Re: On endianness, many #ifdefs, and the need thereof

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: On endianness, many #ifdefs, and the need thereof
Date	2019-10-02 15:05 +0200
Organization	A noiseless patient Spider
Message-ID	<qn27ad$iro$1@dont-email.me> (permalink)
References	<f48b89a2-f66c-4a73-b8ca-c13e8fe90dd1@googlegroups.com> <qn26kg$1spc$1@adenine.netfront.net>

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On 02/10/2019 14:53, Juha Nieminen wrote:
> Daniel <danielaparker@gmail.com> wrote:
>> On the other hand, at least one library merely has
>>  
>> static constexpr bool little_endianess(int num = 1) noexcept
>> {
>>     return *reinterpret_cast<char*>(&num) == 1;
>> }
>>
>> referencing a contribution on stackoverflow, http://stackoverflow.com/a/1001328/266378.
>>
>> So why the many #ifdef's, if this is enough? Or is it enough?
> 
> Endianess cannot be resolved at compile time, no matter what you do, not
> even with modern C++. (Believe me, I have tried. It's not possible.)
> The only way to know endianess at compile time is if an external tool
> (such as some kind of configure script) provides you with a macro or
> constant that says so.
> 
> I haven't tried, but I believe the above example will not be evaluated
> at compile time (assuming it compiles at all, which I don't think it does;
> haven't tried, though).
> 
> There may be a technical reason why the compiler cannot provide
> endianess information at compile time.
> 

The /compiler/ can handle it at compile time - it can figure out the
result of this function at compile time and use that in optimisation.
(Compilers also usually have pre-defined macros for endianness - but
they are not standardised.)  The issue is that the language says this
can't be part of a constant expression.

I suspect the technical reason will be that constant expressions can't
access memory - they need to be independent of the state of the system.

What I don't really understand, is why endianness macros have never been
standardised.  gcc has __BYTE_ORDER__, __ORDER_LITTLE_ENDIAN__,
__ORDER_BIG_ENDIAN__.  Other compilers usually have similar macros.  It
would not have taken a great deal of effort to make __STDC__ equivalents
part of the C and C++ standards.

With C++20, we will have be able to write:

static constexpr bool little_endianess() noexcept
{
	return std::endian::native == std::endian::little;
}

and it will be a constant expression.

Thread

On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-02 04:00 -0700
  Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-02 13:26 +0200
    Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-02 08:59 -0700
  Re: On endianness, many #ifdefs, and the need thereof Öö Tiib <ootiib@hot.ee> - 2019-10-02 05:42 -0700
  Re: On endianness, many #ifdefs, and the need thereof Juha Nieminen <nospam@thanks.invalid> - 2019-10-02 12:53 +0000
    Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-02 15:05 +0200
      Re: On endianness, many #ifdefs, and the need thereof scott@slp53.sl.home (Scott Lurndal) - 2019-10-02 13:54 +0000
        Re: On endianness, many #ifdefs, and the need thereof Keith Thompson <kst-u@mib.org> - 2019-10-02 12:54 -0700
          Re: On endianness, many #ifdefs, and the need thereof scott@slp53.sl.home (Scott Lurndal) - 2019-10-02 20:16 +0000
        Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-03 09:48 +0200
    Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-02 07:52 -0700
      Re: On endianness, many #ifdefs, and the need thereof Öö Tiib <ootiib@hot.ee> - 2019-10-02 08:04 -0700
        Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-02 08:31 -0700
          Re: On endianness, many #ifdefs, and the need thereof Juha Nieminen <nospam@thanks.invalid> - 2019-10-02 21:32 +0000
      Re: On endianness, many #ifdefs, and the need thereof Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-10-03 11:33 +0000
        Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-03 13:46 +0200
          Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-03 05:56 -0700
            Re: On endianness, many #ifdefs, and the need thereof scott@slp53.sl.home (Scott Lurndal) - 2019-10-03 13:17 +0000
              Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-03 16:03 +0200
                Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-03 07:13 -0700
          Re: On endianness, many #ifdefs, and the need thereof Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-10-03 18:51 +0000
          Re: On endianness, many #ifdefs, and the need thereof Keith Thompson <kst-u@mib.org> - 2019-10-03 14:28 -0700
            Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-04 10:11 +0200
              Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-04 09:00 -0700
        Re: On endianness, many #ifdefs, and the need thereof Paavo Helde <myfirstname@osa.pri.ee> - 2019-10-03 14:46 +0300
        Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-03 05:40 -0700
  Re: On endianness, many #ifdefs, and the need thereof Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-02 15:10 +0200
  Re: On endianness, many #ifdefs, and the need thereof Barry Schwarz <schwarzb@dqel.com> - 2019-10-02 08:33 -0700
    Re: On endianness, many #ifdefs, and the need thereof David Brown <david.brown@hesbynett.no> - 2019-10-03 09:54 +0200
  Re: On endianness, many #ifdefs, and the need thereof "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-10-02 19:35 +0200
    Re: On endianness, many #ifdefs, and the need thereof Daniel <danielaparker@gmail.com> - 2019-10-02 15:39 -0700
  Re: On endianness, many #ifdefs, and the need thereof "Oliver S." <Follow.Me@gmx.net> - 2019-10-02 19:59 +0200

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