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. 

Why we 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.

#include <iostream>
using namespace std;

class A {
public:
	int a;
	A() // constructor
	{
		a = 10;
	}
};

class B : public A {
};

class C : public A {
};

class D : public B, public C {
};

int main()
{
	
	D object; // object creation of class d
	cout << "a = " << object.a << endl;

	return 0;
}

When we compile the above program, we will see the following error which is ambiguous of a member function.

g++ /tmp/9QQcpdLW2w.cpp
/tmp/9QQcpdLW2w.cpp: In function 'int main()':
/tmp/9QQcpdLW2w.cpp:26:34: error: request for member 'a' is ambiguous
   26 |         cout << "a = " << object.a << endl;
      |                                  ^
/tmp/9QQcpdLW2w.cpp:6:13: note: candidates are: 'int A::a'
    6 |         int a;
      |             ^
/tmp/9QQcpdLW2w.cpp:6:13: note:                 'int A::a'

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.

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.

#include <iostream>
using namespace std;

class A {
public:
	int a;
	A() // constructor
	{
		a = 50;
	}
};

class B : public virtual A {
};

class C : public virtual A {
};

class D : public B, public C {
};

int main()
{
	D object; // object creation of class d
	cout << "a = " << object.a << endl;

	return 0;
}

Output

a =50