Virtual inheritance
Virtual inheritance is a C++ technique that ensures only one copy of a base class's member variables are inherited by grandchild derived classes. Without virtual inheritance, if two classes B
and C
inherit from a class A
, and a class D
inherits from both B
and C
, then D
will contain two copies of A
's 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 (D
in the example above) the virtual base (A
) acts as though it were the direct base class of D
, not a class derived indirectly through a base (B
or C
).[1][2]
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 {
virtual ~Animal() = default; // Explicitly show that the default class destructor will be made.
virtual void Eat() {}
};
struct Mammal: Animal {
virtual void Breathe() {}
};
struct WingedAnimal: Animal {
virtual void Flap() {}
};
// A bat is a winged mammal
struct Bat: Mammal, WingedAnimal {};
As declared above, a call to bat.Eat
is ambiguous because there are two Animal
(indirect) 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 bat;
Animal& animal = bat; // 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 bat;
Animal& mammal = static_cast<Mammal&>(bat);
Animal& winged = static_cast<WingedAnimal&>(bat);
In order to call Eat
, the same disambiguation, or explicit qualification is needed: static_cast<Mammal&>(bat).Eat()
or static_cast<WingedAnimal&>(bat).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 to model that the relation (Bat
is an Animal
) 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 Bat
implements (the "is a" relationship above really means "implements the requirements of"), 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
. (In the first code example we see that Eat
is not overridden in either Mammal
or WingedAnimal
, so the two Animal
subobjects will actually behave the same, but this is just a degenerate case, and that does not make a difference from the C++ point of view.)
This situation is sometimes referred to as diamond inheritance (see Diamond problem) 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 {
virtual ~Animal() = default;
virtual void Eat() {}
};
// Two classes virtually inheriting Animal:
struct Mammal: virtual Animal {
virtual void Breathe() {}
};
struct WingedAnimal: virtual Animal {
virtual void Flap() {}
};
// A bat is still a winged mammal
struct Bat: Mammal, WingedAnimal {};
The Animal
portion of Bat::WingedAnimal
is now the same Animal
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 (vpointer
, Mammal
, vpointer
, WingedAnimal
, Bat
, Animal
). 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 if the Squirrel
class is the same size as Bat
.
Additional Example of Several Ancestors
This example to illustrates a case where base class A
has a constructor variable msg
and an additional ancestor E
is derived from grandchild class D
.
A / \ B C \ / D | E
Here, A
must be constructed in both D
and E
. Further, inspection of the variable msg
illustrates how class A
becomes a direct base class of its deriving class, as opposed to a base class of any intermediate deriving classed between A
and the final deriving class. The code below may be explored interactively here.
#include <string>
#include <iostream>
class A {
private:
std::string _msg;
public:
A(std::string x): _msg(x) {}
void test(){ std::cout<<"hello from A: "<<_msg <<"\n"; }
};
// B,C inherit A virtually
class B: virtual public A { public: B():A("b"){} };
class C: virtual public A { public: C():A("c"){} };
// since B,C inherit A virtually, A must be constructed in each child
// B() and C() constructors can be omitted
class D: public B,C { public: D():A("d_a"),B(),C(){} };
// D() constructor omitted
class E: public D { public: E():A("e_a"){} };
// breaks without constructing A
// class D: public B,C { public: D():B(),C(){} };
// breaks without constructing A
//class E: public D { public: E():D(){} };
int main(int argc, char ** argv){
D d;
d.test();
// prints: "hello from A: d_a"
E e;
e.test();
// prints: "hello from A: e_a"
}
Pure Virtual Methods
Suppose a pure virtual method is defined in the base class. If a deriving class inherits the base class virtually, then the pure virtual method does not need to be defined in that deriving class. However, if the deriving class does not inherit the base class virtually, then all virtual methods must be defined. The code below may be explored interactively here.
#include <string>
#include <iostream>
class A {
protected:
std::string _msg;
public:
A(std::string x): _msg(x) {}
void test(){ std::cout<<"hello from A: "<<_msg <<"\n"; }
virtual void pure_virtual_test() = 0;
};
// since B,C inherit A virtually, the pure virtual method pure_virtual_test doesn't need to be defined
class B: virtual public A { public: B(std::string x):A("b"){} };
class C: virtual public A { public: C(std::string x):A("c"){} };
// since B,C inherit A virtually, A must be constructed in each child
// however, since D does not inherit B,C virtually, the pure virtual method in A *must be defined*
class D: public B,C {
public:
D(std::string x):A("d_a"),B("d_b"),C("d_c"){}
void pure_virtual_test() override { std::cout<<"pure virtual hello from: "<<_msg <<"\n"; }
};
// it is not necessary to redefine the pure virtual method after the parent defines it
class E: public D {
public:
E(std::string x):A("e_a"),D("e_d"){}
};
int main(int argc, char ** argv){
D d("d");
d.test(); // hello from A: d_a
d.pure_virtual_test(); // pure virtual hello from: d_a
E e("e");
e.test(); // hello from A: e_a
e.pure_virtual_test(); // pure virtual hello from: e_a
}
References
- ^ Milea, Andrei. "Solving the Diamond Problem with Virtual Inheritance". Cprogramming.com. Retrieved 2010-03-08.
One of the problems that arises due to multiple inheritance is the diamond problem. A classical illustration of this is given by Bjarne Stroustrup (the creator of C++) in the following example:
- ^ McArdell, Ralph (2004-02-14). "C++/What is virtual inheritance?". All Experts. Archived from the original on 2010-01-10. Retrieved 2010-03-08.
This is something you find may be required if you are using multiple inheritance. In that case it is possible for a class to be derived from other classes which have the same base class. In such cases, without virtual inheritance, your objects will contain more than one subobject of the base type the base classes share. Whether this is what is the required effect depends on the circumstances. If it is not then you can use virtual inheritance by specifying virtual base classes for those base types for which a whole object should only contain one such base class subobject.