C++ Win32. SelectObject Fails, with GetLastError returning error 1400 (invalid window handle) - c++

Question

So I'm trying to make a clone of Pong in Win32, and things WERE working, but then I did a lot of stuff with the physics, and when I tested it, the sprite bitmaps weren't even displaying any more :/
So, here is how I initialise the rendering stuff:
int InitRenderer(int showCMD)
{
context = GetDC(winHandle);
if(!context)
{
return EXIT_FAILURE;
}
ShowWindow(winHandle, showCMD);
UpdateWindow(winHandle);
CreateDoubleBuffer(&globalBuffer);
ClearWindow(globalBuffer.hdcBack, globalBuffer.scrnRect);
return EXIT_SUCCESS;
}
Here is the CreateDoubleBuffer function:
void CreateDoubleBuffer(BUFFER *buffer)
{
buffer->hwnd = winHandle;
GetClientRect(winHandle, &(buffer->scrnRect));
buffer->hdcFront = GetDC(buffer->hwnd); //get a handle to the DC and plop it into the front buffer.
buffer->hdcBack = CreateCompatibleDC(buffer->hdcFront); //get a compatible DC for the Back buffer.
buffer->hdcBitmap = CreateCompatibleDC(buffer->hdcFront); //get a compatible DC for the bitmap.
buffer->hCompBitmap = CreateCompatibleBitmap(buffer->hdcFront, buffer->scrnRect.right, buffer->scrnRect.bottom); //Create a compatible bitmap as a dummy, and store in the front buffer.
buffer->hOldBitmap = (HBITMAP)SelectObject(buffer->hdcBack, buffer->hCompBitmap);
}
the BUFFER struct, for reference, looks like this:
struct BUFFER // This is our back buffering structure
{
HWND hwnd; // This holds the current window's handle
RECT scrnRect; // This holds the client rectangle of the window
HANDLE hCompBitmap; // This holds the compatible bitmap for the backbuffer
HANDLE hOldBitmap; // This is used for storage to free when the program quits
HANDLE hOldBitmap2; // This is used as storage to swap between selected bitmaps when using selectObject()
HDC hdcFront; // This is the front buffer (The part we see)
HDC hdcBack; // This is the back buffer (the part we draw to, then flip)
HDC hdcBitmap; // This is a temp buffer to swap the bitmap back and forth from
};
So I have a Sprite class which just wraps an HBITMAP and a string for the filename, and some functions to manipulate those. When I want to draw the sprite, this function is called:
void RenderSprite(BUFFER *buffer, HBITMAP bmp, Vec2I pos, Vec2F origin)
{
buffer->hOldBitmap2 = (HBITMAP)SelectObject(buffer->hdcBitmap, bmp); //we put the bitmap into the extra HDC to hold it there.
if(!buffer->hOldBitmap2)
{
std::cout << GetLastError() << "\n";
}
BitBlt(buffer->hdcBack, pos.GetX() + (int)origin.GetX(), pos.GetY() + (int)origin.GetY(), buffer->scrnRect.right, buffer->scrnRect.bottom, buffer->hdcBitmap, 0, 0, SRCCOPY); //blit the bitmap into the backbuffer.
SelectObject(buffer->hdcBitmap, buffer->hOldBitmap2); //put the old handle to the bitmap back where it belongs.
}
And it is at the start of this function where SelectObject fails, so buffer->hOldBitmap2 is null. The error returned by GetLastError is 1400, which means invalid window handle, so I guess winHandle (a global variable, just so you know) is messed up. But I don't see how. Here is how I initialise it:
int Game::Start(HINSTANCE instance, int showCMD)
{
WNDCLASSEX winClass = {0};
winClass.cbSize = sizeof(winClass);
winClass.style = CS_HREDRAW | CS_VREDRAW;
winClass.lpfnWndProc = WndProc;
winClass.hInstance = instance;
winClass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
winClass.lpszClassName = _winClassName;
winClass.hCursor = (HCURSOR)LoadImage(NULL, MAKEINTRESOURCE(OCR_CROSS), IMAGE_CURSOR, 0, 0, LR_SHARED); //using a cross for a cursor because we're hipsters here at cow_co industries.
RegisterClassEx(&winClass);
/**
* WS_EX_CLIENTEDGE gives the client a sunken edge.
**/
winHandle = CreateWindowEx(WS_EX_CLIENTEDGE, _winClassName, "Win32 Pong", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, instance, NULL);
if(!winHandle)
{
return EXIT_FAILURE;
}
//other stuff...
And I haven't changed this since the time when it worked.
In terms of what happens on-screen, I just get a blank white window, so no Crash message or whatever, but it's just...blank.
I've looked around other questions and the solutions to those seem to be related to issues registering the window class etc. I've checked mine and I can't see anything wrong with it. I've debugged, and hOldBitmap2 is the only part of the buffer that's null. The rest's fine.
Any help you sages of the Stack could provide would be much appreciated.

Answer 1

I just ran into this problem.
An application cannot select a single bitmap into more than one DC at
a time. Is the bitmap already selected into another DC? – theB Aug 27
'15 at 18:09
Is close!
OK:
SelectObject( hdc_bitmap, buf_bitmap )
FAILS:
SelectObject( hdc_bitmap, buf_bitmap )
SelectObject( hdc_bitmap, buf_bitmap )
This means if your RenderSprite function tries to draw the same sprite twice in a row,
it will fail.