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Subject: Re: [boost] intrusive_ptr design question
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-07-06 11:50:59

Zachary Turner wrote:
> On Mon, Jul 6, 2009 at 10:32 AM, Stewart,
> Robert<Robert.Stewart_at_[hidden]> wrote:
> > Zachary Turner wrote:
> >>
> >> Ok, so let me rephrase then: Is there a case to be made for
> >> a more flexible shared_ptr (either through modification or
> >> through a new class addition) that allows one to provide
> >> custom reference counting semantics much the same way that
> >> shared_ptr allows one to provide custom deletion semantics?
> >
> > I haven't followed your discussion closely, so I may have the
> > wrong idea, but don't you run the risk of mixing the
> > different ref counting techniques on objects of the same
> > type? If the ref counting scheme isn't part of the smart
> > pointer type, then all instances must do ref counting the
> > same way.
> That's correct. The way I have envisioned is analagous to the
> way shared_ptr allows you to specify a custom deleter.
> Whenever a copy is made, the new copy gets the same deleter.
> That way all copies agree on the deletion semantics. As long
> as all copies of the shared pointer that refer to the same
> underlying instance of pointed-to object agree on reference
> counting semantics, everything should be ok right?

Deleting with the wrong deleter is an obvious failure which is likely to crash an application, but only happens once per object. Manipulating a reference count, tens, hundreds, or more times, exposes a much greater danger. Yes, copies will agree on how to manipulate the reference count, but the same memory can be referenced by separately constructed, non-copied smart pointer instances.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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