Re: [Boost-users] boost::weak_ptr and boost::intrusive_ptr

From: Steven T. Hatton (hattons_at_[hidden])
Date: 2006-12-01 23:37:14

On Friday 01 December 2006 16:15, Peter Dimov wrote:
> Nat Goodspeed wrote:
> > It seems possible to implement weak_ptr notification by building a
> > list of weak_ptr instances referencing a given object. If we don't
> > want that list to consume additional heap memory, the list could
> > itself be intrusive in the weak_ptr objects. If we want it to be
> > efficient, we build a doubly-linked list. (This may call for a
> > policy-based implementation so the consumer can decide which overhead
> > is least noxious.)
>
> This works. It's also completely independent of intrusive_ptr. You can
> implement such a weak pointer (and I know people who have done so) without
> ever using an intrusive_ptr or reference counting at all. The object
> destructor just goes over the list and zeroes the weak pointers. Threads
> are problematic and would probably require a per-object mutex.
>
> Another option is to use
>
> http://boost.org/libs/smart_ptr/sp_techniques.html#weak_without_shared
>
> which also doesn't require intrusive_ptr and works for any object, and
> handles threads nicely. But you pay for a shared_ptr control block, so it
> probably won't satisfy intrusive_ptr users.
>
> Both options are intrusive, but then, so is intrusive_ptr. :-)

My motivation for wanting the intrusive pointer is really design simplicity.  
For a tree I ran into problems trying share the pointer from within the 
pointee.  enable_shared_from_this didn't work in all cases, and could get 
downright confusing, even when it did work.  It also obviates one of the 
primary arguments from not using an intrusive pointer.  If I use it, I 
already have to open up the pointee to modification. The reason I want the 
weak pointer is simply to prevent access to an invalid pointer.  If I had an 
easy to use pair of intrusive/weak pointers, then I would just used them and 
not have to think about the details.