13. Exercise: Virtual Functions

Virtual Functions

Virtual functions are a polymorphic feature. These functions are declared (and possibly defined) in a base class, and can be overridden by derived classes.

This approach declares an interface at the base level, but delegates the implementation of the interface to the derived classes.

In this exercise, class Shape is the base class. Geometrical shapes possess both an area and a perimeter. Area() and Perimeter() should be virtual functions of the base class interface. Append = 0 to each of these functions in order to declare them to be "pure" virtual functions.

A pure virtual function is a virtual function that the base class declares but does not define.

A pure virtual function has the side effect of making its class abstract. This means that the class cannot be instantiated. Instead, only classes that derive from the abstract class and override the pure virtual function can be instantiated.

class Shape {
    public:
        Shape() {}
        virtual double Area() const = 0;
        virtual double Perimeter() const = 0;
};

Virtual functions can be defined by derived classes, but this is not required. However, if we mark the virtual function with = 0 in the base class, then we are declaring the function to be a pure virtual function. This means that the base class does not define this function. A derived class must define this function, or else the derived class will be abstract.

Instructions

1. Create base class called Shape.
2. Define pure virtual functions (= 0) for the base class.
3. Write the derived classes.

• Inherit from class Shape.
• Override the pure virtual functions from the base class.

1. Test in main()