虛擬函式

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


class Shape
{
public:
    virtual double area(){ return 0; }
	Shape(double x = 0, double y = 0){ this->x = x; this->y = y; }
protected:
	double x, y;
};

class Circle :virtual public Shape
{
public:
	Circle(double x = 0, double y = 0, double radius = 0) :Shape(x, y){ r = radius; }
	double area(){ return r*r*3.14159; }
private:
	double r;
};

class Rectangle : virtual public Shape
{
public:
	Rectangle(double x = 0, double y = 0, double x1 = 1, double y1 = 1) :Shape(x, y){ this->x1 = x1; this->y1 = y1; }
	double area(){ return (x1 - x)*(y1 - y); }
private:
	double x1, y1;
};

 void main()
{
	 Shape*sp[3] = { new Shape(), new Circle(1, 1, 3), new Rectangle(1, 2, 3, 4) };
	 for (int i = 0; i < 3; i++)
		 cout << sp[i]->area() << endl;
}