Re: [Boost-Users] Re: Any interest in a weak pointer?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-01-11 08:22:43

From: "terekhov" <terekhov_at_[hidden]>
> --- In Boost-Users_at_y..., "Peter Dimov" <pdimov_at_m...> wrote:
> > OK, I did, but I still have trouble translating this into a
> specific problem
> > with the shared_ptr code. Do you have an example?
>
> Given:
>
> - a mutable shared object OBJ;
> - two threads THREAD1 and THREAD2 each holding
>   a private smart_ptr object pointing to that OBJ.
>
> ----
>
> t1: THREAD1 updates OBJ (thread-safe via some synchronization)
>     and a few cycles later (after "unlock") destroys smart_ptr;
>
> t2: THREAD2 destroys smart_ptr WITHOUT doing any synchronization
>     with respect to shared mutable object OBJ; OBJ destructors
>     are called driven by smart_ptr interface...

Yes, I see it now, thanks.

I'm still not sure whether this is a shared_ptr problem... One might argue
that the destructor of a mutable object is logically an "update" and
therefore should be mutex-protected as the rest of the operations on OBJ
unless it's guaranteed to not compete with them.

On the other hand placing a memory barrier in shared_ptr will probably be a
good thing from user point of view; but I don't want to make shared_ptr
depend on pthreads.

--
Peter Dimov
Multi Media Ltd.