Re: [boost] [function] function wrapping with no exceptionsafetyguarantee

Subject: Re: [boost] [function] function wrapping with no exceptionsafetyguarantee
From: David Abrahams (dave_at_[hidden])
Date: 2010-10-20 21:41:49

At Wed, 20 Oct 2010 18:36:01 -0400,
Daniel Walker wrote:
> 
> On Wed, Oct 20, 2010 at 6:04 PM, David Abrahams <dave_at_[hidden]> wrote:
> > At Wed, 20 Oct 2010 16:51:50 -0400,
> > Daniel Walker wrote:
> >>
> >> First of all, boost::function can be in an empty state for two
> >> reasons: either it has not been assigned a target function or there
> >> has been a problem with the internal target management system.
> >
> > What kind of problem? Â Can you point to code that shows such a problem
> > being detected? Â Or are you merely saying, "somebody invoked UB
> > somewhere and it nulled out the pointer?" Â We don't need to support
> > the latter.
> 
> boost::function clones target function objects. So, there could be a
> failure allocating and/or copy-constructing the target. When this
> happens, vtable is set to null. You can see an example in operator= on
> line 783 of function_template.hpp. There may be other reasons that I'm
> not aware of which could lead to vtable being null. In principle,
> there is nothing in the current system guaranteeing that vtable is
> non-null at any arbitrary point in time.

Of course that happens; it's not a problem.  Instead of setting the
vtable to null it would be set to point to the empty-state function.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com