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

[Giuseppe D'Angelo]

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


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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