John Maddock wrote: <snip>
Cons: * If the functor takes a long time to execute, and there are multiple threads racing to call-once, then the wait will be less efficient than a mutex, this should be a very rare occurance though.
Neat. Though , if f was longer than short, and there was contention, a lot of wasted cycles could occur. This leads to the thought that this spinning approach would suffer from a priority problem if sufficient waiting threads, one on a single cpu context, were at a higher priority than the initialising item. This would cause a virtual deadlock, a term I just made up ;-) pthread32's initial implementation was similar. Your atomic approach is the right approach I think. A suitable wait list approach in the "already initializing" section with subsequent processing after initialisation is complete should cure it and keep the essence of the fast approach but make it a fair bit more complicated. matt.