On May 28, 2008, at 3:02 PM, Peter Dimov wrote:
Howard Hinnant:
I've used this (for example) in implementing recursive mutexes:
class recursive_mutex { unsigned state_; std::thread::id id_;
As already pointed out in the thread
http://www.decadentplace.org.uk/pipermail/cpp-threads/2006-August/001091.htm...
linked from #783, this requires thread::id atomicity, which is not guaranteed. I'm not sure whether instant reuse can also break this particular example, but it definitely breaks other uses of thread::id.
I probably made a mistake in posting code snippets instead of a complete recursive_mutex. In the example I programmed, which was actually a recurisve_timed_mutex instead of a recursive_mutex, there were member mutex and condition_variables. The mutex was locked while manipulating the internal state_ and id_, providing the required atomicity, and preventing "instant reuse". Since "not a thread" values do not exist (as far as I know) in pthreads and Windows, I was trying to make the point that many people probably haven't considered the use of such values in multithreading programming in dealing with the id-reuse-problem. I believe this tool will solve some, but not all, problems which "unique_id" addresses, and with far less expense. -Howard