Re: Expose 'array_length()' macro in <sys/param.h>

From: Florian Weimer <fweimer@redhat.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: Alejandro Colomar <colomar.6.4.3@gmail.com>,
	Ville Voutilainen <ville.voutilainen@gmail.com>,
	gcc@gcc.gnu.org, libstdc++ <libstdc++@gcc.gnu.org>,
	Libc-alpha <libc-alpha@sourceware.org>,
	LKML <linux-kernel@vger.kernel.org>,
	libc-coord@lists.openwall.com
Subject: Re: Expose 'array_length()' macro in <sys/param.h>
Date: Tue, 22 Sep 2020 12:01:02 +0200	[thread overview]
Message-ID: <87k0wmyvtt.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20200922094020.GS6061@redhat.com> (Jonathan Wakely's message of "Tue, 22 Sep 2020 10:40:20 +0100")

* Jonathan Wakely:

> I don't see much point in using std::size here. If you're going to
> provide the alternative implementation for when std::size isn't
> defined, why not just use it always?
>
> template<typename _Tp, std::size_t _Num>
> #if __cplusplus >= 201103L
> constexpr
> #endif
> inline std::size_t
> __array_length(const _Tp(&)[_Num]) __THROW
> {
>   return _Num;
> }
>
> This only requires <cstddef>, not <iterator>.

I agree that this is an advantage.  But the version without constexpr is
not sufficient because __array_length does not produce a constant
expression.

I've seen something like this used instead:

  template<typename _Tp, std::size_t _Num>
  char (&___array_length(const _Tp(&)[_Num]))[_Num];
  #define __array_length(v) (sizeof(___array_length(v)))

If the function type is too cute, a helper struct could be used instead.

Thanks,
Florian
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