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

Subject: Re: [boost] [variant] Heads-up!
From: Nikita Kniazev (nok.raven_at_[hidden])
Date: 2019-04-29 20:07:43

There seems to be misunderstanding of what the thread is about.

The short sum-up of the changes: Nothing changed for users that does not
use recursive variant. The changes were applied mainly for
recursive_wrapper, and affect recursive variant indirectly.

The PR was proposed as a solution for two major recursive variant problems:
* Move-only types support
* Poor move performance (because of this one Spirit has its own variant)

Most of the users are not affected by recursive_wrapper move constructor
change. To hit it you have to visit (either directly, or with applying
comparison operators) a recursive moved-out variant.

> have to agree with Steven. Never empty guarantee is a crucial difference
between Boost.Variant and std::variant.

std::variant has no recursive variant support. In the common field nothing
changed. See the comment above.

> If the one negative is adding UB to valid existing code at the next point
release, doesn't that outweigh any number of positive outcomes?

I do not think there even a single user that visits moved-out variants.
There is an assertation for such usage though.

> Why wouldn't this be an opt-in strategy - this property of moves has been
established for years now.

Because it solves the bigger problems and for much more users that it
potentially breaks.