第十三周任務（三）

#include <iostream>
using namespace std;
const double pi = 3.1415926;
class Shape
{
public:
	virtual double area() const = 0;
};
class Circle:public Shape
{
public:
	Circle(double r):rang(r){}
	~Circle();
	double area() const;
private:
	double rang;
};
class Rectangle :public Shape 
{
public:
	Rectangle(double l,double w):len(l),wid(w){}
	~Rectangle();
	double area() const;
private:
	double len;
	double wid;
};
class Triangle :public Shape
{
public:
	Triangle(double d,double h):di(d),high(h){}
	~Triangle();
	double area() const;
private:
	double di;
	double high;
};
Circle::~Circle(){}
double Circle::area() const
{
	return pi*rang*rang;
}
Rectangle::~Rectangle(){}
double Rectangle::area() const
{
	return len*wid;
}
Triangle::~Triangle(){}
double Triangle::area() const
{
	return 0.5*di*high;
}
int main()  
{  
    Circle c1(12.6), c2(4.9);    //建立Circle類對象c1,c2,參數為圓半徑  
    Rectangle r1(4.5, 8.4), r2(5.0, 2.5);       //建立Rectangle類對象r1,r2,參數為矩形長、寬  
    Triangle t1(4.5, 8.4),t2(3.4, 2.8);    //建立Triangle類對象t1,t2，參數為三角形底邊長與高  
    Shape *pt[6] = {&c1, &c2, &r1, &r2, &t1, &t2}; //定義基類指針數組pt，各元素指向一個派生類對象  
    double areas = 0.0;      //areas為總面積  
    for(int i = 0; i < 6; i ++)  
    {  
        areas = areas + pt[i]->area();  
    }  
    cout << "totol of all areas=" << areas << endl;   //輸出總面積  
    system("pause");  
    return 0;  
}  	
           

感悟：我想問一下那個const，為什麼要加呢？？為麼要定義成純虛函數呢？