Virtual base class in C++

14 Mar, 2019
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 is 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.
Example: To show the need of Virtual Base Class in C++
#include <iostream>using namespace std;  class A {public:    void show()    {        cout << "Hello form A \n";    }};  class B : public A {};  class C : public A {};  class D : public B, public C {};  int main(){    D object;    object.show();}
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
#include <iostream>using namespace std;  class A {public:    int a;    A() // constructor    {        a = 10;    }};  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;}
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 becomes virtual base class and no duplication of data member a is done.
Example 2:
#include <iostream>using namespace std;  class A {public:    void show()    {        cout << "Hello from A \n";    }};  class B : public virtual A {};  class C : public virtual A {};  class D : public B, public C {};  int main(){    D object;    object.show();}
Hello from A

gyanendra371
C++-Class and Object
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