id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
12309,"Is ""Reference counting"" example misleading and wrong?",Adam Badura <adam.f.badura@…>,timblechmann,"Boost.Atomic in its documentation provides ""Reference counting"" example:
http://www.boost.org/doc/libs/1_61_0/doc/html/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters

The example itself is excellent. After all this is a typical use case so providing an example with brief discussion is mostly welcome.

The problem is however whether the example is correct after all. I mean here the part about counter decrement. Example uses `release` order and then a fance with `acquire`:
{{{
friend void intrusive_ptr_release(const X * x)
{
  if (x->refcount_.fetch_sub(1, boost::memory_order_release) == 1) {
    boost::atomic_thread_fence(boost::memory_order_acquire);
    delete x;
  }
}
}}}
while description states later:
  It would be possible to use `memory_order_acq_rel` for the `fetch_sub` operation, but this results in unneeded ""acquire"" operations when the reference counter does not yet reach zero and may impose a performance penalty.

At the same time `boost::shared_ptr` uses `acq_rel` in its `std::atomic` implementation in file `boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp` (http://www.boost.org/doc/libs/1_61_0/boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp).

Also Herb Sutter in his ""C++ and Beyond 2012: Herb Sutter - atomic<> Weapons, 2 of 2"" (https://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-2012-Herb-Sutter-atomic-Weapons-2-of-2) lecture mentions reference counting topic (starting at 1:19:51) where he also calls for `acq_rel`. And it seems that he discourages use fences in this talk.

So which one is good? Because either Boost.Atomic documentation should be updated or `boost::shared_ptr` should be improved.

And let me note here that this Boost.Atomic documentation part seems to be quoted (or refereed to) at various places, including Stack Overflow, quite often. So it is important for it to be correct.

----

BTW (although beyond this ticket) it seems strange to me that `boost::shared_ptr` doesn't use `boost::atomic` and instead provides like 19 different implementations. It smells like code repetition to me. But maybe there are valid reasons for it...",Bugs,closed,To Be Determined,atomic,Boost 1.61.0,Cosmetic,fixed,"atomic, shared_ptr, reference counting, documentation",