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POWhy does uninitialised return value cause Invalid Window Handle err in CreateWindowEx?
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  1. POWhy does uninitialised return value cause Invalid Window Handle err in CreateWindowEx?
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    copied!<p><em>Edit</em>- Added code to do with m_hWndClient and the WndProc that were not originally included. In an attempt to be brief I had incorrectly assumed it was unrelated.</p> <p>After the following is run </p> <pre><code>HWND m_hWndFrame; HWND m_hWndClient; // added in Edit2 ... m_hWndFrame = CreateWindowEx(...) </code></pre> <p><code>m_hWndFrame</code> is NULL and <code>GetLastError</code> gives "Error 1400 - Invalid Window Handle" but this works fine:</p> <pre><code>HWND m_hWndFrame = NULL; HWND m_hWndClient = NULL; // added in Edit2 ... m_hWndFrame = CreateWindowEx(...) </code></pre> <p>My <code>WndProc</code> looks like this:</p> <pre><code>LRESULT CALLBACK ProgramManager::WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) { CLIENTCREATESTRUCT clientCreate; HINSTANCE hInstance = GetModuleHandle(NULL); RECT clientRect; switch (uMsg) { case WM_CREATE: clientCreate.hWindowMenu = NULL; clientCreate.idFirstChild = IDM_FIRSTCHILD ; GetClientRect(hwnd,&amp;clientRect); s_instance-&gt;m_hWndClient = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT ("MDICLIENT"), NULL, WS_CHILD | WS_CLIPCHILDREN | WS_CLIPSIBLINGS, 0, 0, clientRect.right, clientRect.bottom, hwnd, (HMENU)ID_MDI_CLIENT, hInstance, (LPVOID)&amp;clientCreate); return 0 ; case WM_CLOSE: DestroyWindow(hwnd); break; case WM_DESTROY: PostQuitMessage(0); break; } return DefFrameProc(hwnd,m_hWndClient,uMsg,wParam,lParam); } </code></pre> <p>My project now works (after much hair-tearing) but I don't understand why initialising a variable that is only used to hold a return value should matter. </p> <p>Obviously assuming that a variable is NULL or 0 without initialising and then using or testing the contents (eg <code>if (!m_unitialisedVariable)</code>) is going to end in disaster but why should it matter in this instance? There is no requirement for <code>m_hWndFrame</code> to contain anything in particular before calling 'CreateWindowEx' (at least according to the Help in VS2010) so why should it affect the outcome of 'CreateWindowEx'?</p>
 

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