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Re: Concerning the necessity of include guards

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On 25/02/2020 13:50, Bonita Montero wrote:
>> I have used "exotic compilers" regularly over the years - as have a good
>> many people in this group.  I have maybe 10-15 different C compilers
>> installed on my machines, not counting different versions.  A couple
>> were bought on 5¼" floppies.  I don't use the older ones much, but
>> occasionally I need to maintain ancient projects - I keep them all
>> available.
> 
> Maybe years ago, but today almost any compiler supports #pragma once.
> 

You really have no idea, do you?  Most C programmers use one of the 
major compilers (which, as Wikipedia says and you accept without any 
personal experience, support "pragma once").  Most C compilers, on the 
other hand, probably don't support it.  That will include C compilers 
written or bought /today/ as well.  Most C compilers are small, niche 
products for unusual targets or unusual needs.

>> "Pragma once" is, at most, marginally more convenient than an include
>> guard - ..
> 
> With #pragma once theres no need to preprocess-parse the header for an
> #endif, so compiling is faster.

Big, serious compilers have particularly fast handling of include 
guards, and will not pre-process the file beyond matching up the #endif. 
  For #pragma once, the compiler has to make sure it is dealing with the 
same file - depending on how that is determined (since it can be handled 
in many ways - there is no standard here) that can mean generating and 
comparing hashes for the files.  It is unlikely that you will get any 
measurable difference either way.

The exception, perhaps, is MSVC - which (AFAIUI) handles "pragma once" 
measurably faster than include guards.  And even there I think this only 
applies to older versions of the compiler, meaning there is even less 
point is using non-standard and unreliable #pragma once.

(In practice, speed issues only apply to C++, since C headers are 
generally too small to be relevant in the timing.)

> 
>> guard - and comes with much vaguer semantics and lower portability.  If
>> MSVC (where "pragma once" is apparently noticeably faster than include
>> guards) is all you ever see ...
> 
> https://en.wikipedia.org/wiki/Pragma_once#Portability

Yes, I have seen it.  It covers a tiny fraction of the C compilers that 
people use (though it does cover the compilers that a large fraction of 
C programmers use).  But even if it is supported by the compiler, why 
use it?  Include guards are simple, standard, well-defined and efficient.

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Thread

Concerning the necessity of include guards Anton Shepelev <anton.txt@gmail.com> - 2020-02-24 22:08 +0300
  Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-24 20:28 +0100
  Re: Concerning the necessity of include guards Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-24 19:40 +0000
    Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-24 22:37 +0100
      Re: Concerning the necessity of include guards Anton Shepelev <anton.txt@g{oogle}mail.com> - 2020-02-25 14:13 +0300
        Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 13:03 +0100
          Re: Concerning the necessity of include guards Anton Shepelev <anton.txt@gmail.com> - 2020-02-29 01:14 +0300
            Re: Concerning the necessity of include guards Bart <bc@freeuk.com> - 2020-02-28 22:36 +0000
              Re: Concerning the necessity of include guards Anton Shepelev <anton.txt@gmail.com> - 2020-02-29 13:28 +0300
            Re: Concerning the necessity of include guards Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-02-29 00:08 +0000
            Re: Concerning the necessity of include guards Richard Damon <Richard@Damon-Family.org> - 2020-02-29 07:05 -0500
            Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-03-01 15:01 +0100
  Re: Concerning the necessity of include guards Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-02-24 14:52 -0500
    Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-24 21:19 +0100
      Re: Concerning the necessity of include guards Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-02-24 15:48 -0500
        Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-24 21:53 +0100
          Re: Concerning the necessity of include guards Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-02-24 17:26 -0500
            Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 00:08 +0100
              Re: Concerning the necessity of include guards Thiago Adams <thiago.adams@gmail.com> - 2020-02-24 15:24 -0800
                Re: Concerning the necessity of include guards Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-24 22:20 -0800
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 09:52 +0100
            Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-25 08:12 +0100
              Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 09:55 +0100
                Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-25 10:35 +0100
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 13:13 +0100
                Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-25 13:50 +0100
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 18:33 +0100
                Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-25 18:38 +0100
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 18:42 +0100
                Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-25 18:46 +0100
                Re: Concerning the necessity of include guards Bart <bc@freeuk.com> - 2020-02-25 19:35 +0000
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 21:18 +0100
                Re: Concerning the necessity of include guards Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-26 06:40 +0100
                Re: Concerning the necessity of include guards James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-25 12:12 -0800
                Re: Concerning the necessity of include guards James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-25 11:55 -0800
                Re: Concerning the necessity of include guards David Brown <david.brown@hesbynett.no> - 2020-02-25 21:37 +0100
                Re: Concerning the necessity of include guards scott@slp53.sl.home (Scott Lurndal) - 2020-02-26 02:39 +0000
                Re: Concerning the necessity of include guards Real Troll <real.troll@trolls.com> - 2020-02-25 23:10 -0400
                Re: Concerning the necessity of include guards chad <cdalten@gmail.com> - 2020-02-27 05:39 -0800
  Re: Concerning the necessity of include guards Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-02-24 22:23 +0000
  Re: Concerning the necessity of include guards Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-24 21:17 -0800
    Re: Concerning the necessity of include guards Guillaume <message@bottle.org> - 2020-02-25 13:03 +0100

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