"invalid use of incomplete type" and "forward declaration"

#ifndef _A_
#define _A_

#include "b.h"

class A
{
public:
	B b;
	void test();
};

#endif

#ifndef _B_
#define _B_

class A;

class B
{
public:
	A* parent;
	void test();
};

#endif

#include "b.h"

void B::test()
{
	parent->test();
}

#include <stdio.h>

#include "a.h"

void A::test()
{
	printf("Test.\n");
}

int main()
{
	return 0;
}

g++ test.cpp b.cpp
b.cpp: In member function ‘void B::test()’:
b.cpp:5: error: invalid use of incomplete type ‘struct A’
b.h:4: error: forward declaration of ‘struct A’

void B::test()
{
	//parent->test();
}