多态的实现

#include<iostream>
#include<cmath>
using namespace std;
class Shape
{
public:
virtual void draw()=0;
};
class Rectangle:public Shape
{
public :
 void draw()
 {
  cout<<"这是个矩形"<<endl;
 }
};
class Tangle:public Shape
{
public:
 void draw()
 {
  cout<<"这是个三角形"<<endl;
 }
};
class Circle:public Shape
{
public:
 void draw()
 {
  cout<<"这是个圆形"<<endl;
 }
};
Shape *createpicture()
{
 int i;
 i=rand()%3;
 switch(i)
 {
 case 0:
  return new Rectangle;
 case 1:
  return new Tangle;
 default:
  return new Circle;
 }
}
void main()
{
 int j;
 
 Shape *s;

 for(j=0;j<100;j++)
 { 
  s=createpicture();
  s->draw();
 }

}