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

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Message-ID: <87k0wmyvtt.fsf@oldenburg2.str.redhat.com>
Date:   Tue, 22 Sep 2020 12:01:02 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Jonathan Wakely <jwakely@...hat.com>
Cc:     Alejandro Colomar <colomar.6.4.3@...il.com>,
        Ville Voutilainen <ville.voutilainen@...il.com>,
        gcc@....gnu.org, libstdc++ <libstdc++@....gnu.org>,
        Libc-alpha <libc-alpha@...rceware.org>,
        LKML <linux-kernel@...r.kernel.org>,
        libc-coord@...ts.openwall.com
Subject: Re: Expose 'array_length()' macro in <sys/param.h>

* 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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