C++ Virtual Functions

A virtual function is a member function in the base class that we expect to redefine in derived classes.

Basically, a virtual function is used in the base class in order to ensure that the function is overridden. This especially applies to cases where a pointer of base class points to an object of a derived class.

For example, consider the code below:

class Base {
   public:
    void print() {
        // code
    }
};

class Derived : public Base {
   public:
    void print() {
        // code
    }
};

Later, if we create a pointer of Base type to point to an object of Derived class and call the print() function, it calls the print() function of the Base class.

In other words, the member function of Base is not overridden.

int main() {
    Derived derived1;
    Base* base1 = &derived1;

    // calls function of Base class
    base1->print();

    return 0;
}

In order to avoid this, we declare the print() function of the Base class as virtual by using the virtual keyword.

class Base {
   public:
    virtual void print() {
        // code
    }
};

Virtual functions are an integral part of polymorphism in C++. To learn more, check our tutorial on C++ Polymorphism.

Example 1: C++ virtual Function

#include <iostream>
using namespace std;

class Base {
   public:
    virtual void print() {
        cout << "Base Function" << endl;
    }
};

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

int main() {
    Derived derived1;

    // pointer of Base type that points to derived1
    Base* base1 = &derived1;

    // calls member function of Derived class
    base1->print();

    return 0;
}

Output

Derived Function

Here, we have declared the print() function of Base as virtual.

So, this function is overridden even when we use a pointer of Base type that points to the Derived object derived1.

C++ override Identifier

C++ 11 has given us a new identifier override that is very useful to avoid bugs while using virtual functions.

This identifier specifies the member functions of the derived classes that override the member function of the base class.

For example,

class Base {
   public:
    virtual void print() {
        // code
    }
};

class Derived : public Base {
   public:
    void print() override {
        // code
    }
};

If we use a function prototype in Derived class and define that function outside of the class, then we use the following code:

class Derived : public Base {
   public:
    // function prototype
    void print() override;
};

// function definition
void Derived::print() {
    // code
}

Use of C++ override

When using virtual functions, it is possible to make mistakes while declaring the member functions of the derived classes.

Using the override identifier prompts the compiler to display error messages when these mistakes are made.

Otherwise, the program will simply compile but the virtual function will not be overridden.

Some of these possible mistakes are:

• Functions with incorrect names: For example, if the virtual function in the base class is named print(), but we accidentally name the overriding function in the derived class as pint().
• Functions with different return types: If the virtual function is, say, of void type but the function in the derived class is of int type.
• Functions with different parameters: If the parameters of the virtual function and the functions in the derived classes don't match.
• No virtual function is declared in the base class.

Use of C++ Virtual Functions

Suppose we have a base class Animal and derived classes Dog and Cat.

Suppose each class has a data member named type. Suppose these variables are initialized through their respective constructors.

class Animal {
   private:
    string type;
    ... .. ...
    public:
      Animal(): type("Animal") {}
    ... .. ...
};

class Dog : public Animal {
   private:
    string type;
    ... .. ...
    public:
      Animal(): type("Dog") {}
    ... .. ...
};

class Cat : public Animal {
   private:
    string type;
      ... .. ...
    public:
      Animal(): type("Cat") {}
    ... .. ...
};

Now, let us suppose that our program requires us to create two public functions for each class:

1. getType() to return the value of type
2. print() to print the value of type

We could create both these functions in each class separately and override them, which will be long and tedious.

Or we could make getType() virtual in the Animal class, then create a single, separate print() function that accepts a pointer of Animal type as its argument. We can then use this single function to override the virtual function.

class Animal {
    ... .. ...
   public:
    ... .. ...
    virtual string getType {...}
};

... .. ...
... .. ...

void print(Animal* ani) {
    cout << "Animal: " << ani->getType() << endl;
}

This will make the code shorter, cleaner, and less repetitive.

Example 2: C++ virtual Function Demonstration

// C++ program to demonstrate the use of virtual function

#include <iostream>
#include <string>
using namespace std;

class Animal {
   private:
    string type;

   public:
    // constructor to initialize type
    Animal() : type("Animal") {}

    // declare virtual function
    virtual string getType() {
        return type;
    }
};

class Dog : public Animal {
   private:
    string type;

   public:
    // constructor to initialize type
    Dog() : type("Dog") {}

    string getType() override {
        return type;
    }
};

class Cat : public Animal {
   private:
    string type;

   public:
    // constructor to initialize type
    Cat() : type("Cat") {}

    string getType() override {
        return type;
    }
};

void print(Animal* ani) {
    cout << "Animal: " << ani->getType() << endl;
}

int main() {
    Animal* animal1 = new Animal();
    Animal* dog1 = new Dog();
    Animal* cat1 = new Cat();

    print(animal1);
    print(dog1);
    print(cat1);

    return 0;
}

Output

Animal: Animal
Animal: Dog
Animal: Cat

Here, we have used the virtual function getType() and an Animal pointer ani in order to avoid repeating the print() function in every class.

void print(Animal* ani) {
    cout << "Animal: " << ani->getType() << endl;
}

In main(), we have created 3 Animal pointers to dynamically create objects of Animal, Dog and Cat classes.

// dynamically create objects using Animal pointers
Animal* animal1 = new Animal();
Animal* dog1 = new Dog();
Animal* cat1 = new Cat();

We then call the print() function using these pointers:

1. When print(animal1) is called, the pointer points to an Animal object. So, the virtual function in Animal class is executed inside of print().
2. When print(dog1) is called, the pointer points to a Dog object. So, the virtual function is overridden and the function of Dog is executed inside of print().
3. When print(cat1) is called, the pointer points to a Cat object. So, the virtual function is overridden and the function of Cat is executed inside of print().