形状类族中的纯虚函数

////  main.cpp//  project////  Created by 徐伟 on 5/31/16.//  Copyright © 2016 fizz_i. All rights reserved.//#include <iostream>using namespace std;class Shape{public:    Shape(double L,double W):L(L),W(W){}    virtual double area()const=0;protected:    double L;    double W;    };class Circle:public Shape{public:    Circle(double L=0,double W=0):Shape(L,W){}    double area()const;};double Circle::area()const{    return L*L*3.14;}class Rectangle:public Shape{public:    Rectangle(double L=0,double W=0):Shape(L,W){}    double area()const;    };double Rectangle::area()const{    return L*W;}class Triangle:public Shape{public:    Triangle(double L=0,double W=0):Shape(L,W){}    double area()const;};double Triangle::area()const{    return L*W/2;}int main(){    Circle c1(12.6),c2(4.9);//建立Circle类对象c1,c2,参数为圆半径    Rectangle r1(4.5,8.4),r2(5.0,2.5);//建立Rectangle类对象r1,r2,参数为矩形长、宽    Triangle t1(4.5,8.4),t2(3.4,2.8); //建立Triangle类对象t1,t2，参数为三角形底边长与高    Shape *pt[6]= {&c1,&c2,&r1,&r2,&t1,&t2}; //定义基类指针数组pt，使它每一个元素指向一个派生类对象    double areas=0.0; //areas为总面积    for(int i=0; i<6; i++)    {        areas=areas + pt[i]->area();    }    cout<<"totol of all areas="<<areas<<endl;   //输出总面积    return 0;}