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Message-ID: <CAJgzZorYJs4DgzYgWs9bhRj4mD6H1XO3qQb9WG33cTMMQrEjFQ@mail.gmail.com>
Date: Mon, 21 Sep 2020 08:47:50 -0700
From: enh <enh@...gle.com>
To: libc-coord@...ts.openwall.com
Cc: Florian Weimer <fweimer@...hat.com>, Libc-alpha <libc-alpha@...rceware.org>, gcc@....gnu.org,
libstdc++@....gnu.org
Subject: Re: Re: Expose 'array_length()' macro in <sys/cdefs.h>
or <sys/param.h>
Why would C++ programmers need this given
https://en.cppreference.com/w/cpp/iterator/size ?
On Mon, Sep 21, 2020, 05:54 Alejandro Colomar <colomar.6.4.3@...il.com>
wrote:
> [[ CC += libc-coord at lists.openwall.com ]]
>
> On 2020-09-21 12:33, Florian Weimer wrote:
> > * Alejandro Colomar:
> >
> >> [[
> >> CC += libc-coord at sourceware.org
> >> CC += gcc at gcc.gnu.org
> >> CC += libstdc++ at gcc.gnu.org
> >> ]]
> >>
> >> Hi Florian,
> >>
> >> On 2020-09-21 10:38, Florian Weimer wrote:
> >>> * Alejandro Colomar via Libc-alpha:
> >>>
> >>>> I'd like to propose exposing the macro 'array_length()' as defined in
> >>>> 'include/array_length.h' to the user.
> >>>
> >>> It would need a good C++ port, probably one for C++98 and another one
> >>> for C++14 or later.
> >>
> >> For C++, I use the following definition:
> >>
> >>
> >> #include <cassert>
> >> #include <sys/cdefs.h>
> >> #include <type_traits>
> >>
> >>
> >> #define is_array__(a) (std::is_array <__typeof__(a)>::value)
> >
> > Should be decltype.
>
> Thanks.
>
> >
> >> However, there are a few problems:
> >>
> >> 1) This doesn't work for VLAs (GNU extension).
> >> I couldn't find a way to do it. Maybe I should file a bug in GCC.
> >
> > I do not think VLA support is critical. C++ programmers will be used to
> > limited support in utility functions.
> >
> >> 2) Also, this requires C++11; I don't know how to do it for older C++.
> >> Again, support from the compiler would be great.
> >
> > I think limited C++98 support is possible using a function template,
> > where the array length N is a template parameter. To enable use in
> > constant expressions, you can return a type of char[N], and the macro
> > wrapper should then apply sizeof to the function result.
>
> Sorry, I don't know much C++, and I don't know how to do this.
>
> >
> >> 3) The macro can't be used in the same places as the C version,
> >> because of the `({})`.
> >> The `0 * sizeof(struct{...})` trick doesn't work in C++ due to:
> >> error: types may not be defined in 'sizeof' expressions
> >
> > For C++11, you can use a constexpr function instead of a macro.
> >
> > array_length should not be a macro in current C++ modes, so that we
> > retain compatibility if a future C++ standard adds array_length (or
> > nitems) on its own. This is not a concern for legacy C++98 mode.
>
> See above.
>
> >
> >>> Maybe also ask on the libc-coord list.
> >>
> >> Ok. Added CCs.
> >
> > libc-coord is not hosted on sourceware:
> >
> > <https://www.openwall.com/lists/libc-coord/2020/01/30/1>
> >
> > The discussion here veered off into C++ territory anyway.
>
> I added the correct list now.
>
> >
> > Thanks,
> > Florian
> >
>
> Thanks,
>
> Alex.
>
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