Re: [Boost-users] Thread Safety of shared_ptr

Subject: Re: [Boost-users] Thread Safety of shared_ptr
From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2011-05-04 04:39:16

On Wed, May 4, 2011 at 12:17 AM, Cory Nelson <phrosty_at_[hidden]> wrote:

> On Tue, May 3, 2011 at 12:59 PM, Ovanes Markarian
> <om_boost_at_[hidden]> wrote:
> >
> >
> > On Tue, May 3, 2011 at 2:18 PM, Peter Dimov <pdimov_at_[hidden]> wrote:
>
> >
> > Peter,
> > can you please give an example how that can happen, given the
> shared_counter
> > is atomic. I thought the counter is first incremented than the rest of
> the
> > machinery deals with pointer copying etc. and in case of underlying
> object
> > destruction, the counter is first decremented and than the object is
> > destroyed. How is that possible, that shared_counter reaches 0,
> destructor
> > is called and afterwards the underlying pointer will be assigned to the
> new
> > shared instance?
> > With Kind Regards,
> > Ovanes
> >
>
> He means that while the reference count is thread safe, the pointer is
> not.  Here's an example that will cause problems:
>
> Thread A: ptr1 = ptr2;
> Thread B: ptr2 = ptr;
>
> Note that ptr2 is being concurrently read and modified.
>

Oh, I see. But this looks for me as the wrong usage of shared_ptr. ptr2
should not be passed to Thread A by reference, but should be copied into the
Thead A's context. Otherwise I can't imagine how this can happen. The same
can happen in the single threaded environment if one uses references to
shared_ptr instances.

Best Regards,
Ovanes