Re: Any interest in a weak pointer?

From: terekhov (terekhov_at_[hidden])
Date: 2002-01-11 09:34:05

--- In Boost-Users_at_y..., "Peter Dimov" <pdimov_at_m...> wrote:
[...]
> 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.

http://groups.google.com/groups?as_umsgid=3BCAAB99.1157302E%40web.de

"> If you need to access the object to clean it up before deleting it,
 > you can lock it, to make sure you have the latest value. Do not 
assume
 > that just because the refcount is zero, you don't have to lock.

 Yeah, that would solve the visibility problem for 
 cleanup of mutable objects. However, with a lock 
 protecting more than one object we could still 
 block -- which would cause an unnecessary decrease 
 in concurrency, increased contention. Also, the 
 memory access reordering constrain (MBAR) injected 
 by unlock seems to be absolutely redundant to me 
 in this situation (and we would need unlock even 
 in the situation with a dedicated lock per object, 
 to ensure that lock destruction would not fail 
 with e.g. EBUSY, AFAIK)."

> On the other hand placing a memory barrier in shared_ptr will 
probably be a
> good thing from user point of view; 

I think so too, but how about portability...
also, given rather close to NULL amount of interest 
(wrt the proposal for having portable interface
specifically for ref.counting) I've observed on c.p.t, 
I am not sure at all... maybe I am missing something 
subtle and the whole idea of a single "RMB" might be 
a BS... I do not know, really.

> but I don't want to make shared_ptr depend on pthreads.

Yeah, pthreads is not a boost library, but other than
that argument and perhaps some licensing issues wrt LGPL 
nature of pthreads-win32, I do not see why boosters just 
ignore pthreads. It is a standard and it is available on 
almost every platform out there (I mean basic threading 
stuff).

regards,
alexander.