On Thu, 13 Mar 2025 at 09:58, Jonathan Wakely <jwakely....@gmail.com> wrote:
>
> 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.
All three of libstdc++, libc++ and MSVC STL support a stateful hash
function and use the value passed to the constructor:
https://godbolt.org/z/cqj4fG6jP
