Virtual Base Class How To?

Virtual base class is a base class acts 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: 
               A
             /   \
            B    C
             \   /
               D

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 subobject of class A.