Re: [boost] Outcome/expected/etc/etc/etc

Subject: Re: [boost] Outcome/expected/etc/etc/etc
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-06-05 16:19:32

2017-06-05 18:08 GMT+02:00 Peter Dimov via Boost <boost_at_[hidden]>:

> Gottlob Frege wrote:
>
> It just means I need to double buffer higher up.  I really want my drawing
>> program to just fail (rollback) the transaction if it can't complete - that
>> is what users really want.
>>
>
> That's true, I understand that, and I tried to communicate the need for
> variant to support the strong guarantee on assignment for this reason.
>

So it looks like there are three choices for variant:
1. valueless_by_exception
2. with never-empty guarantee
3. with strong guarantee

If you implement variant2 with only never-empty guarantee, you will still
leave a group of people unsatisfied with either std::variant or variant2.
Maybe you should just go for the strong guarantee, given that you are
potentially doing double buffering anyway?

>
> Normally, if your type was, say, vector<>, you don't need all vectors to
> be double-buffered. You do
>
>    vector<> tmp( v );
>    process( tmp );
>    v.swap( tmp ); // commit transaction
>
> at the places where you want the strong guarantee.
>

Interestingly, this is also the case for `optional`: swap may throw.

>
> The problem with variant<> is that the above doesn't quite produce the
> strong guarantee when the types aren't nothrow move constructible. I also
> tried to provide a solution to this problem in the form of pilfering,
> because types can typically be pilfered even when they can't be
> nothrow-moved-from, but this didn't go anywhere.
>

Didn't go anywhere in LWG; but maybe this idea can be tested in Boost.

Regards,
&rzej;