Re: [boost] [variant] Heads-up!

Subject: Re: [boost] [variant] Heads-up!
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2019-04-29 18:04:17

пн, 29 апр. 2019 г. в 20:44, Steven Watanabe via Boost <boost_at_[hidden]>:
>
> AMDG
>
> 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.

You can restore the old slow pre-rvalue era behavior by defining
BOOST_VARIANT_NO_RECURSIVE_WRAPPER_POINTER_STEALING.

-- 
Best regards,
Antony Polukhin