Virtual Functions in Derived Classes in C++

A virtual function is a member function of a base class that is overridden by a derived class. When you use a pointer or a reference to the base class to refer to a derived class object, you can call a virtual function for that object and have it run the derived class’s version of the function.

In C++, once a member function is declared as a virtual function in a base class, it becomes virtual in every class derived from that base class. In other words, it is not necessary to use the keyword virtual in the derived class while declaring redefined versions of the virtual base class function.

For example, the following program prints “C::fun() called“ as “B::fun()“ becomes virtual automatically.

CPP

// C++ Program to demonstrate Virtual
// functions in derived classes
#include <iostream>
using namespace std;

class A {
public:
    virtual void fun() { cout << "\n A::fun() called "; }
};

class B : public A {
public:
    void fun() { cout << "\n B::fun() called "; }
};

class C : public B {
public:
    void fun() { cout << "\n C::fun() called "; }
};

int main()
{
    // An object of class C
    C c;

    // A pointer of class B pointing
    // to memory location of c
    B* b = &c;

    // this line prints "C::fun() called"
    b->fun();

    getchar(); // to get the next character
    return 0;
}

Output

 C::fun() called 

Note: Never call a virtual function from a CONSTRUCTOR or DESTRUCTOR