why this program can be compiled, but cannot be linked?

[Andy]

Hi,

I just write a small  program on nested class. It can be compiled, but
cannot be linked. How shall I solve this problem? The program is as
follows:

#include <iostream>
#include <string>
using namespace std;

class A{
public:
  static string str;

  class B{
    public:
       void method(){  str = "assigned in class B"; }
  };

  class B b;
};

int main()
{
  A a;
  a.b.method();
  cout<<"a.str = "<<A::str<<endl;
}

Thanks a lot!

Andy