Polygon abstract class with Area()

A class with at least one pure virtual function is an abstract class; objects of it cannot be created. Polygon declares Area() as pure virtual, and Rectangle and Triangle override it.

#include <iostream>
using namespace std;

class Polygon {
protected:
    float Length, Height;
public:
    Polygon(float l, float h) : Length(l), Height(h) {}
    virtual float Area() = 0;          // pure virtual function
    virtual ~Polygon() {}
};

class Rectangle : public Polygon {
public:
    Rectangle(float l, float h) : Polygon(l, h) {}
    float Area() { return Length * Height; }          // l x h
};

class Triangle : public Polygon {
public:
    Triangle(float l, float h) : Polygon(l, h) {}
    float Area() { return 0.5f * Length * Height; }   // 1/2 x b x h
};

int main() {
    Polygon *p;                 // base pointer, no Polygon object
    Rectangle r(10, 5);
    Triangle t(10, 5);

    p = &r;
    cout << "Area of rectangle = " << p->Area() << endl;
    p = &t;
    cout << "Area of triangle  = " << p->Area() << endl;
    return 0;
}

Output

Area of rectangle = 50
Area of triangle  = 25

The call p->Area() is resolved at run time through the virtual table, so the area function of the actual object is executed.