As discussed previously, the inheritance process creates a new derived class that is made up of the members of the base class(es) plus any new members added by the derived class. In a multiple-inheritance, it is possible to construct an inheritance graph where the same base class is part of more than one of the derived classes. Figure 9.4 shows such a graph.
In Figure 9.4, pictorial representations of the components of CollectibleString and CollectibleSortable are shown. However, the base class, Collectible, is in CollectibleSortableString through the CollectibleString path and the CollectibleSortable path. To eliminate this redundancy, such classes can be declared as virtual base classes when they are inherited.
For information about declaring virtual base classes and how objects with virtual base classes are composed, see Virtual Base Classes.
Figure 9.4 Multiple Instances of a Single Base Class