Exception in Window Framework Win32 API - c++

Question

I'm new to Win32 API and want to create a window. I have a class (as follows) but I get an exception at the line return p_this->HandleMessages(msg, wParam, lParam);.
Window.h (I got this in some msdn website and modified to some extent):
#include "BaseWin.h"
template<class D_CLASS>
class Window
:public BaseWin
{
public:
int returnValue;
private:
static constexpr LPCSTR className = "Best Window in the UNIVERSE!";
public:
// declare this fn in the inherited class and use as wnd proc
virtual LRESULT HandleMessages(UINT msg, WPARAM wParam, LPARAM lParam) = 0;
static LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
D_CLASS* p_this = nullptr;
if (msg == WM_NCCREATE) {
CREATESTRUCT* create = (CREATESTRUCT*)lParam;
p_this = (D_CLASS*)create->lpCreateParams;
SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR)p_this);
}
else
p_this = (D_CLASS*)GetWindowLongPtr(hwnd, GWLP_USERDATA);
if (p_this)
return p_this->HandleMessages(msg, wParam, lParam); // exception here
else
return DefWindowProc(hwnd, msg, wParam, lParam);
}
Window(HWND parent, LPCSTR title, int id, const Point& pos, const Size& _size,
int styles = WS_OVERLAPPEDWINDOW, int retVal = 0x45, int stylesEx = 0
)
:BaseWin(parent, pos, _size, id), returnValue(retVal)
{
WNDCLASSEX wc = { 0 };
wc.cbSize = sizeof(wc);
wc.style = CS_OWNDC;
wc.lpfnWndProc = D_CLASS::WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = GetModuleHandle(nullptr);
wc.hIcon = nullptr;
wc.hCursor = LoadCursor(nullptr, IDC_ARROW);
wc.hbrBackground = nullptr;
wc.lpszMenuName = nullptr;
wc.lpszClassName = className;
wc.hIconSm = nullptr;
RegisterClassEx(&wc);
HMENU _id = nullptr;
if (!((styles & WS_CHILD) != WS_CHILD || id == -1))
_id = (HMENU)ID;
hWnd = CreateWindowEx(
stylesEx,
className, title,
styles,
Position.x, Position.y, size.x, size.y,
parent, _id, GetModuleHandle(nullptr), this
);
}
~Window()
{
UnregisterClass(className, GetModuleHandle(nullptr));
Destroy();
}
};
I'm using Visual Studio and whenever the line is reached is reached, a breakpoint is placed there are no details through which I can deduce the resaon (It's: main.exe has encountered a breakpoint).
main.cpp:
#include "Window.h"
#include "Button.h"
class MainWindow
:public Window<MainWindow>
{
private:
HMENU menuBar;
HMENU menuIt;
enum
{
BTN_KILL,
M_QUIT,
M_ADD_LOG
};
public:
MainWindow()
:Window<MainWindow>(nullptr, "Useless Manger", -1, Point(CW_USEDEFAULT, CW_USEDEFAULT), Size(640, 480), normWinStyle)
{
Button* btn = new Button(hWnd, BTN_KILL, "Kill Me", Point(), Size(300, 100));
menuBar = CreateMenu();
menuIt = CreateMenu();
AppendMenu(menuIt, MF_ENABLED, M_ADD_LOG, "Add Log");
AppendMenu(menuBar, MF_ENABLED | MF_STRING, M_QUIT, "Quit");
AppendMenu(menuBar, MF_POPUP, (UINT_PTR)menuIt, "Add");
SetMenu(hWnd, menuBar);
}
LRESULT HandleMessages(UINT msg, WPARAM wParam, LPARAM lParam)
{
switch (msg)
{
case WM_DESTROY:
PostQuitMessage(0);
break;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
FillRect(hdc, &ps.rcPaint, (HBRUSH)(COLOR_WINDOW + 1));
EndPaint(hWnd, &ps);
break;
}
case WM_COMMAND:
if (LOWORD(wParam) == BTN_KILL)
{
PostQuitMessage(this->returnValue);
break;
}
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}
};
int CALLBACK WinMain(
HINSTANCE hInst,
HINSTANCE h_p_Inst,
LPSTR nCmdLine,
int nCmdShow)
{
MainWindow* window = new MainWindow();
window->Show();
MSG msg;
while (GetMessage(&msg, nullptr, 0, 0) > 0)
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
BaswWin.h is a class holding the current HWND, position, size, ID, etc with some functions like Show().
How do I fix this?

Answer 1

MainWindow::HandleMessages is called before superclass Window<T> is fully constructed. Before Window<T> is fully constructed, entry for Window<T>::HandleMessages refers to special function witch reports that pure function was called. When Window<T> is fully constructed, this entry is replaced by MainWindow::HandleMessages.
In C++ calling virtual functions when superclasses are still constructed needs special atention.
You can replace Window<T>::HandleMessage with unpure implementation.
virtual LRESULT HandleMessages(UINT msg, WPARAM wParam, LPARAM lParam)
{
return DefWindowProc(hWnd, msg, wparam, lparam);
}
Initialize hWnd early, because WndProc can be called before CreateWindowEx returns.
static LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
D_CLASS* p_this = nullptr;
if (msg == WM_NCCREATE) {
CREATESTRUCT* create = (CREATESTRUCT*)lParam;
p_this = (D_CLASS*)create->lpCreateParams;
// initialize early
p_this->hWnd = hwnd;
SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR)p_this);
}
else
p_this = (D_CLASS*)GetWindowLongPtr(hwnd, GWLP_USERDATA);
if (p_this)
// if you don't initialize early, skip when hWnd == nullptr
// if (p_this && p_this->hWnd)
return p_this->HandleMessages(msg, wParam, lParam);
else
return DefWindowProc(hwnd, msg, wParam, lParam);
}
Probably you would also miss WM_DESTROY message, when MainWindow class is already destroyed, but WM_DESTROY would be generated in Window<T>::~Window<T>.
Calling PostQuitMessage(this->returnValue); in MainWindow::~MainWindow could solve this problem.