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Subject: Re: [boost] Mother of all Variants
From: Jonathan Müller (jonathanmueller.dev_at_[hidden])
Date: 2019-04-16 09:28:40

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.

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