虚函数的使用

#include<iostream.h>
class shape
{
public:
virtual float area()           //将area定义为虚函数
{
return 0.0;
}
};
class tri:public shape
{
public:
tri(float h,float w)
{
H=h;
W=w;
}
float area()
{
return (float)(H*W*0.5);
}
private:
float H,W;
};
class cir:public shape
{
public:
cir(float r)
{
R=r;