[boost] Using Boost.Atomic for shared_ptr

Subject: [boost] Using Boost.Atomic for shared_ptr
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-12-20 14:16:03

Dear All,

I have just had a quick look at how easily shared_ptr could be adapted 
to use Helge's proposed Boost.Atomic.  To me it looks very simple, but 
maybe someone who knows more about all this could have a look?  I took 
the non-threaded implementation in sp_counted_base_nt.hpp as a starting 
point, and simply changed the two counts from long to 
boost::atomic<long> :

     boost::atomic<long> use_count_;        // #shared
     boost::atomic<long> weak_count_;       // #weak + (#shared != 0)

Then, the majority of the methods don't need to change since the atomic 
template provides atomic versions of e.g. operator++.  The one that 
does need more work is add_ref_lock, which I've implemented as follows:

     bool add_ref_lock() // true on success
     {
         while (1) {
             long c = use_count_;
             if (c == 0) return false;
             if (use_count_.compare_exchange_weak(c,c+1)) return true;
         }
     }

There is also the question of memory barriers.  I understand that 
things like operator++ provide the most pessimistic memory barriers, so 
ideally these should be replaced with calls that use the right barriers.

Does anyone have any thoughts about how this would compare with the 
current custom assembler implementations?

Cheers,  Phil.