Visual C++ Errors C2146, C4430 - visual-c++

Question

Please help to figure out whats wrong with this code.
main.cpp create Object3D which create Box and pass Object3D* pointer to Box.
And there are errors until i remove Object3D declarations from Box.
main.cpp
#include <iostream>
#include "stdafx.h"
#include "Object3D.h"
int _tmain(int argc, _TCHAR* argv[])
{
Object3D obj;
char c;
std::cin >> c;
return 0;
}
Object3D.cpp
#include "Object3D.h"
#include "Box.h"
Object3D::Object3D()
{}
Object3D::~Object3D()
{}
Object3D.h
#ifndef OBJECT3D_H
#define OBJECT3D_H
#include "Box.h"
class Object3D
{
public:
Object3D();
~Object3D();
private:
Box _box_obj; //<<<---ERROR HERE (C2146, C4430)
};
#endif
Box.cpp
#include "Box.h"
#include "Object3D.h"
int Box::Init(Object3D* _obj)
{
obj = _obj;
}
Box::Box()
{}
Box::~Box()
{}
Box.h
#ifndef BOX_H
#define BOX_H
#include "Object3D.h"
class Box
{
public:
Object3D* obj; //<<<---ERROR HERE (C2143, C4430)
int Init(Object3D* _obj); //<<<---ERROR HERE (C2061)
Box();
~Box();
};
#endif

Answer 1

Change Box.h:
#ifndef BOX_H
#define BOX_H
// forward reference possible since the class is not dereferenced here.
class Object3D;
class Box
{
public:
Object3D* obj;
int Init(Object3D* _obj);
Box();
~Box();
};
#endif
The class definition does not use any member of Object3D. Therefore you don't need to know the definition of Object3D but just the fact that it is a class. The forward reference is sufficient and an appropriate tool to resolve a circular dependency.
BTW: The circular reference of object usually includes some "master" objects that own the other objects. It makes sense to change the member name to show the relation rather the type. I would suggest a
Object3D* owner;
when the box owns the obj.