function ptr using class currently being defined.

I'm having problems getting two classes to get along. Basically each one needs the other in their headers. I am using #pragma once to avoid the natural redefinition errors, but that's not the problem (or at least I don't think it is). (The actual code is fairly large, I'm doing my own GUI library as a learning exercise.) The basic idea is:

#pragma once
#include "B.h"

typedef void (*FUNPTR)(B*);

class A
{
private: 
     FUNPTR p;
};
-------
#pragma once
#include "A.h"

class B
{
public:
    /* stuff */
    A an_a;
};

The error I get is "C2460: 'A::FUNPTR' : uses 'A' which is being defined." which doesn't make any sense. Can someone help me with this? Thanks in advance :)