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

AMDG On 4/29/19 12:04 PM, Antony Polukhin wrote:

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

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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.

That doesn't make it okay. Look, this optimization would be fine with a different variant, one that doesn't provide the never-empty guarantee. That isn't boost::variant. This isn't some simple optimization. It's a major breaking interface change. In Christ, Steven Watanabe