Virtual Functions

Virtual functions are functions in a parent class that are meant to be overwritten in the children classes.

Example

#include <iostream> using namespace std; class Vehicle { public: // virtual function virtual double getInsuranceCost(int months) { return 0; } }; class Car : public Vehicle { public: // overwrite of virtual function double getInsuranceCost(int months) { return months * 15; } }; void main() { Car car; cout << "Car's monthly insurance: " << car.getInsuranceCost(12) << " $"; }

Output:

Car's monthly insurance: 180 $

Virtual Functions VS Non-virtual Functions

The difference can be spotted when pointers are used. Virtual functions have late binding. Non virtual functions have early binding.

This can be understood by an example:

#include <iostream> using namespace std; class Figure { public: // virtual function: virtual double getPerimeter() { return 0; } // non virtual function: double getArea() { return 0; } }; class Square : public Figure { private: double side; public: Square(double sideValue) { side = sideValue; } // overwrite of virtual function double getPerimeter() { return 4 * side; } double getArea() { return side * side; } }; void main() { Square *square_1 = new Square(5); cout << "Perimeter = " << square_1->getPerimeter() << endl; cout << "Area = " << square_1->getArea() << endl; cout << "----" << endl; Figure *square_2 = new Square(3); cout << "Perimeter = " << square_2->getPerimeter() << endl; cout << "Area = " << square_2->getArea(); }

Output:

Perimeter = 20 Area = 25 ---- Perimeter = 12 Area = 0

Couple of things to spot:

• Two pointers are used - square_1 with type Square and square_2 with type Figure.
• getPerimeter function is declared as virtual. This means that the code the child class (Square) executes for both pointers.
• getArea function is non-virtual function. So for the pointer square_2 the code from the parent class (Figure) will be executed.