Re: [boost] Mother of all Variants

Subject: Re: [boost] Mother of all Variants
From: Richard Hodges (hodges.r_at_[hidden])
Date: 2019-04-16 21:56:59

Are there real-world examples of well-designed classes who's move
assignment operators throw?

I think if I saw that in a code review I'd be inclined to demand a
different design choice.

Is there a good reason that variants should support such classes?

On Tue, 16 Apr 2019 at 17:42, Robert Ramey via Boost <boost_at_[hidden]>
wrote:

> On 4/16/19 5:41 AM, Larry Evans via Boost wrote:
> > On 4/16/19 4:28 AM, Jonathan Müller via Boost wrote:
> >> On 16.04.19 03:51, Robert Ramey via Boost wrote:
> >>> I'm way out of my depth in the variant discussion.  Seems to me it
> >>> revolves around all the trade offs regarding design choices.  Could
> >>> we perhaps decide not to decide.  Suppose we create a type which
> >>> looks like:
> >>>
> >>> template<
> >>>    bool BasicGuaranteeSupported,
> >>>    bool StrongGuaranteeSupported,
> >>>    bool EmptyStateProhibited,
> >>>    bool AssignmentSupported,
> >>>    bool MoveSupported,
> >>>    class T ...
> >>>  >
> >>> struct mother_of_variants{
> >>>    // some trivial mp11 code
> >>> };
> >>>
> >>
> >> I did something similar back when I did type_safe::variant: You have a
> >> `VariantPolicy` that controls whether the variant can be empty and a
> >> has a `change_value()` function that switches the type in whatever way
> >> it seems fit.
> >>
> >> With that I have a `rarely_empty_variant` that is like `std::variant`,
> >> a `never_empty_variant` that assumes move is no-throw (and calls
> >> terminate() if they throw) and an `optional_variant` that fully
> >> embraces an empty state.
> >>
> >> At the time I thought it was a good trade-off, but haven't
> >> re-evaluated it since then.
> >>
> >
> > I like this; however, years ago there was some argument
> > about how providing such a policy for smart pointers
> > would be confusing to users.  However, I didn't really
> > agree with that argument and the alternative is, AFAICT,
> > not much better:
> >    std::variant, or boost::variant2 or libx::variant3 or ...
>
> Right.  We could have it both ways.  That is the mother of all variants
> could be used to generate a custom type given the requirements.  For
> those who don't want/need to delve into all the details, we provide a
> list of specialization for commonly used variants which would probably
> address 99.9% of users needs.
>
> Robert Ramey
>
>
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-- 
Richard Hodges
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