C++ Virtual Base Class

The child has two base classes parent1 and parent2 and these two have a common base class grandparent. All the public and protected members of grandparent are inherited into child twice, first from parent1 and again from parent2. This means that child have duplicate sets of members inherited from grandparent. It
causes ambiguity in the program.

 

This kind of problem can be resolved by making the common base class as a virtual base class. It can be done in the following way:

class grandparent
{
    . . . . . .
    . . . . . .
};

class parent1: virtual public grandparent
{
    . . . . . .
    . . . . . .
};


class parent2: public virtual grandparent
{
    . . . . . .
    . . . . . .
};


class child: public parent1, public parent2
{
    . . . . . .
    . . . . . .
};

If a class is made a virtual base class, only one copy of that class is inherited in derived class. Remember one thing that the keyword virtual and public may be used in either order.

Below I have written a program that implements the concept of the virtual base class.

#include<iostream>

using namespace std;

class student
{
    protected: int roll_no;
    
    public: void get_no(int x)
    {
        roll_no=x;
    }
    
    void put_no()
    {
        cout<<"Roll Number:"<<roll_no;
    }
};

class test: virtual public student
{
    protected: float sub_marks;
    
    public: void get_submarks(float y)
    {
        sub_marks=y;
    }
    
    void put_submarks()
    {
        cout<<"\nSubject Marks:"<<sub_marks;
    }
};

class sports: public virtual student
{
    protected: float sp_marks;
    
    public: void get_spmarks(float z)
    {
        sp_marks=z;
    }
    
    void put_spmarks()
    {
        cout<<"\nSports Marks:"<<sp_marks;
    }
};

class result: public test, public sports
{
    float total_marks;
    
    public: void put_result()
    {
        total_marks=sub_marks+sp_marks;
        put_no();
        put_submarks();
        put_spmarks();
        cout<<"\nTotal Marks:"<<total_marks;
    }
};

int main()
{
    result R;
    
    R.get_no(20);
    R.get_submarks(75.6);
    R.get_spmarks(81.2);
    R.put_result();
    return 0;
}

In the above example student class is an abstract class, as it is not used to create an object. It is used only for deriving other classes.

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