Re: A final word on casting.

From	Ike Naar <ike@iceland.freeshell.org>
Newsgroups	comp.lang.c
Subject	Re: A final word on casting.
Date	2018-12-24 07:56 +0000
Organization	A noiseless patient Spider
Message-ID	<slrn3nhoq2148v.5ra.ike@iceland.freeshell.org> (permalink)
References	<af569436-b2e1-492b-8379-9f927013c590@googlegroups.com> <i8nv1ehts9uvoeef9oo3n6ipa4ka2skcsk@4ax.com>

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On 2018-12-23, Barry Schwarz <schwarzb@dqel.com> wrote:
> That being said, there are cases where a cast is essential.  
>
> One example is in the compare function that is passed to qsort.  The
> function takes two constant pointers to void, const void*.  The
> function needs to compare the two objects those pointers point to. For
> sake of discussion, assume the two objects are elements of an array of
> int and the pointers are named p1 and p2.  You cannot code
>      if (p1 < p2)
> because that would compare the addresses of the objects which is not
> what you want.  You cannot code
>      if (*p1 < *p2)
> because you cannot dereference a void*.  Even if you could, how would
> the compiler know you were comparing int values and not double values?
> So you need to copy the values of p1 and p2 into int pointers (named
> i1 and i2).  You cannot code
>      int *i1 = p1;
> because that would remove the const attribute.  Similarly you cannot
> code
>      const int *i1 = p1;
> because there is no implicit conversion from const void* to const
> int*.  So you are essentially forced to code
>      const int *i1 = (const int*)p1;
> or the slightly less rigorous
>      int *i1 = (int*)p1;
> prior to performing the comparison or something equivalent such as
>     if (*(int*)p1 < *(int*)p2;

It can be done without casts:

  int const * const pi1 = p1, * const pi2 = p2;
  if (*pi1 < *pi2)

Thread

A final word on casting. chanrasjid@gmail.com - 2018-12-23 03:02 -0800
  Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 12:11 +0000
    Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 13:08 +0000
      Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 14:43 +0000
        Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 14:56 +0000
          Re: A final word on casting. chanrasjid@gmail.com - 2018-12-23 07:08 -0800
            Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 15:55 +0000
              Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 16:07 +0000
              Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 08:21 -0800
              Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 17:56 +0000
                Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 19:23 +0000
                Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 20:12 +0000
                Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 14:06 -0800
                Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:34 -0800
                Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-28 20:29 +0000
                Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 15:07 -0800
                Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-28 20:56 -0500
                Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-29 09:22 -0800
                Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2019-01-01 13:27 -0800
                Re: A final word on casting. Bart <bc@freeuk.com> - 2019-01-02 01:07 +0000
                Re: A final word on casting. Bart <bc@freeuk.com> - 2019-01-03 18:55 +0000
                Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2019-01-03 22:27 -0500
                Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-01-04 16:25 -0500
                Re: A final word on casting. Ike Naar <ike@iceland.freeshell.org> - 2018-12-28 19:10 +0000
                Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-28 12:33 -0800
                Re: A final word on casting. "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-23 14:11 -0800
                Re: A final word on casting. Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-12-24 02:54 -0800
                Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:24 -0800
                Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:29 -0800
                Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-24 03:45 -0800
                Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-24 13:44 +0000
                Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-24 08:51 -0500
              Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-23 14:53 -0500
                Re: A final word on casting. Robert Wessel <robertwessel2@yahoo.com> - 2018-12-24 09:39 -0600
                Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-24 13:38 -0500
              Re: A final word on casting. Manfred <noname@add.invalid> - 2018-12-23 21:22 +0100
              Re: A final word on casting. Manfred <noname@add.invalid> - 2018-12-23 21:33 +0100
            Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 10:56 -0500
              Re: A final word on casting. Sjouke Burry <burrynulnulfour@ppllaanneett.nnll> - 2018-12-23 17:18 +0100
        Re: A final word on casting. Bart <bc@freeuk.com> - 2018-12-23 20:03 +0000
    Re: A final word on casting. "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-28 14:47 -0500
  Re: A final word on casting. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 12:24 +0000
  Re: A final word on casting. Richard Damon <Richard@Damon-Family.org> - 2018-12-23 07:28 -0500
    Re: A final word on casting. Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 07:50 -0800
  Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:35 -0800
    Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:42 -0800
      Re: A final word on casting. fir <profesor.fir@gmail.com> - 2018-12-23 04:52 -0800
  Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-23 10:28 -0500
  Re: A final word on casting. Barry Schwarz <schwarzb@dqel.com> - 2018-12-23 11:39 -0800
    Re: A final word on casting. Ike Naar <ike@iceland.freeshell.org> - 2018-12-24 07:56 +0000
  Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 14:03 -0800
    Re: A final word on casting. Barry Schwarz <schwarzb@dqel.com> - 2018-12-24 14:39 -0800
      Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 14:50 -0800
        Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 15:34 -0800
      Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 15:23 -0800
        Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 20:34 -0800
    Re: A final word on casting. Keith Thompson <kst-u@mib.org> - 2018-12-24 14:52 -0800
      Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 20:25 -0800
    Re: A final word on casting. James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-24 17:52 -0500
    Re: A final word on casting. luser droog <luser.droog@gmail.com> - 2018-12-24 21:27 -0800
      Re: A final word on casting. luser droog <luser.droog@gmail.com> - 2018-12-24 21:39 -0800
      Re: A final word on casting. chanrasjid@gmail.com - 2018-12-24 22:36 -0800

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