On Sun, Nov 22, 2009 at 11:49 AM, Vicente Botet Escriba <vicente.botet@wanadoo.fr> wrote:
Hi,
I have changed the ticket from threads to thread
As this works as expected, Anthony could just add on the documentation why reentrant call_once functions are not a good idea. The explanation of Steven could be a good starting point:
"I'll also point out that there is no good behavior for a recursive call to call_once. If it calls the function again, then f will be called twice. If it does not, then whatever initialization that call_once is doing may be incomplete. In either case, you have to know whether a call can be recursive and carefully design for it. If it happens accidentally, you're probably toast regardless of how it behaves. In the face of recursive calls it is impossible to maintain the post-condition that the function has run exactly once."
After this modification IMO the ticket can be closed.
Best, Vicente
First of all, I agree that a recursive call_once makes little sense. However, I was in the process of writing a (better?) version of call_once that avoided creating the system wide mutex (on Windows), using a process-local Event instead (and even that is not created unless there is contention - ie the second thread in creates the event). While tackling this, I also decided to tackle ways of handling exceptions (should 'f' be recalled if the first attempt threw an exception?), as well as optionally allowing recursion. The code is currently Windows-only, although it could be ported most anywhere. I'd be interested to know: - if the exception handling and/or recursion handling are worthwhile options and more importantly - if the avoidance of creating a named mutex is worthwhile (I think so) Code attached. Not 100% tested yet (or 'boostified'), but I'm pretty sure the algorithm is sound. Tony