In the C++ programming language, virtual inheritance is a kind of inheritance that solves some of the problems caused by multiple inheritance (particularly the "diamond problem") by clarifying ambiguity over which ancestor class members to use. It is used when inheritance is representing restrictions of a set rather than composition of parts. A multiply-inherited base class is denoted as virtual with the
Consider the following class hierarchy.
But how does
eat()? As declared above, a call to
bat.eat() is ambiguous. One would have to call either
This situation is sometimes referred to as diamond inheritance because the inheritance diagram is in the shape of a diamond. Virtual inheritance can help to solve this problem.
Bat::WingedAnimalis the same
Animalas the one used by
Bat::Mammal, which is to say that a
Bathas only one
Animalin its representation and so a call to
This is implemented by providing
WingedAnimal with a vtable pointer since, e.g., the memory offset between the beginning of a
Mammal and of its
Animal part is unknown until runtime. Thus Bat becomes (vtable*,Mammal,vtable*,WingedAnimal,Bat,Animal). Two vtable pointers per object, so the object size increased by two pointers, but now there is only one Animal and no ambiguity. There are two vtables pointers: one per inheritance hierarchy that virtually inherits Animal: One for Mammal and one for WingedAnimal. All objects of type Bat will have the same vtable *'s, but each Bat object will contain its own unique Animal object. If another class inherits Mammal, such as Squirrel, then the vtable* in the Mammal object in a Squirrel will be different from the vtable* in the Mammal object in a Bat, although they can still be essentially the same in the special case that the squirrel part of the object has the same size as the Bat part, because then the distance from the Mammal to the Animal part is the same. The vtables are not really the same, but all essential information in them (the distance) is.