6). The multiple inheritance is simply pushed down into the framework of the door.

Like all frameworks, this framework captures information relevant to the design of doors as well as the code that implements doors. By adding a new derived class to one of the contained base classes, we create the possibility for a number of different object constructions. For example, by adding a steel hinge to our domain, we create the possibility of having wooden doors with steel hinges, steel doors with steel hinges, etc.