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Subject: Re: [boost] [variant] Heads-up!
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2019-04-29 20:23:07

On Mon, Apr 29, 2019 at 1:44 PM Steven Watanabe via Boost
<boost_at_[hidden]> wrote:
> On 4/29/19 11:30 AM, Antony Polukhin via Boost wrote:
> >
> > I've merged a very cool optimization by Nikita Kniazev into the master
> > branch. From now on boost::variant does pointer stealing for recursive
> > variants.
> >
> > This significantly improves the performance of the variants move constructors.
> >
> > However if you use a variant variable after the std::move for anything
> > except destruction and assignment then you're getting an UB. Beware!
> >
> boost::variant goes to great lengths to prevent
> exactly this situation. You just broke it. This
> change is unacceptable. Please revert it. This
> optimization can be used iff. you have a way to
> construct a valid object in the rhs.

Stressing that, as Steven points out, this wouldn't violate the
never-empty guarantee *if* after stealing you were able to
default-construct something on the right-hand side and update the
discriminator accordingly -- similar to what is done in other parts of
variant. I think that if people want an alternative to this that is
closer to what the change does, we'd probably want a template
different from recursive_wrapper that explicitly has an empty state,
leaving recursive_wrapper and variant's invariants intact. In other
words, an additional template that is just an `optional` equivalent
that always uses dynamic storage (at least for types that can be
incomplete). This would be a different type from recursive_wrapper so
as to not weaken the invariants of recursive_wrapper, but it could
still be a type that variant knows about in much the same way, if that
were to prove desirable. Such an optional-equivalent would be more
generally useful anyway.

-Matt Calabrese

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