Dialog wrapper class attempt

/* ------- Dialog.h ------- */
#pragma once

#ifndef DIALOG_H_
#define DIALOG_H_

#define WIN32_LEAN_AND_MEAN
#include <Windows.h>
#include <map>
using namespace std;

// All dialogs share this same procedure. This method actually
// uses 'hWnd' to search for the correct member procedure and returns it.
class Dialog;
extern map<HWND,Dialog*> g_DlgProcs;
INT_PTR CALLBACK g_MainDlgProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);

class Dialog
{
private:
    int _id;
    HWND _handle, _parent;

    // Make non-copyable
    Dialog(const Dialog& other);
    Dialog& operator= (const Dialog&);

    // Dialog procedure
    INT_PTR CALLBACK cb_procedure(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
protected:
    Dialog(int id, HWND parent = HWND_DESKTOP);
public:
    INT_PTR CALLBACK cb_proc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);

    // Properties
    int ID(void);
    HWND Handle(void);
    HWND Parent(void);

    // Methods
    LRESULT Send(UINT msg, WPARAM wParam = 0, LPARAM lParam = 0);
    void Show(void);
    void Show(int nCmdShow);
    void Hide(void);
    void Destroy(void);

    // Events
    virtual bool ProcessMsg(UINT msg, WPARAM wParam, LPARAM lParam);
    virtual void OnClose(void);
    virtual void OnDestroy(void);
    virtual void OnInitialize(void);
    virtual void OnMenuCommand(bool isAccel, int id);
    virtual void OnCtrlCommand(int code, int id, HWND from);
    virtual void OnNotification(int code, int id, HWND from);
};

#endif // DIALOG_H_

/* ------- Dialog.cpp -------- */
#include "Dialog.h"
#include "Macros.h" // Just for Alert(string msg) messagebox macro

map<HWND,Dialog*> g_DlgProcs;

INT_PTR CALLBACK g_MainDlgProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) {
    if (g_DlgProcs.find(hWnd) != g_DlgProcs.end()) {
        Alert("blah"); // Gets executed repeatedly
        return g_DlgProcs[hWnd]->cb_proc(hWnd, msg, wParam, lParam);
    } else {
        Alert("blah"); // Removing the above alert, this gets
                       // executed repeatedly, erm, as well.. O.o strange
        return FALSE;
    }
}

Dialog::Dialog(int id, HWND parent /* = HWND_DESKTOP */) {
    _id = id;
    _parent = parent;

    // Tried this before CreateDialogParam
    g_DlgProcs.insert(make_pair(_handle, this));

    _handle = CreateDialogParam(
        (HINSTANCE)GetModuleHandle(NULL),
        MAKEINTRESOURCE(id), _parent,
        (DLGPROC)g_MainDlgProc, NULL
    );

    // Then tried it after CreateDialogParam
    g_DlgProcs.insert(make_pair(_handle, this));
}

INT_PTR CALLBACK Dialog::cb_proc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) {
    Alert("blah"); // Never gets executed

    bool handled = true;

    switch (msg)
    {
    case WM_INITDIALOG:
        OnInitialize();
        break;
    case WM_COMMAND:
        if (HIWORD(wParam) == 0 || HIWORD(wParam) == 1) {
            OnMenuCommand((HIWORD(wParam) == 1), (int)LOWORD(wParam));
        } else {
            OnCtrlCommand((int)HIWORD(wParam), (int)LOWORD(wParam), (HWND)lParam);
        }
        break;
    case WM_NOTIFY:
        {
            LPNMHDR head = (LPNMHDR)lParam;
            OnNotification(head->code, head->idFrom, head->hwndFrom);
        }
        break;
    case WM_CLOSE:
        OnClose(); // DestroyWindow(_handle)
        break;
    case WM_DESTROY:
        OnDestroy(); // PostQuitMessage(0)
    default:
        handled = ProcessMsg(msg, wParam, lParam);
    }

    // Convert bool to Windows BOOL enum
    return ((handled == true) ? TRUE : FALSE);
}