pass variable value from one dialogbox to other/ one class to other class in vc++ MFC

[GCDEF]

comletely wrong unless you mean static variables
Instead of trying to acces private variables of another class
better try to obtain current object of that class and invoke it's(public) functons

//CClass.cpp
...

//Main.cpp global scope ,CClass.h included
CClass cl;
...

//CClassZ.cpp
#include "CClass.h"
extern CClass cl;

CClassZ::Somestuff()
{
cl.whatsoever
}

BUT you cant do it this way with MFC dialogs(coz CDialog-derived constructor does need intra-instance stuff and fails at global scope)
so..
//CDialClass.cpp
...

//Main.cpp global scope ,CDialClass.h included
HWND hd;

CDialApp::InitInstance()
{
CDialClass cl;
...
//after cl window is created somehow
hd=cl.m_hWnd;
//you can do this also inside CDialClass ithelf with hd=this->m_hWnd;
}
...

//CDialClassZ.cpp
#include "CDialClass.h"
extern HWND hd;

CClassZ::Somestuff()
{
//hard
((CDialClass *)CDialog::FromHandle(hd))->whatsoever;
}

The more you add the worse it gets. I really can't even tell what you're trying to say here, how it helps the OP, or how it's an improvement over the previous answers.