Virtual inheritance is a C++ technique that ensures only one copy of a base classs member variables are inherited by grandchild derived classes. Without virtual inheritance, if two classes B and C inherit from a classA, and a classD inherits from both B and C, then D will contain two copies of As member variables: one via B, and one via C. These will be accessible independently, using scope resolution. Instead, if classes B and C inherit virtually from class A, then objects of class D will contain only one set of the member variables from class A. This feature is most useful for multiple inheritance, as it makes the virtual base a common subobject for the deriving class and all classes that are derived from it. This can be used to avoid the diamond problem by clarifying ambiguity over which ancestor class to use, as from the perspective of the deriving class the virtual base acts as though it were the direct base class of D, not a class derived indirectly through a base. It is used when inheritance represents restriction of a set rather than composition of parts. In C++, a base class intended to be common throughout the hierarchy is denoted as virtual with the virtual keyword. Consider the following class hierarchy. struct Animal ; struct Mammal: Animal ; struct WingedAnimal: Animal ; // A bat is a winged mammal struct Bat: Mammal, WingedAnimal ; Bat bat;
As declared above, a call to bat.Eat is ambiguous because there are two Animal base classes in Bat, so any Bat object has two different Animal base class subobjects. So an attempt to directly bind a reference to the Animal subobject of a Bat object would fail, since the binding is inherently ambiguous: Bat b; Animal& a = b; // error: which Animal subobject should a Bat cast into, // a Mammal::Animal or a WingedAnimal::Animal?
To disambiguate, one would have to explicitly convert bat to either base class subobject: Bat b; Animal& mammal = static_cast; Animal& winged = static_cast;
In order to call Eat, the same disambiguation, or explicit qualification is needed: static_cast.Eat or static_cast.Eat or alternatively bat.Mammal::Eat and bat.WingedAnimal::Eat. Explicit qualification not only uses an easier, uniform syntax for both pointers and objects but also allows for static dispatch, so it would arguably be the preferable method. In this case, the double inheritance of Animal is probably unwanted, as we want tomodel that the relation exists only once; that a Bat is a Mammal and is a WingedAnimal does not imply that it is an Animal twice: an Animal base class corresponds to a contract that Batimplements, and a Bat only implements the Animal contract once. The real world meaning of "is a only once" is that Bat should have only one way of implementing Eat, not two different ways, depending on whether the Mammal view of the Bat is eating, or the WingedAnimal view of the Bat. 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.
The solution
We can re-declare our classes as follows: struct Animal ; // Two classes virtually inheriting Animal: struct Mammal: virtual Animal ; struct WingedAnimal: virtual Animal ; // A bat is still a winged mammal struct Bat: Mammal, WingedAnimal ;
The Animal portion of Bat::WingedAnimal is now the sameAnimal instance as the one used by Bat::Mammal, which is to say that a Bat has only one, shared, Animal instance in its representation and so a call to Bat::Eat is unambiguous. Additionally, a direct cast from Bat to Animal is also unambiguous, now that there exists only one Animal instance which Bat could be converted to. The ability to share a single instance of the Animal parent between Mammal and WingedAnimal is enabled by recording the memory offset between the Mammal or WingedAnimal members and those of the base Animal within the derived class. However this offset can in the general case only be known at runtime, thus Bat must become. There are two vtable pointers, one per inheritance hierarchy that virtually inherits Animal. In this example, one for Mammal and one for WingedAnimal. The object size has therefore increased by two pointers, but now there is only one Animal and no ambiguity. All objects of type Bat will use the same vpointers, but each Bat object will contain its own unique Animal object. If another class inherits from Mammal, such as Squirrel, then the vpointer in the Mammal part of Squirrel will generally be different to the vpointer in the Mammal part of Bat though they may happen to be the same should the Squirrel class be the same size as Bat.
