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

Subject: Re: [boost] [variant] Heads-up!
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2019-04-29 19:05:40

On 4/29/19 9:04 PM, Antony Polukhin via Boost wrote:
> 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.

I would prefer to avoid config macros, especially if they affect ABI or 
can cause ODR issues.

I have to agree with Steven. Never empty guarantee is a crucial 
difference between Boost.Variant and std::variant. Its consequences may 
be unfortunate for the move support, but that is that and one has to 
bite the bullet if he wants the guarantee. If you want something else - 
use a different variant.