Virtual base class in C++

Virtual base classes are used in virtual inheritance in a way of preventing multiple "instances" of a given class appearing in an inheritance hierarchy when using multiple inheritances. 

Need for Virtual Base Classes: Consider the situation where we have one class A . This class A is inherited by two other classes B and C. Both these class are inherited into another in a new class D as shown in figure below. 

As we can see from the figure that data members/function of class A are inherited twice to class D. One through class B and second through class C. When any data / function member of class A is accessed by an object of class D, ambiguity arises as to which data/function member would be called? One inherited through B or the other inherited through C. This confuses compiler and it displays error. The C++ Course covers this topic thoroughly, ensuring you grasp how to implement virtual inheritance correctly in your applications.

Example: To show the need of Virtual Base Class in C++ 

Compile Errors:

prog.cpp: In function 'int main()':
prog.cpp:29:9: error: request for member 'show' is ambiguous
object.show();
^
prog.cpp:8:8: note: candidates are: void A::show() void show()
^
prog.cpp:8:8: note: void A::show()

How to resolve this issue?
To resolve this ambiguity when class A is inherited in both class B and class C, it is declared as virtual base class by placing a keyword virtual as : 

Syntax for Virtual Base Classes:

Syntax 1:
class B : virtual public A
{
};
Syntax 2:
class C : public virtual A
{
};

Note:
virtual can be written before or after the public. Now only one copy of data/function member will be copied to class C and class B and class A becomes the virtual base class. Virtual base classes offer a way to save space and avoid ambiguities in class hierarchies that use multiple inheritances. When a base class is specified as a virtual base, it can act as an indirect base more than once without duplication of its data members. A single copy of its data members is shared by all the base classes that use virtual base. 

Example 1

a = 10

Explanation :
The class A has just one data member a which is public. This class is virtually inherited in class B and class C. Now class B and class C use the virtual base class A and no duplication of data member a is done; Classes B and C share a single copy of the members in the virtual base class A.

Example 2: