If we store the return value along with the once_flag, then we really ought to destroy it properly, when the once_flag is destroyed. If a once_flag has static storage duration, and has a non-trivial destructor that does cleanup, then we have a classic singleton cleanup problem, as John Maddock pointed out --- the order of static destruction can now be important.
You mean an extra non-POD version? That's not your problem... as long as you provide a POD version + call_once_reset/destroy().
So, you're suggesting that we store the result with the once_flag, and then leave it up to users to schedule a call to call_once_reset, to ensure the result gets cleared up?
Hmm.
I'd still like to know how the user can safely schedule the call to call_once_reset, so it doesn't cause problems. I suppose there's an element to which it doesn't really matter; the result will just be reinitialized if there's a subsequent call_once.
I don't like leaving things as "not my problem", unless I can see that there is a safe solution.
Me neither, and I still don't see what having a return value really gains us in this case (I would much rather be able to pass parameters to the call_once procedure, but that's a whole other ball game). If a return value is truly required, then we could use aligned_storage to provide raw storage for the return value and construct the object on first call to the one procedure, but that still doesn't solve the destruction problem. John.