Russell Hind <rhind@mac.com> wrote:

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.

It isn't fundamentally necessary to have two counters, though; see <http://groups.google.com/groups?threadm=slrnbhbnpc.154.do-not-spam -benh%40tin.bwsint.com>.