求教一个C++中虚数问题. c++高手请进......

请看下面的代码:
#include <iostream.h>

class a
{
public :
virtual void good() ;

};

class b :public a
{
public :
void good()
{
cout<<"this is b"<<endl ;
};

};
/*
class c:public b
{
public :
void good()
{
cout<<"this good :"<<endl ;
};
};
*/

void main()
{
b *cc ;
cc = new b;
cc->good() ;
delete cc ;
return ;
};

为何编译通过,但运行时却出现以下错误:
Linking...
text2.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall a::good(void)" (?good@a@@UAEXXZ)
Debug/text1.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

text1.exe - 2 error(s), 0 warning(s)

请高手解决一下........