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Home Forums What is virtual function and why is it used? with example. – MindStick
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1 Answers

Anubhav Sharma
Anubhav Sharma 06 Nov 2025
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A virtual function is a function in a base class that can be overridden in derived classes.
It allows runtime polymorphism — meaning the function that gets called is determined at runtime based on the type of object being referred to, not the type of pointer or reference.

Why it is used

Virtual functions are used to:

  • Achieve runtime polymorphism (dynamic binding).
  • Allow derived classes to provide their own implementation of a base class method.
  • Make code flexible and extensible — you can add new derived types without changing existing code.

Syntax

In C++:

class Base {
public:
    virtual void show() {    // virtual function
        cout << "Base class show function" << endl;
    }
};

class Derived : public Base {
public:
    void show() override {   // override keyword is optional
        cout << "Derived class show function" << endl;
    }
};

Example — Without and With Virtual Function

Without virtual function:

#include <iostream>
using namespace std;

class Base {
public:
    void show() {  // Not virtual
        cout << "Base class show" << endl;
    }
};

class Derived : public Base {
public:
    void show() {
        cout << "Derived class show" << endl;
    }
};

int main() {
    Base* ptr;
    Derived obj;
    ptr = &obj;

    ptr->show();  // Calls Base::show() because it's not virtual
    return 0;
}

Output:

Base class show

With virtual function:

#include <iostream>
using namespace std;

class Base {
public:
    virtual void show() {  // Virtual function
        cout << "Base class show" << endl;
    }
};

class Derived : public Base {
public:
    void show() override {
        cout << "Derived class show" << endl;
    }
};

int main() {
    Base* ptr;
    Derived obj;
    ptr = &obj;

    ptr->show();  // Calls Derived::show() because of virtual function
    return 0;
}

Output:

Derived class show

Key Points:

  • Virtual functions must be declared in the base class using the virtual keyword.
  • They enable late binding (runtime decision).
  • If a class has at least one virtual function, it should have a virtual destructor to ensure proper cleanup.
  • The override keyword (C++11 onward) ensures that the function actually overrides a base class virtual function.
  • Virtual functions are implemented internally using a vtable (virtual table).
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