c++ - Window won't open - Stack Overflow

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I am writing some code using the Windows API to open a window. But, for some reason, the window won't open. I tried to use breakpoints to check the code line by line, but even breakpoints aren't working. When checking the output window, it says:

The program exited with code 0x1

I tried to use the MessageBox() function to gather some further information, but even MessageBox() isn't showing. The program instantly closes.

#include "app_core.h"

LRESULT CALLBACK _Window_Procedure_(HWND, UINT, WPARAM, LPARAM);

_App_Core_::_App_Core_(const std::string_view &winName) 
    : _My_Name_(winName), _Core_Hwnd_(nullptr), _Core_Hinstance_(GetModuleHandle(NULL)) 
{
    // _Class_Name_
    std::wstring _Class_Name_ = _Convert_Char_To_Wchar_(_My_Name_.data(), TRUE, "_Class_");

    // _Window_Class_
    WNDCLASSEX _Window_Class_Ex_ = {};
    {
        _Window_Class_Ex_.cbSize        = sizeof(WNDCLASSEX);
        _Window_Class_Ex_.hInstance     = _Core_Hinstance_;
        _Window_Class_Ex_.lpszClassName = _Class_Name_.data();
        _Window_Class_Ex_.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
        _Window_Class_Ex_.hCursor       = LoadCursor(NULL, IDC_ARROW);
        _Window_Class_Ex_.hIcon         = LoadIcon(NULL, IDI_WINLOGO);
        _Window_Class_Ex_.lpfnWndProc   = _Window_Procedure_;
        _Window_Class_Ex_.lpszMenuName  = 0;
        _Window_Class_Ex_.style         = 0;
    }

    // _Register_Class_Ex_
    RegisterClassEx(&_Window_Class_Ex_);

    // _Create_Window_Ex_
    _Core_Hwnd_ = CreateWindowEx(
        0,
        _Class_Name_.data(),
        _Convert_Char_To_Wchar_(_My_Name_.data(), false, "").c_str(),
        WS_OVERLAPPED | WS_OVERLAPPEDWINDOW,
        0, 0, 480, 360,
        NULL, NULL, _Core_Hinstance_, NULL
    );

    // _Show_Window_Update_Window_
    _Show_Window_Update_Window_(_Core_Hwnd_);
    MessageBoxA(NULL, "A", "A", MB_OK); // Debug: Doesn't work (Window closes before showing it).

    // _Get_Message_
    for (MSG _Window_Message_ = {}; GetMessage(&_Window_Message_, NULL, 0, 0) > 0;) 
    {
        DispatchMessage(&_Window_Message_);
        TranslateMessage(&_Window_Message_);
    }
}

HWND _App_Core_::_Parent_() const
{
    // _Core_Hwnd_
    return _Core_Hwnd_;
}

VOID _App_Core_::_Destroy_()
{
    // _Destroy_Window_
    if (_Core_Hwnd_)
        DestroyWindow(_Core_Hwnd_);

    _Core_Hwnd_ = nullptr;
    _Core_Hinstance_ = nullptr;
}

LRESULT CALLBACK _Window_Procedure_(
    HWND _Core_Hwnd_, 
    UINT _Un_Msg_, 
    WPARAM _Wide_Param_, 
    LPARAM _L_Wide_Param_) 
{
    // _Switch_Un_Msg_
    switch (_Un_Msg_) 
    {
        // _Wm_Destroy_
        case 0x0002: // WM_DESTROY
            PostQuitMessage(0);
            return 0;

        case 0x000F: // WM_PAINT
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(_Core_Hwnd_, &ps);

            // All painting occurs here, between BeginPaint and EndPaint.
            FillRect(hdc, &ps.rcPaint, (HBRUSH)(COLOR_WINDOW + 1));

            EndPaint(_Core_Hwnd_, &ps);
        }
        return 0;
    }
    return DefWindowProc(_Core_Hwnd_, _Un_Msg_, _Wide_Param_, _L_Wide_Param_);
}

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