On Thu, 13 Mar 2025 at 09:54, Jonathan Wakely <jwakely....@gmail.com> wrote:
>
> On Thu, 13 Mar 2025 at 09:24, Florian Weimer <fwei...@redhat.com> wrote:
> >
> > * Jonathan Wakely:
> >
> > > On Thu, 13 Mar 2025 at 06:50, Florian Weimer <fwei...@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).
In the original SGI STL, hash functions were required to be stateless:
https://www.boost.org/sgi/stl/HashFunction.html
I don't see any such requirement in C++11 and later, which added std::hash.
