A virtual function is a member function in the base class that can be overridden in derived classes.
Virtual functions are a key part of polymorphism in C++. They let different objects respond differently to the same function call.
Why Use Virtual Functions?
Without virtual , C++ decides which function to call based on the pointer type, not the actual object type.
With virtual , it checks the actual object the pointer is pointing to.
- Without virtual : the base function runs, even if the object is from a child class.
- With virtual : the child's version runs, like you expect.
Without Virtual Function
Example (No virtual keyword)
class Animal {
public: void sound() {
cout << "Animal sound\n";
}
};
class Dog : public Animal {
public: void sound() {
cout << "Dog barks\n";
}
};
int main() {
Animal* a; // Declare a pointer to the base class (Animal)
Dog d; // Create an object of the derived class (Dog)
a = &d; // Point the base class pointer to the Dog
object
a->sound(); // Call the sound() function using the pointer. Since sound() is
not virtual, this calls Animal's version return 0; }Even though a points to a Dog , it still calls Animal::sound() because the function is not virtual.
With Virtual Function
Example (With virtual keyword)
class Animal {
public: virtual
void sound() {
cout << "Animal sound\n";
}
};
class Dog : public Animal {
public: void sound() override {
cout << "Dog barks\n";
}
};
int main() {
Animal* a;
Dog d;
a = &d;
a->sound(); // Outputs: Dog barks
return 0;
}Now it works! Because sound() is virtual , the call uses the actual object's function and not just the pointer type.
- Use virtual only in the base class
- Use override (optional, but recommended) in the derived class for clarity
The -> Operator in C++
You might be wondering why we used -> in the examples above.
The -> operator is used to access members (like functions or variables) through a pointer .
It's a shortcut for writing (*pointer).member :
Animal* a = new Animal();
a->sound(); // Same as (*a).sound();Tip
If you are using a pointer to an object, use -> to access its members.