c++ creating a window in a new thread - c++

Question

I have a basic window program, the problem is when i try to create a window in a new thread after the message loop has already started the window displays for a second and disappears. does anyone no the reason for this? can a window be created in a separate thread?
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
::hInstance =hInstance; // initialize global variables
::nCmdShow =nCmdShow;
// start thread
HANDLE threadHandle = startThread(StartUp);
MSG msg;
while(GetMessage(&msg, 0, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
::CloseHandle(threadHandle);
return static_cast<int>(msg.wParam);
}
DWORD WINAPI StartUp(LPVOID lpParam) // new thread runs here
{
//code to create a new window...
}
what i figured out so far is that the GetMessage(&msg, 0, 0, 0) returns false if there are no windows in the current thread that it is in... is there a way to get a round this?

Answer 1

GetMessage() does not return FALSE if there are no windows. It is only looking for messages in the calling thread's message queue. You are specifying NULL for its hWnd parameter, so it will not care how messages get queued, whether by PostMessage() to a window, or by PostThreadMessage() to the thread's ID.
Each thread has its own local message queue and thus requires its own message loop. You can most certainly create a new window in a worker thread after the main thread has started its message loop. They are independent of each other. So whatever problem you are having in the main thread is not related to creating a window in a worker thread. Something else is going on.
With that said, keep in mind that GetMessage() returns a BOOL, which is actually an int, not a true bool. GetMessage() can return one of 3 different return values:
-1 if an error occurs
0 if a WM_QUIT message is retrieved
>0 if any other message is retrieved
You are only checking for 0 and != 0, so if GetMessage() returns -1 on error, you are treating it as a success instead of a failure. Even MSDN says not to do that:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644936.aspx
Because the return value can be nonzero, zero, or -1, avoid code like this:
while (GetMessage( lpMsg, hWnd, 0, 0)) ...
The possibility of a -1 return value means that such code can lead to fatal application errors. Instead, use code like this:
BOOL bRet;
while( (bRet = GetMessage( &msg, hWnd, 0, 0 )) != 0)
{
if (bRet == -1)
{
// handle the error and possibly exit
}
else
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}