Re: [boost] [review][variant2] Variant2 Review Starts April 1

Subject: Re: [boost] [review][variant2] Variant2 Review Starts April 1
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2019-03-26 18:10:12

On Tue, Mar 26, 2019 at 2:03 PM Larry Evans via Boost
<boost_at_[hidden]> wrote:
>
> On 3/26/19 12:08 PM, Andrzej Krzemienski via Boost wrote:
> [snip]
> >> As long as there are non-trivial trade-offs for any type/template, there
> >> will be reasonable users who would opt for different trade-offs when in
> >> different domains. We just have to decide which combinations of the
> >> trade-offs are worth collecting into a facility that is present in boost or
> >> the standard or elsewhere.
> >>
> >
> > True. And maybe names of different variants of variant should reflect the
> > different trade-offs. Maybe noempty::variant rather than
> > variant2::variant?
> >
>
> What about:
>
>    template<typename... T>
>    struct boost::variant2<T...>: std::variant<T...>
>    { ... };
>
> ?
>
> After all, isn't boost::variant2<T...>
> simply more restrictive than std::variant<T...> in that
> boost::variant2 has the
> the never-valueless requirement
> but std::variant does not.

That cannot work for a variety of reasons. Even if it were possible to
implement it that way and we were okay with a dependence on the
standard library having std::variant, it also would mean that
interactions with the variant via the base class would be able to
break the child class's never-empty guarantee (meaning it would not be
a guarantee). So an inheritance relationship here would not make
sense.

-- 
-Matt Calabrese