Shape.h

#ifndef SHAPE_H
#define SHAPE_H

class Shape
{
public:
	Shape();
	~Shape();
	virtual double calcArea();
};

#endif

Shape.cpp

#include "Shape.h"
#include <iostream>
using namespace std;

Shape::Shape()
{
	cout << "Shape" << endl;
}

Shape::~Shape()
{
	cout << "~Shape" << endl;
}

double Shape::calcArea()
{
	cout << "Shape -> calcArea" << endl;
	return 0;
}
 

Rect.h

#ifndef RECT_H
#define RECT_H

#include "Shape.h"

class Rect : public Shape
{
public:
	Rect(double width, double height);
	~Rect();
	virtual double calcArea();
protected:
	double m_dWidth;
	double m_dHeight;
};

#endif

Rect.cpp

#include "Rect.h"
#include <iostream>
using namespace std;

Rect::Rect(double width, double height) : m_dWidth(width), m_dHeight(height)
{
	cout << "Rect" << endl;
}


Rect::~Rect()
{
	cout << "~Rect" << endl;
}

double Rect::calcArea()
{
	cout << "Rect -> calcArea" << endl;
	return m_dWidth * m_dHeight;
}
 

Circle.h

#ifndef CIRCLE_H
#define CIRCLE_H

#include "Shape.h"

class Circle : public Shape
{
public:
	Circle(double r);
	~Circle();
	virtual double calcArea();
protected:
	double m_dR;
};


#endif

Cirlce.cpp

#include "Circle.h"
#include <iostream>
using namespace std;

Circle::Circle(double r) : m_dR(r)
{
	cout << "Circle" << endl;
}

Circle::~Circle()
{
	cout << "~Circle" << endl;
}

double Circle::calcArea()
{
	cout << "Circle -> calcArea" << endl;
	return 3.14 * m_dR * m_dR;
}
 

demo.cpp

#include <iostream>
using namespace std;

#include "Shape.h"
#include "Rect.h"
#include "Circle.h"

int main()
{
	Shape *shape1 = new Rect(3,6);
	Shape *shape2 = new Circle(5);

	shape1->calcArea();
	shape2->calcArea();

	delete shape1;
	shape1 = NULL;
	delete shape2;
	shape2 = NULL;
	
	system("pause");
	return 0;
}