Re: [boost] intrusive_ptr design question

Subject: Re: [boost] intrusive_ptr design question
From: Ulrich Eckhardt (doomster_at_[hidden])
Date: 2009-07-12 17:24:46

On Wednesday 08 July 2009 21:33:50 Neil Groves wrote:
> On Wed, Jul 8, 2009 at 8:21 PM, Ulrich Eckhardt <doomster_at_[hidden]> wrote:
> > On Monday 06 July 2009 17:23:41 Zachary Turner wrote:
> > > 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?
> >
> > Maybe it's already there: shared_ptr doesn't care if the passed deleter
> > actually deletes anything. If you now take an object with an existing
> > reference count, you just increment that count and pass the function that
> > decrements it as deleter function to a shared_ptr constructor. This
> > creates a parallel reference count, but it should be safe. Nothing can
> > pull the object from under the shared_ptr's feet because it holds a
> > reference. The shared_ptr doesn't destroy the object either, at most it
> > decrements the reference count.
>
> It isn't already there, because unfortunately, in general, this doesn't
> work. You need the reference count to increase when the shared pointer
> is copied, for example.

There is no "the reference count", but actually two of them. The internal 
reference count of the object doesn't increase but the external one, shared by 
the shared pointers. That external one counts how many shared pointers share a 
single internal reference.

 // first pointer, increases internal refcount
 intrusive_ptr<InternallyRefcounted> ptr1(raw_ptr);

 // increment refcount manually
 inc_refcount(raw_ptr);
 // second pointer, living on the above manual reference
 shared_ptr<InternallyRefcounted> ptr2(raw_ptr, &dec_refcount);

Reset either of ptr1 or ptr2. For the intrusive_ptr, it would decrement the 
internal reference count. For the shared_ptr, it would decrement its own 
reference count and if that reaches zero, it would also decrement the object's 
internal one as part of invoking the deleter.

Should work, unless I'm missing something. Also, I'm not 100% sure that this 
is what the OP is actually looking for.

Uli