Nested definition problem for C + + classes--two classes refer to each other

A.h file
Class A
{
Public:void MethodA ();
...
};

B.h file
Class B
{
Public:void MethodB ();
...
}

It is now in Class A to invoke the method in Class B, and in Class B there is a method to invoke in Class A.

Failed methods tested:

Method 1:

A.h file
#include "B.h"
Class B;
Class A
{
Private:b M_CLASSB;
Public:void MethodA ();
...
};

B.h file:
#include "A.h"
Class B
{
Private:a M_classa;
Public:void MethodB ();
...
};
Leads to a dead loop.
Probable cause: When constructing A, B should be constructed first, and B must be constructed first to cause a dead loop;

Method 2:

A.h file
#include "B.h"
Class B;
Class A
{
Private:b *M_PCLASSB;
Public:void MethodA ();
...
};

B.h file:
#include "A.h"
Class B
{
Private:a *m_pclassa;
Public:void MethodB ();
...
};

Then add the Class A constructor in A.cpp, respectively: M_PCLASSB = new class B;
In the Class B constructor in B.cpp, add: M_pclassa = new class A;
Possible reasons: Same Method 1.

Method 3:

A.h file
#include "B.h"
Class B;
Class A
{
Friend class B;
Private:b *M_PCLASSB;
Public:void MethodA ();
...
};

B.h file:
#include "A.h"
Class B
{
Private:a *m_pclassa;
Public:void MethodB ();
...
};
Then add the Class A constructor in A.cpp, respectively: M_PCLASSB = new class B;
In the Class B constructor in B.cpp, add: M_pclassa = new class A;
The results and causes are the same.

The right approach:

A.h file
#include "B.h"
Class B;
Class A
{
Friend class B;
Private:b *M_PCLASSB;
Public:void MethodA ();
...
};

B.h file:
#include "A.h"
class B
{
public:a *m_pclassa;
public:void MethodB ();
...
}; The
is then added to the Class A constructor in A.cpp: M_PCLASSB = new Class B;m_pclassb->m_pclassa = this;
Add in Class B constructors in B.cpp: M_pclassa = new class A;  

Message.h:interface for the "message class."
//////////////////////////////////////////////////////////////////////

#if!defined (AFX_MESSAGE_H__ Da2e5287_a04c_445c_82f5_c685f492613a__included_)
#define Afx_message_h__da2e5287_a04c_445c_82f5_ C685f492613a__included_

#if _msc_ver > 1000
#pragma once
#endif//_msc_ver > 1000

#include " Folder.h "

class folder;

Class message  
{public
:
	void Remove (Folder &folder);
	void Save (Folder &folder);
	Message (const-message &mess);
	message& operator = (const message &mess);
	Message (const string &STR = ""): Content (str) {};
	Virtual ~message ();

Private:
	string content;
	Set<folder *> folders;
	void Put_msg_in_folders (const set<folder *> &folds);
	void Rem_msg_in_folders ();

#endif//!defined (AFX_MESSAGE_H__DA2E5287_A04C_445C_82F5_C685F492613A__INCLUDED_)

Message.cpp:implementation of the message class. #include "stdafx.h" #include "Message.h"/ Construction/destruction///////////////

Message::~message () {rem_msg_in_folders ();} Message::message (const message &mess): Content (mess.content), folders (mess.folders) {put_msg_in_folders (folders
);

		} message& message::operator = (Const message &mess) {if (this!= &mess) {rem_msg_in_folders ();
		Content = Mess.content;
		folders = Mess.folders;
	Put_msg_in_folders (Folders);
return *this;
	} void Message::save (Folder &folder) {Folders.insert (&folder);
Folder.addmsg (this);
	} void Message::remove (Folder &folder) {folders.erase (&folder);
Folder.remmsg (this); } void Message::P ut_msg_in_folders (const set<folder *> &folds) {for (Set<folder *>::const_iterator it = Folds.begin (); it!= folds.end (); ++it)
	{(*it)->addmsg (this); } void Message::rem_msg_in_folders () {for (Set<folder *>::const_iterator it = Folders.begin (); it!= Folders.en D ();
	++it) {(*it)->remmsg (this);
 }
}

Folder.h:interface for the Folder class.
//////////////////////////////////////////////////////////////////////

#if!defined (AFX_FOLDER_H__ A34ce0d3_ddd3_4c9f_b97c_c872ac65ecea__included_)
#define Afx_folder_h__a34ce0d3_ddd3_4c9f_b97c_c872ac65ecea __included_

#if _msc_ver > 1000
#pragma once
#endif//_msc_ver > 1000

//#include "Message.h"

class message;

Class Folder  
{
	friend class message;
Public:
	Folder ();
	Virtual ~folder ();
Private:
	void remmsg (message *m);
	void addmsg (Message *m);
	Set<message *> messages;

#endif//!defined (AFX_FOLDER_H__A34CE0D3_DDD3_4C9F_B97C_C872AC65ECEA__INCLUDED_)

Folder.cpp:implementation of the Folder class.
//////////////////////////////////////////////////////////////////////

#include "stdafx.h"
# Include "Folder.h"

//////////////////////////////////////////////////////////////////////
// Construction/destruction
//////////////////////////////////////////////////////////////////////

Folder :: Folder ()
{

}

Folder::~folder ()
{

}

void folder::addmsg (message *m)
{
	Messages.insert (m);
}

void folder::remmsg (Message *m)
{
	messages.erase (m);
}