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

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

пн, 29 апр. 2019 г. в 20:44, Steven Watanabe via Boost :

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AMDG

On 4/29/19 11:30 AM, Antony Polukhin via Boost wrote:

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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.