Another solution would be pattern matching, if done right

variant< Bitmap_File, JPEG_File > object;

Pseudocode:

object match Image img => img.FlipHorizontally();

Image being either a concept or base clase.

On Fri, Sep 16, 2022 at 9:43 AM Lénárd Szolnoki via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> On 15 September 2022 22:57:46 BST, Frederick Virchanza Gotham via
> Std-Proposals <std-proposals_at_[hidden]> wrote:
> >On Thu, Sep 15, 2022 at 9:33 PM Zhihao Yuan <zy_at_[hidden]> wrote:
> >>
> >> I agree that variant<monostate, Cat, Dog>
> >> can be a model for
> >> variant<specify_base<Animal>, Cat, Dog>,
> >> but I also want variant<Cat, Dog> to
> >> implement an Animal interface.
> >> To summarize, I think
> >>
> >> nonnull
> >> nullable
> >> throwing
> >>
> >> All make sense.
> >>
> >> How about this:
> >>
> >> template<class T>
> >> struct implements
> >> {
> >> template<class... Ts>
> >> using model = std::variant<Ts...>;
> >> };
> >>
> >> template<class T>
> >> struct implements_nullable
> >> {
> >> template<class... Ts>
> >> using model = std::variant<std::monostate, Ts...>;
> >>
> >> static auto default_proxy() noexcept { return nullptr; }
> >> };
> >>
> >> template<class T>
> >> struct implements_or_throw
> >> {
> >> template<class... Ts>
> >> using model = std::variant<std::monostate, Ts...>;
> >> };
> >
> >
> >It isn't immediately apparent to me how you would use those. Something
> >like the following?
> >
> > implements<Animal>::model<Dog,Cat,Fish> obj;
> >
> > obj->Eat();
> >
> >The whole point of my proposal is to have this simple syntax:
> >obj->BaseClassMethod();
> >
> >I want it to all be inside "std::variant" so that the object can be
> >used in templates that expect an std::variant,
>
> I would prefer a "variant-like" concept for this, that defines the common
> interface between variant and your type.
>
> > and also so that the
> >"std::variant" object can be copied and it will still retain all of
> >its info (which wouldn't happen if we made a new class derived from
> >"std::variant").
>
> I didn't propose deriving from variant, bet even so, all the data members
> are in the variant, slicing wouldn't lose information here.
>
> Cheers,
> Lénárd
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