Re: [boost] [singleton] call_once

From: John Maddock (john_at_[hidden])
Date: 2005-09-13 12:14:36

> I should probably elaborate on my problem a little bit more.  I wanted
> to create a function that makes a given call atomic, but doesn't enforce
> that the function be called only once.  I thought I could do so by using
> a once_flag allocated on the stack immediately before invoking
> call_once, so the flag would always be reinitialized to BOOST_ONCE_INIT
> and call_once could just offer me it's atomic guarantees, but apparently
> that won't work.  I guess I'll just have to find some place to put my
> own mutex where it can be constructed safely.

I forgot to mention:

1) I had the same issue in Boost.Regex and wrote a statically initialised 
mutex as a solution (see boost/regex/pending/static_mutex.hpp), it might 
make a useful addition to the threading lib, however, I really should 
rewrite the Win32 implementation to use a mutex so you don't get a priority 
inversion and busy waiting (see 
http://www.netrino.com/Publications/Glossary/PriorityInversion.html).

2) It's possible to do what you want with call_once, but it's one of those 
things you might think is impossible until someone shows you how, and then 
it's embarrassingly easy, see 
http://lists.boost.org/Archives/boost/2005/07/90756.php for an explanation.

HTH,  John.