title: C/C++ - Windows - how to keep track of HWND context data
body: <p>In Windows, suppose you have multiple windows (HWNDs) of the same window class open. How do you keep track of the context data in the window procedure so that, for example, window 1 does not get modified when the user tried to type in window 2?</p>
  
  <p>CreateWindow() does not return until after the WndProc() has been called several times, so you can't simply set the resulting HWND to the context data and do a lookup in the WndProc(); you do need to set it in the WndProc().</p>
  
  <p>WndProc() doesn't directly have the context information passed to it except on window creation messages, but unfortunately window creation messages aren't exactly the first messages to be passed to WndProc(). Nay, I find things such as WM_SIZE, WM_NCSIZE, and even some others are passed before I ever see WM_CREATE.</p>
  
  <p>Storing the HWND in a linked list type of storage mechanism would be inefficient with large amounts of windows: each control in a window is simply another type of window and therefore another HWND of which you need to keep track; after a few hundred controls, searching the linked list for the HWND will be a major bottleneck in the program after a few dozen messages are passed to the program in a short amount of time!</p>
  
  <p>From what I hear, some people use SetWindowLong() - but I also hear that some libraries like to use that too to store their own context information separate from the program and that window data collisions can sometimes occur. How can that be avoided?</p>