[PATCH][_Hashtable] Fix hash code cache usage

[Jonathan Wakely]

On Thu, 13 Mar 2025 at 11:51, Florian Weimer <fweimer@redhat.com> wrote:
>
> * Jonathan Wakely:
>
> > On Thu, 13 Mar 2025 at 09:24, Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * Jonathan Wakely:
> >>
> >> > On Thu, 13 Mar 2025 at 06:50, Florian Weimer <fweimer@redhat.com> wrote:
> >> >>
> >> >> * François Dumont:
> >> >>
> >> >> > +      // Get hash code for a node that comes from another _Hashtable.
> >> >> > +      // Reuse a cached hash code if the hash function is stateless,
> >> >> > +      // otherwise recalculate it using our own hash function.
> >> >> > +      __hash_code
> >> >> > +      _M_hash_code_ext(const __node_value_type& __from) const
> >> >> > +      {
> >> >> > +     if constexpr (__and_<__hash_cached, is_empty<_Hash>>::value)
> >> >> > +       return __from._M_hash_code;
> >> >> > +     else
> >> >> > +       return this->_M_hash_code(_ExtractKey{}(__from._M_v()));
> >> >> > +      }
> >> >>
> >> >> Does C++ support stateful hash functions?  I don't think so, and I don't
> >> >> see it documented as a GNU extension, either.
> >> >
> >> > It does, yes. That's why the hash function isn't required to be
> >> > default constructible, and has to be stored in the container and why
> >> > doing swap on two containers has to swap the hash functions as well.
> >>
> >> Interesting.  I have trouble reconciling this with the Cpp17Hash
> >> requirement that “The value” h(k) “shall depend only on the argument k
> >> for the duration of the program.”
> >
> >
> > That's for a given value of the type, h. For any two values of the
> > type h and h2, it's not required that h2(k) == h(k).
>
> But the wording for the comp object for ordered containers is very
> different and makes it clear that the instance matters.
>
> I still think this is more confusing than necessary.  If there isn't
> some general rule that empty objects can be considered stateless, that
> should be added somewhere, too. 8-)

Could you submit an issue (or two)?
https://cplusplus.github.io/LWG/lwg-active.html#submit_issue