Virtual base class is a base class act as an indirect base for more than one without duplication
of its data members.
A single copy of its data members is shared by all the base classes that use it as a virtual base.

For example:

class A { /* … */ }; // indirect base class
class B : virtual public A { /* … */ };
class C : virtual public A { /* … */ };
class D : public B, public C { /* … */ }; // valid
Using the keyword virtual in this example ensures that an object of class D inherits only one
sub-object of class A.