> On Jan 31, 2022, at 1:10 PM, Peter Dimov <pdimov_at_[hidden]> wrote:
>
> Daveed Vandevoorde wrote:
>>> In Boost.Describe, I support a "modifier" mod_inherited, which
>>> includes the inherited members in the "members_of" list. I don't
>>> currently support it for the base classes, but I could. So when you
>>> have
>>>
>>> struct B {};
>>> struct D1: B {};
>>> struct D2: B {};
>>> struct C: D1, D2 {};
>>>
>>> and you say describe_bases<C, mod_any_access | mod_inherited>, you
>>> would get a list of base class descriptors [desc(D1), desc(B), desc(D2),
>> desc(B)].
>>
>> Okay, so like the alternative API I described where you also get indirect bases.
>> Do the descriptors maintain information about the derivation path?
>
> Not at the moment.
>
> In a prototype of mine (before the official Describe) I had base descriptors
> have something like
>
> struct Desc
> {
> static constexpr auto pointer = [](C* p) -> B*
> {
> return (B*)(D1*)p;
> };
> };
>
> with the derivation path implicitly encoded in the lambda. But in Describe
> I don't provide `pointer` for base descriptors at all, having, err, postponed it.
>
> A similar issue actually exists for inherited members. If you have
>
> struct B { int m; };
> struct D1: B {};
> struct D2: B {};
> struct C: D1, D2 {};
>
> and you get the all members list for C, including inherited, that would
> be two descriptors for B::m. At the moment in Describe both of these have
>
> struct Desc
> {
> static constexpr auto pointer = &B::m;
> };
>
> but that's wrong; they should actually have two different member pointers
> of type `int D::*`, one pointing to the first `m`, other to the second.
>
> I don't think this is achievable without compiler support, though.
> Both &C::D1::m and &C::D2::m give &B::m.

Right. Maybe if you had something like:

 template<auto PM, typename D, typename … Path> struct X {
   static constexpr auto pointer = PM;
   static auto select(D *ptr) { … }
 }

where the “select” operation would apply casts down Path and then "pointer” could select from that?

Seems convoluted, admittedly.


>
>> IIRC, in the context of P1240 someone once suggested that we should be able
>> to write something like:
>>
>> D *p;
>> auto *pb = p->[:bases_of(^D)[0]:];
>>
>> (assuming D has a base class). That’s similar to your suggestion of a more
>> general pointer-to-member value concept, but it has the advantage of
>> generalizing to bit fields as well.
>
> That would indeed be a way to do it.

Okay, I’ll se if we can pursue that possibility then.

 Daveed