Frederick,

I also failed to mention Herb Sutter's metaclasses.
https://herbsutter.files.wordpress.com/2017/07/p0707r1.pdf
vtables can be built by hand but there is a lot of boiler plate though one
done their awesome
Herb showed that with reflection and once it matures to a point vtables can
be built with a library that looks and feels as if it was a language
feature.
In short, without an explicit language feature, it is only a matter of time.
Give it another 25 years on top of the existing 25 years.
In the meantime, I'll keep using https://github.com/boost-ext/te,
function_ref, move_only_function and copyable_function.


On Tue, Jan 16, 2024 at 2:45 AM Frederick Virchanza Gotham via
Std-Proposals <std-proposals_at_[hidden]> wrote:

> On Mon, Jan 15, 2024 at 1:57 AM Thiago Macieira via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
> > then the vtable linker symbol is
> > > '_ZTV5Donkey'.
> >
> > And this one is _ZTV6Donkey.
>
>
> Another possibility would be to get the vtable pointer from the
> 'std::type_info', as follows:
>
> template<class Class> requires std::is_polymorphic_v<Class>
> void (*const *GetVtable(void))(void)
> {
> // The type_info struct for T is located
> // right after the vtable for T.
> // The vtable for T contains a pointer to the type_info.
> // So take the address of the type_info struct,
> // and keep decrementing and dereferencing it until we
> // see the address of the type_info struct -- the vtable
> // is right after it.
>
> static constexpr void const *pv = &typeid(Class);
> static constexpr void const *const *ppv = static_cast<void
> const *const *>(pv);
>
> for ( void const *const *p = ppv; ; --p )
> {
> if ( pv != *p ) continue;
>
> return
> static_cast<void(*const*)(void)>(
> static_cast<void const*>(
> static_cast<char const*>(
> static_cast<void const*>(p)
> ) - 0xe8
> )
> );
> }
> }
>
> I have it working here: https://godbolt.org/z/fe9Wd5EW1
>
> I wonder if there will come a point when we should mention vtables in
> the Standard? We don't have to call them vtables, maybe just call them
> "polymorphic facilitators" or some other generic term that leaves the
> door open for compiler implementers, and then we could get the vtable
> pointer from the 'type_info' as follows:
>
> namespace std {
> class type_info {
> void const *get_polymorphic_facilitator(void) noexcept;
> };
> }
>
> Let it return a nullptr if !std::is_polymorpic_v<T>.
> --
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>