[PATCH v2] libstdc++: hashing support for chrono value classes (P2592R2)

[Tomasz Kaminski]

On Fri, Jun 6, 2025 at 9:07 AM Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
wrote:

> Hi Tomasz,
>
> Thank you for reviewing the original patch!
>
> I'm attaching a second version, hopefully addressing what you've
> highlighed. I've also pushed it on Forge:
>
> https://forge.sourceware.org/gcc/gcc-TEST/pulls/52

Posted review there, but major change is, for type that are wrappers over
integer:
  * use hash<Integer> to hash a value, instead of jus returning it
  * specialize __is_fash_hash as true for them.

>
>
>
> On 24/04/2025 15:30, Tomasz Kaminski wrote:
> > Hi,
> >
> > I am reattaching the original patch below, as I wasn't on the mailing
> > list when it was sent.
> > Thank you for submitting the patch and apologies for the late response.
> >
> > The major comment I have is that these are new C++26 classes, so we can
> > use requires __is_hash_enabled_for<_Tp> and define only enabled
> > specialization. In other cases we will be using a primary template that
> > is disabled.
> >
> > Also, argument_type and result_type typedefs are no longer present since
> > C++20, so you can remove them, and also remove __hash_base base classes
> > that were responsible for providing them.
>
> Sounds good to me, I've done this cleanup.
>
> > +
> >  > +    template<typename _Arg, typename... _Args>
> >  > +      static size_t __hash(const _Arg& __arg, const _Args&... __args)
> >  > +      {
> >  > + const size_t __arg_hash = hash<_Arg>{}(__arg);
> >  > + size_t __result = _Hash_impl::hash(__arg_hash);
> > ## COMMENT
> > I think you can implement this using fold expression, as:
> > (_Hash_impl::__hash_combine(hash<_Arg>{}(__args), __result), ...);
>
> Ok, I just wanted to make it work on C++11, but there's no real need for
> that I guess...
>
> >  > + __hash_combine(__result, __args...);
> >  > + return __result;
> >  > +      }
> >  > +  };
> >  > +#endif // C++11
> >  > +
>
> Thank you,
>
> --
> Giuseppe D'Angelo
>
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