[Boost-users] Re: shared_ptr: LWM versus atomic_count?

From: Russell Hind (rhind_at_[hidden])
Date: 2004-07-14 09:12:42

Caleb Epstein wrote:

> I've been perusing the source code of <boost/shared_ptr.hpp> and
> noticed that it uses the <boost/detail/shared_count.hpp> class to do
> reference counting and not <boost/detail/atomic_count.hpp> (unless
> BOOST_NO_MEMBER_TEMPLATES or BOOST_MSVC6_MEMBER_TEMPLATES are
> defined).
> 
> Can anyone comment on the reasoning behind using a lightweight
> mutex-protected counter instead of an atomic counter?  I'd think that
> an atomic counter would be faster, but I suspect that there is a good
> reason for protecting the count with a mutex.  This then begs the
> question of why use the atomic counter in the situations mentioned
> above?
> 

If you look at shared_count, you'll see it has to keep track of 2 
values, strong_count and weak_count, you can't do this with 'atomic 
count' so therefore it uses lwm.

Cheers

Russell