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

On Mon, Apr 29, 2019 at 4:23 PM Matt Calabrese wrote:

On Mon, Apr 29, 2019 at 1:44 PM Steven Watanabe via Boost wrote:

...

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.

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

Continuing on this thought, you don't necessarily have to default-construct on the right-hand side to satisfy the never-empty guarantee here. If the left-hand side also happens to currently contain a recursive_wrapper, or something that is nothrow-move-constructible, it can be moved into or swapped with rhs. -- -Matt Calabrese