C++ Code for Function Overriding Using Virtual Keyword

Example

Function Overriding in OOP occurs when a derived class has its own implementation of a function which already exists in the base class.

The base class virtual function should be declared and redefined in the derived class.

This also allows Runtime Polymorphism where the relevant function is chosen at run time based on the type of object used.

#include <iostream>
using namespace std;
class Animal {
public:
    virtual void sound() {   // Virtual function
        cout << "Animal makes a sound" << endl;
    }
};
class Dog : public Animal {
public:
    void sound() override {   // Overriding base class function
        cout << "Dog barks" << endl;
    }
};
class Cat : public Animal {
public:
    void sound() override {
        cout << "Cat meows" << endl;
    }
};
int main() {
    Animal* a1;
    Dog d;
    Cat c;
    a1 = &d;
    a1->sound();   // Calls Dog's sound()
    a1 = &c;
    a1->sound();   // Calls Cat's sound()
    return 0;
}

Output

Dog barks
Cat meows

Explanation

1. Animal is a base class that includes virtual sound function.

2. Derived classes Dog and Cat use the same function by using virtual keyword.

3. Our base class pointer (Animal* a1) is pointed to both Dog and Cat objects in main.

4. Due to the virtual nature of this function, the call a1->sound() is not resolvable at compile-time (but only at runtime).

5. When a1 says to the Dog what it sounds like, it calls to the Dog what it sounds like.

   • Instead, when a1 points at a Cat, it calls the sound () of Cat.

   • When a1 is pointing at a Cat, then it calls the sound() of Cat.

This demonstrates Function Overriding in C++ as a form of Runtime Polymorphism.

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