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From: William Kempf (sirwillard_at_[hidden])
Date: 2000-11-27 10:34:31

--- In boost_at_[hidden], "Michael D. Crawford" <crawford_at_g...> wrote:
> > And
> > then there's multithreading.
> I know it take extra effort, but you can do reference counting with
> multithreading. The ZRef reference counted smart pointer in ZooLib
> multithreading correctly.
> You could probably lift out ZRef and ZAtomic from ZooLib and use
them on
> their own for multithreaded reference counting without using the
rest of
> ZooLib.
> It does this by using atomic addition and subtraction on the
> counts when it's getting operated on. So if you have two threads
> incrementing and decrementing the reference count it does the right

I've not looked at ZooLib (yet!), but I can tell you that if all that
was done were to use atomic increment and decrement on the ref-count
then the smart pointer is *NOT* thread safe. Copies also must copy
the pointer, and if the whole operation isn't protected I don't see
how it can be thread safe (though maybe I'm missing some subtle
ordering of operations that might achieve this).

> It wasn't easy for Andy Green to get it right, considering also the
> platform issues (and we found a small compiler bug when doing the
> operations) but it does seem to work correctly.

Then we should look here for ideas for Boost Threads as well.

Bill Kempf

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