Re: [boost] [variant2] documentation request

Subject: Re: [boost] [variant2] documentation request
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2019-03-07 11:58:58

śr., 6 mar 2019 o 17:39 Peter Dimov via Boost <boost_at_[hidden]>
napisał(a):

> Andrzej Krzemienski wrote:
> > I am interested in the latter ("Otherwise") case, so, going to emplace:
> >
> > template<size_t I, class... A>
> >   constexpr variant_alternative_t<I, variant<T...>>&
> >     emplace( A&&... a );
> >
> >    - Requires:
> >    I < sizeof(T…).
> >
> >    Effects:
> >    Destroys the currently contained value, then initializes a new
> > contained
> >    value as if using the expression Ti(std::forward<A>(a)…).
> >
> > This doesn't mention any double buffering. No tricks, no "monostate". It
> > reads as if the variant is left with no contained object whatsoever.
>
> Yes, we mentioned that upthread. At minimum, I'll need to add Remarks that
> on exception, the variant is left in a valid but unspecified state.
>
> Rigorously specifying the exact behavior of emplace will be rather
> verbose,
> and I'm not yet sure how to go about it.
>

There is a number of tricks to put back the variant to a non-empty state
after a throw, and you may wish to reserve the right to change them in the
future (and guarantee only that one of the Ts is stored with a valid but
unspecified value). However, it looks like you also want to guarantee some
smaller things:
* If one of the types is `monostate` you guarantee that upon throw, the
type stored is `monostate`.
* If at least one type satisfies the noexcept requirements you guarantee
that the size of variant<T...> is no more that the biggest of T plus the
discriminator.

Regards,
Andrzej