c++ - Class method for WndProc

Question

This article explains brilliantly the options to call a class member WndProc. I've seen this response in stackoverflow but the main problem associating class member WndProc after CreateWindow is that some messages will be lost (including the important WM_CREATE) as explained in the mentioned article.

My question: I would like to hear the opinion from an expert on which of the methods exposed below or new one is the best one (performance, maintanability, ...) to create a class member WndProc.

Briefing the two final solutions exposed in the article (suposing that it exists a Window class with WndProc method):

1. Per-window data with this global pointer storage, protecting it with CRITICAL_SECTION to make it thread safe (extracted from here):

   // The helper window procedure
   // It is called by Windows, and thus it's a non-member function
   // This message handler will only be called after successful SetWindowLong call
   // We can assume that pointer returned by GetWindowLong is valid
   // It will route messages to our member message handler
   LRESULT CALLBACK WndProc2(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
     // Get a window pointer associated with this window
     Window *w = (Window *) GetWindowLong(hwnd, GWL_USERDATA);
     // It should be valid, assert so
     _ASSERT(w);
     // Redirect messages to the window procedure of the associated window
     return w->WndProc(hwnd, msg, wp, lp);
   }
   // The temporary global this pointer
   // It will be used only between CreateWindow is called and the first message is processed by WndProc
   // WARNING: it is not thread-safe.
   Window *g_pWindow;
   
   // Critical section protecting the global Window pointer
   CRITICAL_SECTION g_WindowCS;
   
   // The helper window procedure
   LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
     // Stash global Window pointer into per-window data area
     SetWindowLong(hwnd, GWL_USERDATA, (long) g_pWindow);
     // Unlock global critical section
     g_pWindow->HaveCSLock = false;
     LeaveCriticalSection(&g_WindowCS);
     // Reset the window message handler
     SetWindowLong(hwnd, GWL_WNDPROC, (long) WndProc2);
     // Dispatch first message to the member message handler
     return WndProc2(hwnd, msg, wp, lp);
   }
   

   And now we can create the window:

   InitializeCriticalSection(&g_WindowCS);
   // Enter the critical section before you write to protected data
   EnterCriticalSection(&g_WindowCS);
   
   // Set global Window pointer to our Window instance
   // Moved the assignment here, where we have exclusive access to the pointer
   g_pWindow = &w;
   
   // Set a flag indicating that the window has the critical section lock
   // Note: this must be executed after the above assignment
   g_pWindow->HaveCSLock = true;
   
   // Create window
   // Note: lpParam is not used
   HWND hwnd = CreateWindow(TEXT("BaseWnd"), TEXT("Hello, World!"), WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, 0, 0, hinst, 0);
   
   // Leave critical section if window creation failed and our window procedure hasn't released it
   if (g_pWindow->HaveCSLock)
     LeaveCriticalSection(&g_WindowCS);
   // Destroy critical section
   // In production code, you'd do this when application terminates, not immediately after CreateWindow call
   DeleteCriticalSection(&g_WindowCS);
   

2. Using CBT hook procedure (extracted from here):

   // The helper window procedure
   // It is called by Windows, and thus it's a non-member function
   // This message handler will only be called after successful SetWindowLong call from the hook
   // We can assume that pointer returned by GetWindowLong is valid
   // It will route messages to our member message handler
   LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp)
   {
     // Get a window pointer associated with this window
     Window *w = (Window *) GetWindowLong(hwnd, GWL_USERDATA);
     // It should be valid, assert so
     _ASSERT(w);
     // Redirect messages to the window procedure of the associated window
     return w->WndProc(hwnd, msg, wp, lp);
   }
   
   // The CBT hook procedure
   // It is called during CreateWindow call before WndProc receives any messages
   // Its job is to set per-window Window pointer to the one passed through lpParam to CreateWindow
   LRESULT CALLBACK CBTProc(int code, WPARAM wp, LPARAM lp)
   {
     if (code != HCBT_CREATEWND) {
       // Ignore everything but create window requests
       // Note: generally, HCBT_CREATEWND is the only notification we will get,
       // assuming the thread is hooked only for the duration of CreateWindow call.
       // However, we may receive other notifications, in which case they will not be passed to other CBT hooks.
       return 0;
     }
     // Grab a pointer passed to CreateWindow as lpParam
     std::pair<Window *, HHOOK> *p = (std::pair<Window *, HHOOK> *) LPCBT_CREATEWND(lp)->lpcs->lpCreateParams;
     // Only handle this window if it wasn't handled before, to prevent rehooking windows when CreateWindow is called recursively
     // ie, when you create windows from a WM_CREATE handler
     if (p->first) {
       // Stash the associated Window pointer, which is the first member of the pair, into per-window data area
       SetWindowLong((HWND) wp, GWL_USERDATA, (long) p->first);
       // Mark this window as handled
       p->first = 0;
     }
     // Call the next hook in chain, using the second member of the pair
     return CallNextHookEx(p->second, code, wp, lp);
   }
   

   And now we can create the window:

   // Install the CBT hook
   // Note: hook the thread immediately before, and unhook it immediately after CreateWindow call.
   // The hook procedure can only process window creation nofitications, and it shouldn't be called for other types of notifications
   // Additionally, calling hook for other events is wasteful since it won't do anything useful anyway
   HHOOK hook = SetWindowsHookEx(WH_CBT, CBTProc, 0, GetCurrentThreadId());
   _ASSERT(hook);
   
   // Create window
   // Pass a pair consisting of window object pointer and hook as lpParam
   std::pair<Window *, HHOOK> p(&w, hook);
   HWND hwnd = CreateWindow(TEXT("BaseWnd"), TEXT("Hello, World!"), WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, 0, 0, hinst, &p);
   
   // Unhook first
   UnhookWindowsHookEx(hook);
   

Answer 1

I personally would not use either of these methods. The global variable approach works, but feels dirty. Especially with the lock. And the CBT hook is, well over the top. Although it points in the right direction.

The standard way to pass state information to your window procedure during creation is through lpParam parameter of CreateWindow or CreateWindowEx. So the technique is as follows:

1. Pass your instance pointer in the lpParam parameter of CreateWindow or CreateWindowEx.
2. Read this value in your WM_NCCREATE handler. That message supplies the information as part of the CREATESTRUCT struct.
3. Still in WM_NCCREATE call SetWindowLongPtr to set the user data of the window to the instance pointer.
4. All future calls to the window procedure can now obtain the instance pointer by calling GetWindowLongPtr.

Raymond Chen illustrates the details here: How can I make a WNDPROC or DLGPROC a member of my C++ class?