10_88.cpp

书记所附的小程序

#include<iostream.h>
const double PI=3.1416;
class shape
{
protected:
	double r;
public:
	shape(double x){r=x;}
	virtual double area()
	{
		cout<<"No area of this class."<<endl;
		return 0.0;
	}
	virtual double perimeter()
	{
		cout<<"No perimeter of this class."<<endl;
		return 0.0;
	}
};
class circle:public shape
{
public:
	circle(double x):shape(x){}
	double area()
	{
		return PI*r*r;
	}
	double perimeter()
	{
		return 2*PI*r;
	}
};
class in_s:public shape
{
public:
	in_s(double x):shape(x){}
	double area()
	{
		return 2*r*r;
	}
	double perimeter()
	{
		return 4*1.414*r;
	}
};
class ex_s:public shape
{
public:
	ex_s(double x):shape(x){}
	double area()
	{
		return 4*r*r;
	}
	double perimeter()
	{
		return 8*r;
	}
	friend double operator+(double d,ex_s &e);
};
void main()
{
	shape *ptr;
	circle c1(11);
	in_s c2(12);
	ex_s c3(13);
	ptr=&c1;
	cout<<"The circle's area is:";
	cout<<ptr->area();  cout<<endl;
	cout<<"The circle's perimeter is:";
	cout<<ptr->perimeter(); cout<<endl;
	ptr=&c2;
	cout<<"The internal square' area is:";
	cout<<ptr->area(); cout<<endl;
	cout<<"The internal square'perimeter is:";
	cout<<ptr->perimeter();cout<<endl; 
	ptr=&c3;
	cout<<"The external square' area is:";
	cout<<ptr->area();cout<<endl;
	cout<<"The external square'perimeter is:";
	cout<<ptr->perimeter();cout<<endl;
}