Re: [boost] intrusive_ptr design question

Subject: Re: [boost] intrusive_ptr design question
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2009-07-12 20:44:03

On Sunday, July 12, 2009, Ulrich Eckhardt <doomster_at_[hidden]> wrote:
> 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
>

It does work. I've used this technique before with some internally ref
counted that I inherited.

Tony