GetDC() for HWND before LRESULT function - c++

Question

I am trying to build software with 3d rendering capabilities without using external libraries. I'm a little confused with GetDC() and ReleaseDC(). If I write the following code after my LRESULT() loop, the program runs for several seconds and then freezes. (I have created two dc's for double buffering with BitBlt())
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
HDC hdc = GetDC(hWnd);
HDC memdc = CreateCompatibleDC(hdc);
HBITMAP membmp = CreateCompatibleBitmap(hdc, width, height);
SelectObject(memdc, membmp);
...
switch (message) {
...
}
}
However, according to this linkabout SetPixel() crashing after a time, I should move GetDC() outside of the loop. However, I must be doing it wrong, because if I do that...
HDC hdc = GetDC(hWnd);
HDC memdc = CreateCompatibleDC(hdc);
HBITMAP membmp = CreateCompatibleBitmap(hdc, width, height);
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
TCHAR greeting[] = _T("program_name");
PAINTSTRUCT ps;
SelectObject(memdc, membmp);
...
}
... I realize that I cannot SelectObject() outside of the LRESULT loop. In fact, visual studio isn't even going to live with GetDC(hWnd) before the LRESULT loop, I expect because it is created in the first line of the loop. So, woefully, my only option seems to be to replace hWnd with NULL.
HDC hdc = GetDC(NULL);
HDC memdc = CreateCompatibleDC(hdc);
HBITMAP membmp = CreateCompatibleBitmap(hdc, width, height);
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
TCHAR greeting[] = _T("program_name");
PAINTSTRUCT ps;
SelectObject(memdc, membmp);
...
}
Now I can no longer clear the window, as that relies on InvalidateRect(hWnd, NULL, TRUE). So it runs indefinitely, yet draws outside of the window and cannot erase.
I am new to programming and if anyone can point me in a direction or let me know what's going on, I would appreciate it. :)

Answer 1

If you're new to programming and c++ you should first learn the basics of the language and not start with using the WinAPI.
Also your "LRESULT() loop" is not a loop but rather a callback function called by your message loop.
Concerning your question: Your application should draw its windows content when it receives a WM_PAINT-message.

Answer 2

Your code is setup all wrong. Try this instead:
HDC hmemdc = NULL;
HBITMAP membmp = NULL;
HBITMAP oldbmp = NULL;
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
switch (message) {
case WM_CREATE: {
HDC hdc = GetDC(hWnd);
membmp = CreateCompatibleBitmap(hdc, width, height);
memdc = CreateCompatibleDC(hdc);
oldbmp = (HBITMAP) SelectObject(memdc, membmp);
ReleaseDC(hWnd, hdc);
break;
}
case WM_DESTROY: {
SelectObject(memdc, oldbmp);
DeleteDC(memdc); memdc = NULL;
DeleteObject(membmp); membmp = NULL;
break;
}
case WM_ERASEBKGND: {
return 1;
}
case WM_PAINT: {
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
BitBlt(hdc, 0, 0, width, height, memdc, 0, 0, SRCCOPY);
EndPaint(hWnd, &ps);
break;
}
...
}
return DefWindowProcc(hWnd, message, wParam, lParam);
}
Then, you can simply draw your 3D images into hmemdc/membmp as needed, and call InvalidateRect() whenever you want to signal to the OS that the window needs to be redrawn with the latest bitmap.
Though, if you are going to use a memory bitmap for your window, you might consider using UpdateLayeredWindow() instead, and get rid of the WM_PAINT/WM_ERASEBKGND handlers completely.