Cannot create main window?

Question

I tried to make a simple MFC GUI application with C++, but it seems to have problem at generating the main window(dialog?). When I'm trying to compile the code, following message shows :

Unhandled exception at 0x00E7A9DC in GUI_Employee_0501.exe: 0xC0000005: Access violation reading location 0xFEFEFF66.

and the break point stops inside winmain.cpp, at pThread->m_pMainWnd->DestroyWindow();. The value of pThread->m_pMainWnd is NULL, which I suspect as the cause of the problem.

Can you specify what is the problem here? I have a sample code and that is very similar to mine but that works, so I truly have no idea what is happening!

#include <afxwin.h>
#include "resource.h"
#include <iostream>
using namespace std;

#pragma comment(linker, "/entry:WinMainCRTStartup /subsystem:console")

CEdit *pFNAME;
CEdit *pLNAME;
CEdit *pSALARY;
CEdit *pDDAY;
CEdit *pMMONTH;
CEdit *pYYEAR;

CComboBox *pGENDER;

CButton *pADD;
CButton *pDELETE;
CButton *pSAVE;
CButton *pLOAD;

class ENTRY_FORM : public CDialog
{
public:
    ENTRY_FORM(CWnd* pParent = NULL) : CDialog(ENTRY_FORM::IDD, pParent) { }
    enum { IDD = dialog_main };
protected:
    virtual void DoDataExchange(CDataExchange* pDX) { CDialog::DoDataExchange(pDX); }

    virtual BOOL OnInitDialog()
    {
        CDialog::OnInitDialog();
        SetUpInterfacePointers();
        return true;
    }

    void SetUpInterfacePointers(){ ... does its own thing ... }

public:
    afx_msg void add() { PRESS_ADD(); }

    void PRESS_ADD() { ... does its own thing ... }

    DECLARE_MESSAGE_MAP()
};

class Program : public CWinApp
{
public:
    Program(){ }

public:
    virtual BOOL InitInstance()
    {
        CWinApp::InitInstance();
        cout << "CWinAPP:initInstance" << endl;
        ENTRY_FORM dlg;
        m_pMainWnd = &dlg;                  
        cout << "mpMainWnd" << endl;
        INT_PTR nResponse = dlg.DoModal();  
        return FALSE;
    }
};

//
BEGIN_MESSAGE_MAP(ENTRY_FORM, CDialog)
    ON_COMMAND(button_add, add)
END_MESSAGE_MAP()
//

Program theApp;

Answer 1

Change in InitInstance():

return FALSE;

to

return TRUE;

This is because CWinApp::InitInstance should return FALSE only in case of failure, and TRUE if all initialization went OK. In case of failure, AfxWinMain will try to destroy window pointed by m_pMainWnd, which is not possible (it causes Undefined Behaviour) because you assign to m_pMainWnd a local object (which is destroyed once InitInstance ends).

[edit]

S.B Bae - to investigate it further and find a root cause, you will need to debug place where m_pMainWnd should be set to NULL once your dialog ends. This should be in CWnd::OnNcDestroy() in wincore.cpp. There is a line pThread->m_pMainWnd = NULL; which apparently is not being executed in your application.

Answer 2

Late to the party, but I saw the exact problem when creating an MFC Dialog application. I found I had accidentally changed the application dialog Style (in Properties) from 'Popup' to 'Child'. Toggling this can consistently toggle this error. 'Overlapped' seems to work just as well as 'Popup'.

https://docs.microsoft.com/en-us/cpp/mfc/reference/styles-used-by-mfc?view=vs-2019#window-styles