Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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>
References: <946e9377-0558-3adf-3eb9-38c507afe2d0@gmail.com>
        <874knr8qyl.fsf@oldenburg2.str.redhat.com>
        <dbcf5c85-c468-72f8-0f83-92ec2a6a2991@gmail.com>
        <875z875si2.fsf@oldenburg2.str.redhat.com>
        <20200921140100.GA449323@redhat.com>
        <e734429a-d543-7e75-48e9-a8297a94b035@gmail.com>
        <20200921220443.GP6061@redhat.com>
        <CAFk2RUbEtvgFb_FZmcM9L4-g1kG_E7S2p9gveM0Z5Fe=zEDm9w@mail.gmail.com>
        <34bae433-f67e-877d-f871-928cb55d2a87@gmail.com>
        <20200922094020.GS6061@redhat.com>
Date:   Tue, 22 Sep 2020 12:01:02 +0200
In-Reply-To: <20200922094020.GS6061@redhat.com> (Jonathan Wakely's message of
        "Tue, 22 Sep 2020 10:40:20 +0100")
Message-ID: <87k0wmyvtt.fsf@oldenburg2.str.redhat.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

* 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
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill