Virtual Functions

A virtual function is a member function in a base class that can be overridden in a derived class and called dynamically through a base-class pointer or reference.

The keyword is:

virtual

The Problem Without virtual

#include <iostream>
using namespace std;

class Animal {
public:
    void sound() const {
        cout << "Animal sound" << endl;
    }
};

class Dog : public Animal {
public:
    void sound() const {
        cout << "Bark" << endl;
    }
};

int main() {
    Dog dog;
    Animal* animal = &dog;

    animal->sound();

    return 0;
}

Output:

Animal sound

Why? The pointer type is Animal*, so without virtual, the compiler binds the call to Animal::sound().

The Solution With virtual

#include <iostream>
using namespace std;

class Animal {
public:
    virtual ~Animal() = default;

    virtual void sound() const {
        cout << "Animal sound" << endl;
    }
};

class Dog : public Animal {
public:
    void sound() const override {
        cout << "Bark" << endl;
    }
};

int main() {
    Dog dog;
    Animal* animal = &dog;

    animal->sound();

    return 0;
}

Output:

Bark

Why override Is Useful

override tells the compiler:

This function must override a virtual function from the base class.

If the signature is wrong, the compiler gives an error.

This protects against mistakes like:

void sound(); // missing const, so it does not override void sound() const

Rules for Virtual Functions

• Virtual functions are declared in the base class.
• Derived classes override them using the same signature.
• Runtime polymorphism works through base-class pointers or references.
• Constructors cannot be virtual.
• Destructors can be virtual and should be virtual in polymorphic base classes.
• override is optional but strongly recommended.

Viva Answer

A virtual function is a base-class function that supports runtime polymorphism. When it is called through a base-class pointer or reference, C++ checks the actual object type at runtime and calls the overridden derived-class version.

Quick Check

1. What does virtual change in the example?
2. Why does the base pointer call Dog::sound() after adding virtual?
3. Why should we use override?