Re: [Boost-users] [SmartPtr] enable_shared_from_raw

Subject: Re: [Boost-users] [SmartPtr] enable_shared_from_raw
From: Frank Mori Hess (fmh6jj_at_[hidden])
Date: 2014-07-24 18:50:59

On Thu, Jul 24, 2014 at 12:53 AM, Gavin Lambert <gavinl_at_[hidden]> wrote:
> Hi all,
>
> Firstly, any chance this could get some documentation?  Is the author still
> around?

I'm still around, although not super-active.  I'm personally neutral
towards the code and whether its use should be encouraged.  It has
some pitfalls for the unwary, but there also seems to be recurring
desire for the functionality.  Also, it doesn't need CRTP they way
enable_shared_from_this does.

> I'm thinking about using this at the moment for two separate reasons (and
> ways in which enable_shared_from_this seems to fall short):
>
> 1. shared_from_this can't be used from the constructor (requiring
> factory-method-init shenanigans); reportedly
> (https://groups.google.com/forum/#!topic/boost-list/8JWFZ56ry6k)
> shared_from_raw can, although I can't find specific details on the caveats
> mentioned.

The two dangers that come to mind are:

1) you throw out of your constructor after handing out
shared_ptrs/weak_ptrs, which reference an object which ultimately was
never constructed.
2) you call shared_from_raw, but never actually give ownership of the
object to an external shared_ptr.  This results in the object holding
a shared_ptr to itself, potentially indefinitely.

>
> 2. What I *really* want to be able to do is to create a child object (most
> commonly a callback functor) that contains a weak_ptr, and it seems silly
> (and badly performant) to use the usual factory-init pattern to call
> shared_from_this just to degrade it back to a weak_ptr.
>
> And it should be possible to obtain that weak_ptr before any shared_ptr
> exists, and "fix" it later to point at the real shared_ptr, especially if it
> is known that the weak_ptr will never actually be used until that real
> shared_ptr exists.
>
> enable_shared_from_raw almost seems to fit the bill (according to
> http://stackoverflow.com/questions/22184772/why-is-boostenable-shared-from-raw-so-undocumented),
> but I encountered the same problem as in this post
> (http://lists.boost.org/boost-users/2013/05/78833.php); it works with
> shared_from_raw but not weak_from_raw.
>
> However this change:
>   https://github.com/boostorg/smart_ptr/pull/8
> fixes that.  It looks like the two calls went down different code paths for
> some reason I can't fathom.

I don't remember what the rationale was for the current behavior of
weak_from_raw, or if I was the one who decided on that behavior.
However, it seems like the current behavior could be useful in that it
provides a way to query if an external shared_ptr ever actually took
ownership of the object (or at least shared_from_raw has been used,
which commits the coder to giving ownership to an external shared_ptr
at some point).