Re: [Boost-Users] shared_ptr and counted_base

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-09-26 14:55:39

From: "Bobby Thomale" <bobby-thomale_at_[hidden]>
> I was looking through the latest boost documentation, trying to find a way
> to get a shared_ptr from a this pointer of an object that already is
> contained by a shared_ptr somewhere, without knowing about that other
> shared_ptr out there.
>
> I noticed this intriguing comment in the latest docs:

[...]

> That looks like EXACTLY what I am looking for.  But a couple things aren't
> clear to me:
>
> 1.  Usage - The docs tell me its there but don't really say explicitly how
> to use it.  It looks like, to use this, all I have to do is subclass
> boost::counted_base in my class that is being shared.  Then I can just
call
> shared_from_this to get another shared_ptr out of myself and it should
just
> work.  If that is all I have to do, then awesome!  Is it really that easy
or
> is there another step I am missing?

Yes, it should be that simple, if you use only shared_ptrs; weak pointers to
counted_base-derived objects are a bit more complicated:

"> If you derive from counted_base, what happens with weak_ptr?
> It seems as though every live weak_ptr ends up keeping the object alive...

Weak pointers to intrusive counted objects require a bit of help from the
user. As the count and the object are inseparable, a weak_ptr will keep the
whole object alive. However, counted_base::dispose will be called when the
use count drops to zero and only weak pointers to the object remain. If the
object manages any resources, it should override dispose() and release them.
An important special case is that any shared_ptr members should be reset(),
or potential cycles won't be broken.

The current implementation has a defect since shared_ptr::reset() is not
guaranteed to never throw, but counted_base::dispose is. This will be fixed
in the next release."

> 2.  The word "experimental" above scares me.  :-)  If I use the
counted_base
> subclass and shared_from_this function, is that code going to break later
on
> if you guys change your minds?

I think that you'll be pretty safe. counted_base will most likely become
"official" in 1.30.