Class` member changes unwillingly somehow

I got the two following CDialog-class created for creating dialog windows from templates and a class CMainDialog that is dereived from CDialog and has some methods do manipulate controls on the dialogs.

class CDialog
    {
    public:
        CDialog(DWORD dwTemplate) : m_dwTemplateID(dwTemplate), m_hWnd(NULL) {};
        virtual ~CDialog() {};

        static INT_PTR CALLBACK DialogProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

        virtual BOOL Create(HINSTANCE hInstance, HWND hParent = NULL);

        BOOL Show(BOOL bShow);
    private:
        DWORD m_dwTemplateID;

    protected:
        HWND m_hWnd;
        virtual INT_PTR HandleMessages(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
    };

//Base Dialog Implementation

#include "BaseDialog.h"

//statics

INT_PTR CALLBACK Inc::CDialog::DialogProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    Inc::CDialog* pDialog = nullptr;

    if(uMsg == WM_INITDIALOG)
    {
        //save address of the CDialog-object into the dialog´s userdata
        pDialog = (Inc::CDialog*)lParam;
        SetWindowLongPtr(hWnd, GWLP_USERDATA, (LONG_PTR)pDialog);
    }
    else
    {
        //get the pointer
        pDialog = (Inc::CDialog*)GetWindowLongPtr(hWnd, GWLP_USERDATA);
    }

    if(pDialog)
    {
        //handle messages
        return pDialog->HandleMessages(hWnd, uMsg, wParam, lParam);
    }

    return FALSE;       //!pDialog
}

INT_PTR Inc::CDialog::HandleMessages(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(uMsg)
    {
    case WM_CLOSE:
            DestroyWindow(m_hWnd);
        return TRUE;
    case WM_DESTROY:
            PostQuitMessage(0);
            return TRUE;
    };

    return FALSE;       //Message not handled => system will take action
}

BOOL Inc::CDialog::Create(HINSTANCE hInstance, HWND hParent)
{
    m_hWnd = CreateDialogParam(hInstance, MAKEINTRESOURCE(m_dwTemplateID), hParent, DialogProc, (LPARAM)this);
    if(m_hWnd == NULL)
        return FALSE;

    return TRUE;
}

//return values: TRUE => window was previously visible, FALSE otherwise
BOOL Inc::CDialog::Show(BOOL bShow)
{
    if(bShow)
        return ShowWindow(m_hWnd, SW_SHOWNORMAL);
    return ShowWindow(m_hWnd, SW_HIDE);
}

//CMainDialog

class CMainDialog :
    开发者_JAVA百科    public Inc::CDialog
    {
    public:
        CMainDialog(DWORD dwTemplateID);
        virtual ~CMainDialog(void);

        virtual INT_PTR HandleMessages(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

    private:
        static const char m_szDates[8][14];

        //Dialog control item
        HWND m_hDateComboBox;
        HWND m_hItemListBox;
        HWND m_hDescriptionEditBox;
        HWND m_hButtonClose;
        HWND m_hButtonSave;
        HWND m_hButtonDelete;

        //get save the control item handles
    public:
        void GetControlHandles();
    public:
        void PopulateDateComboBox();
    };

//CMain Dialog Implementation

#include "MainDialog.h"
#include <WindowsX.h>
#include "resource.h"
#include <stdexcept>

//statics
const char Inc::CMainDialog::m_szDates[8][14] = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", "Single Event"};

Inc::CMainDialog::CMainDialog(DWORD dwTemplateID) : CDialog(dwTemplateID)
{
}


Inc::CMainDialog::~CMainDialog(void)
{
}

//adds the entries in m_szDates to the Combo Box for choosing the date
void Inc::CMainDialog::PopulateDateComboBox()
{
    for(unsigned short s = 0; s < 8; s++)
    {
        if(ComboBox_AddString(m_hDateComboBox, m_szDates[s]) <= CB_ERR)
            throw(std::runtime_error("ComboBox_AddString() failed"));
    }
}

INT_PTR Inc::CMainDialog::HandleMessages(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(uMsg)
    {
    case WM_INITDIALOG:
            this->GetControlHandles();    // It happens here !!!!!!! <====
            this->PopulateDateComboBox();
        return TRUE;
    }

    return Inc::CDialog::HandleMessages(hWnd, uMsg, wParam, lParam);        //if message isnt handled here, default handling
}

void Inc::CMainDialog::GetControlHandles()
{
    //Get Control window handles
    m_hDateComboBox = GetDlgItem(m_hWnd, IDC_COMBO_DAY);
    m_hItemListBox = GetDlgItem(m_hWnd, IDC_LIST);
    m_hDescriptionEditBox = GetDlgItem(m_hWnd, IDC_EDIT_DESCRIPTION);
    m_hButtonClose = GetDlgItem(m_hWnd, IDC_BUTTON_CLOSE);
    m_hButtonSave = GetDlgItem(m_hWnd, IDC_BUTTON_SAVE_CHANGE);
    m_hButtonDelete = GetDlgItem(m_hWnd, IDC_BUTTON_DELETE_ITEM);
}

The problem that I do encounter is, that in the CMainDialog::HandleMessages() member function when the WM_INITDIALOG should be handled the m_hWnd member is NULL even tho the CDialog::Create() function did succeed and return the window handle to m_hWnd.

The CDialog::DialogProc-procedure seems to work in the way, that it gets the right address from the WM_INITDIALOG´s LPARAM and let the pDialog-pointer point to the right object and call it´s member functions.

Maybe You see what I did miss or where I did wrong.

Thank You in advance

your WM_INITDIALOG message is processed during the call to CreateDialog ,while the assignment to m_hWnd (the result of CreateDialog) is done after CreateDialog ends.

Solution : pass hWnd to GetControlHandle( HWND hWnd ) or set m_hWnd in WM_INITDIALOG of the Base class.

The fix for this is to add pDialog->m_hWnd = hWnd; in the handling of WM_INITDIALOG:

if(uMsg == WM_INITDIALOG)
{
    //save address of the CDialog-object into the dialog´s userdata
    pDialog = (Inc::CDialog*)lParam;
    SetWindowLongPtr(hWnd, GWLP_USERDATA, (LONG_PTR)pDialog);
    pDialog->m_hWnd = hWnd;
}

This ensures m_hWnd is set before any call to HandleMessage