Safer subclassing

Raymond Chen


So what was wrong with our
little subclassing sketch

Most people figured this out.

Consider what would happen if somebody else had subclassed the window during the “…
do stuff …” section. When we unsubclassed the window, we would have removed two subclasses,
the one we installed, and the one that was installed after us. If the other subclass
allocated memory (which is very common), then that memory got leaked, in addition
to the subclass failing to do whatever it was trying to do.

Do not assume that subclasses are added and removed in a purely stack-like manner.
If you want to unsubclass and find that you are not the window procedure at the top
of the chain you cannot safely unsubclass. You will have to leave
your subclass attached until it becomes safe to unsubclass. Until that time, you just
have to keep passing the messages through to the previous procedure.

This is quite a cumbersome process, so the shell team wrote some helper functions
to do all this for you. The SetWindowSubclass
does all the grunt work of installing a subclass procedure, remembering
the previous one, and passing reference data to the subclass procedure you provide.
You use the DefSubclassProc
to forward the message to the previous subclass procedure, and when you’re
done, you use the RemoveWindowSubclass
to remove yourself from the chain. RemoveWindowSubclass does all the
work to do the right thing if you are not the window procedure at the top of the chain.

One gotcha that isn’t explained clearly in the documentation is that you must
remove your window subclass before the window being subclassed is destroyed
This is typically done either by removing the subclass once your temporary need has
passed, or if you are installing a permanent subclass, by inserting a call to RemoveWindowSubclass
inside the subclass procedure itself:

  RemoveWindowSubclass(hwnd, thisfunctionname, uIdSubclass);
  return DefSubclassProc(...);

One comment expressed concern that a message could be sent between the call to SubclassWindow and
the store of the previous window procedure into the OldWndProc variable.
This is actually a safe operation provided that you are doing the work from the thread
that owns the window you are subclassing. Remember that message delivery occurs only
when the thread is in a receiving state, such as when it calls GetMessage or PeekMessage.
If somebody sends a message when the thread is not in a receiving state, the message
merely waits until the thread finally calls GetMessage (for example)
before being delivered. Since we don’t make any message-receiving function calls between
the SubclassWindow and the store into OldWndProc, there
is no risk of an untimely message arriving before the store to OldWndProc has

There was another comment that claimed that the SubclassWindow macro
is undocumented. Actually this macro is so old that the
documentation for it has faded almost into obscurity
. (You forced my hand;

I wasn’t
going to dig into this header file until tomorrow

Raymond Chen
Raymond Chen

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