m_pMainWnd is the public member of CWinThread. This is mainly used when dealing with a UI thread and there is a top level main window. Every MFC application should have a main application thread. This main thread is implemented by CWinApp which is derived from CWinThread. Every application should create a main overlapped window and assign this main window pointer to m_pMainWnd. m_pMainWnd is the member to pass main window pointer from application to MFC framework which is used in CWinApp or CWinThread.

Source Code

#include <windows.h>

#include <afxwin.h>

class CMyWnd : public CFrameWnd
      Create(NULL, _T("My MFC Window"));
    afx_msg void CMyWnd::OnClose();


afx_msg void CMyWnd::OnClose()


class CMyApp : public  CWinApp 

     virtual BOOL InitInstance()
      m_pMainWnd = new CMyWnd();
      return TRUE;
CMyApp app;

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Further readings

Where is WinMain() function in MFC application ?

MFC hides WinMain in its framework and includes source file on WinMain(). This explains how framework calls global CWinApp::Initinstance() from entry WinMain.

What is the utility of CWinApp class?

This is constructed during global C++ objects are constructed and is already available when Windows calls the WinMain function, which is supplied by the ...

Basic steps in Win32 GUI Application with source code.

Define a custom Window class structure, Register the class name, CreateWindow, Show windows and write message get and dispatch loop statements. Define the Window CallBack procedure and write the handlers.

What is a Window CallBack procedure and what is its utility?

DispatchMessage() is a API which indirectly triggers the Window CallBack procedure. Message structure members from this function are passed to the CallBack procedure. CallBack procedure should implement event handlers depending on the need of the application.

What are LPARAM and WPARAM in window proc function?

LPARAM and WPARAM are the two parameters in Window CallBack procedure. They signifies parameters of various events. They are used in handing individual events.

What are the basic steps of a typical MFC based application?

We need to write WinMain and need to follow all these in a Win32 application. However we need not to write much if we are writing an application with MFC ...