多态应用实例一：几何形体程序

用基类指针数组存放派生类对象的指针地址，是多态的常用应用，遍历该指针数组就能对各个派生类对象做各种操作。

problem

输入不同几何形体及其参数，要求按照面积大小顺序输出其对应的面积，以及集合形体形状。

code

#include <iostream>
#include <math.h>
#include<stdlib.h>
using namespace std;

class CShape{
	public:
		virtual double Area() = 0;//纯虚函数
		virtual void PrintInfo() = 0;
};

class CRectangle: public CShape{
	
	public:
		double l,w;
		double Area(){
			return l*w;
		}
		void PrintInfo(){
			cout<<"Rectangle: "<<Area()<<endl;
		}
};

class CTriangle: public CShape{
	
	public:
		double a,b,c;
		double Area(){
			double p = (a+b+c)/2.0;
			return sqrt(p*(p-a)*(p-b)*(p-c));
		}
		void PrintInfo(){
			cout<<"Triangle: "<<Area()<<endl;
		}
};

class CCircle: public CShape{
	
	public:
		double r;
		double Area(){
			return 3.14159*r*r;
		}
		void PrintInfo(){
			cout<<"Circle: "<<Area()<<endl;
		}
};
int comp(const void *s1,const void *s2){
	CShape **p1;
	CShape **p2;
	p1 = (CShape **)s1;
	p2 = (CShape **)s2;
	double d1,d2;
	d1 =  (*p1)->Area();
	d2 =  (*p2)->Area();
	return d1>d2?1:-1; 
}
int main(){
	CShape *pShapes[100];
	int n;
	cin>>n;
	char c;
	CRectangle *pr; CTriangle *pt; CCircle *pc;
	for(int i=0;i<n;i++){
		cin>>c;
		switch(c){
			case'R':
				pr = new CRectangle();
				cin>>pr->l>>pr->w;
				pShapes[i] = pr;
				break;
			case'T':
				pt = new CTriangle();
				cin>>pt->a>>pt->b>>pt->c;
				pShapes[i] = pt;
				break;
			case'C':
				pc = new CCircle();
				cin>>pc->r;
				pShapes[i] = pc;
				break;
			default: break;
		}
	}
	qsort(pShapes,n,sizeof(CShape *),comp);
	for(int i=0;i<n;i++){
		pShapes[i]->PrintInfo();
	}
	return 0;
}