Re: Is this really necessary

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c++
Subject	Re: Is this really necessary
Date	2021-06-07 09:43 -0700
Organization	A noiseless patient Spider
Message-ID	<86pmwxd3qq.fsf@linuxsc.com> (permalink)
References	(5 earlier) <s9iqno$ajd$1@dont-email.me> <87r1hevbc8.fsf@nosuchdomain.example.com> <aFevI.570005$ST2.277494@fx47.iad> <87y2bmphoo.fsf@nosuchdomain.example.com> <KWfvI.25213$v01.23286@fx07.iad>

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Richard Damon <Richard@Damon-Family.org> writes:

> On 6/6/21 9:49 PM, Keith Thompson wrote:
>
>> Richard Damon <Richard@Damon-Family.org> writes:
>>
>>> On 6/6/21 7:10 PM, Keith Thompson wrote:
>>>
>>>> Paavo Helde <myfirstname@osa.pri.ee> writes:
>>>>
>>>>> 05.06.2021 16:06 Bonita Montero kirjutas:
>>>>>
>>>>>>> No.  Subtraction of pointers is defined as the difference in
>>>>>>> their indexes within a single array.
>>>>>>
>>>>>> That coudn't be true because you can cast any pointer-pair to
>>>>>> char *, subtract them and use the difference for memcpy().
>>>>>
>>>>> This holds only for linear memory model.  While this is a
>>>>> dominant memory model nowadays, the C++ language is old enough
>>>>> to take also other memory models (like segmented ones) into
>>>>> account.  In segmented memory models, pointer arithmetic only
>>>>> works in a single segment, and accordingly the arrays are
>>>>> limited to a single segment.  There is no such limitation for
>>>>> struct members.
>>>>
>>>> I believe there is.  Without that limitation, the offsetof macro
>>>> wouldn't work.
>>>>
>>>>> As an example, with Intel 386 you could have a 16-bit program
>>>>> working simultaneously with at least 4 different 64 kB segments,
>>>>> which might have been fully separate in the physical memory.
>>>>> Good luck with forming a difference of pointers in a 16-bit
>>>>> size_t variable when the segments are more than 64kB separate in
>>>>> the physical memory!
>>>>
>>>
>>> Actually, one reason offsetof is a Standard Macro so it can be
>>> made to use implementation dependent tricks to make it work here.
>>>
>>> The implementation has enough information to handle a bigger than
>>> a segment structure if it wanted to.  It might only be able to
>>> make it work easily for 'real' mode where segments offset from the
>>> current segment can just be computed, or it might need special
>>> support from the OS to make multiple overlapping segments to build
>>> a net address space bigger than one segment long.
>>>
>>> If you reference an member of the structure whose offset is big
>>> enough that it won't fit in the first segment of the structure,
>>> the implementation just needs to make a segment offset from the
>>> start of the object that does hold that full member.
>>>
>>> I will say that I never heard of a compiler that did that for a
>>> structure, there were implementation that did that for specified
>>> arrays, but pointers to elements of that array had a non-standard
>>> type to keep track of the fact that segment updates might be
>>> needed for pointer arithmetic.  These weree __huge__ arrays, with
>>> __huge__ pointers.
>>
>> Sure, and it can do the same thing for arrays.
>>
>> Both array objects and struct objects has to *act like* they occupy
>> a contiguous range of memory addresses.  If the implementation has
>> to play some tricks to make it act that way, that's fine.  And if
>> it restricts object to a single segment, that's fine too (as long
>> as the segment size is big enough -- 65535 bytes for hosted
>> implementations, C99 and later).
>>
>> And if an implementation plays tricks for arrays but not for
>> structures, that's probably fine too.  The standard doesn't
>> mention segments.
>
> The problem with doing it for arrays is that either you need to do
> it for ALL pointers, or you need to make big array use a special
> syntax.
>
> The key thing for structs is that the pointer to the big structure
> is already a special type, so is easy to recognize without cost to
> other code.
>
> In essence, you can treat a BigStruct* pointer special when you
> apply the member selection operation to it.
>
> Given a int* pointer into a big array, you don't know if it IS a
> big array, unless you pessimistically do for all, or make big
> arrays an extension that creates a special type of pointer that
> needs a non-standard type definition (like __huge__)

In C a pointer to any object can be converted to unsigned char *
which then can be used as though it were pointing into a
character array whose extent covers the entire object.

Do you have some reason to believe that property does not apply
to C++?

Thread

Is this really necessary Bonita Montero <Bonita.Montero@gmail.com> - 2021-06-05 12:53 +0200
  Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 14:19 +0200
    Re: Is this really necessary Bonita Montero <Bonita.Montero@gmail.com> - 2021-06-05 14:27 +0200
      Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 14:33 +0200
        Re: Is this really necessary Bonita Montero <Bonita.Montero@gmail.com> - 2021-06-05 15:06 +0200
          Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 15:29 +0200
            Re: Is this really necessary Bonita Montero <Bonita.Montero@gmail.com> - 2021-06-05 17:38 +0200
              Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 17:51 +0200
              Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-05 11:56 -0400
            Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-06-05 19:08 -0400
              Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-06 11:24 +0200
              Re: Is this really necessary Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2021-06-06 10:33 +0100
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-06 07:56 -0400
                Re: Is this really necessary Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2021-06-06 15:00 +0100
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-06 16:20 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-07 00:44 -0700
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-07 12:13 -0700
                Re: Is this really necessary Bo Persson <bo@bo-persson.se> - 2021-06-07 21:58 +0200
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-07 10:25 -0700
                Re: Is this really necessary Bo Persson <bo@bo-persson.se> - 2021-08-07 21:21 +0200
          Re: Is this really necessary Öö Tiib <ootiib@hot.ee> - 2021-06-05 06:34 -0700
          Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-05 20:03 -0400
            Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-07 12:59 -0700
              Re: Is this really necessary MrSpud_85yGi2@1ahbfz.gov - 2021-08-08 09:18 +0000
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-08 09:23 -0400
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-08 15:04 +0000
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-08-08 13:14 -0400
                Re: Is this really necessary MrSpud_1dgrmf@dx58865qyrdw30lqllb.info - 2021-08-09 08:21 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-09 08:53 +0000
                Re: Is this really necessary "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-08-09 19:53 +0200
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-10 05:22 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 14:54 +0000
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-10 11:41 -0700
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-11 05:39 +0000
                Re: Is this really necessary Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2021-08-08 20:01 +0100
                Re: Is this really necessary MrSpud_ifhov@nldls6_1kg3nl2qnwv.biz - 2021-08-09 08:19 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-09 08:55 +0000
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-09 12:16 -0400
                Re: Is this really necessary MrSpud_HG@_0b772d8ha3yjo0xb.edu - 2021-08-09 16:23 +0000
                Re: Is this really necessary Vir Campestris <vir.campestris@invalid.invalid> - 2021-08-09 21:50 +0100
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-08-09 20:57 -0400
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 01:12 +0000
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-08-09 21:21 -0400
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-10 07:13 -0700
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-10 05:29 +0000
                Re: Is this really necessary MrSpud_12tus_Atff@bya886olr5o8dhu9.ac.uk - 2021-08-10 07:28 +0000
                Re: Is this really necessary Mike Terry <news.dead.person.stones@darjeeling.plus.com> - 2021-08-10 15:34 +0100
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 14:59 +0000
                Re: Is this really necessary MrSpud_wPgnov999o@vv7a6.tv - 2021-08-10 15:54 +0000
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-10 11:29 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-11 08:45 +0000
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-11 07:05 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-11 14:18 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 14:57 +0000
                Re: Is this really necessary MrSpud_pb9bpp7Ej@6urvt16b9fax.gov.uk - 2021-08-10 07:23 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 15:02 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-10 06:11 -0700
                Re: Is this really necessary Bo Persson <bo@bo-persson.se> - 2021-08-10 16:13 +0200
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-11 12:40 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-12 08:20 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-12 03:50 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-12 14:58 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-12 08:29 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-12 15:39 +0000
                Re: Is this really necessary Paavo Helde <myfirstname@osa.pri.ee> - 2021-08-12 20:07 +0300
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-13 07:16 +0000
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-13 09:26 +0000
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-13 10:30 -0400
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-13 14:58 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-14 08:07 +0000
                Re: Is this really necessary "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-08-14 10:40 -0700
                Re: Is this really necessary "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-08-14 10:38 -0700
                Re: Is this really necessary "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-08-13 15:30 +0200
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-13 11:18 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 03:20 -0700
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-14 13:24 -0700
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-14 20:33 +0000
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-15 00:40 -0400
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-15 07:02 -0700
                Re: Is this really necessary "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-08-15 21:19 +0200
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-08-15 15:42 -0400
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-21 12:38 -0700
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-14 08:13 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 02:58 -0700
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-13 14:32 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 02:28 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 03:00 -0700
                Re: Is this really necessary Bart <bc@freeuk.com> - 2021-08-14 14:03 +0100
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-14 15:06 +0000
                Re: Is this really necessary "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-08-12 12:06 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-13 09:19 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-13 14:35 +0000
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-13 14:58 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-10 15:03 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-11 12:54 -0700
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-11 20:20 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-12 03:42 -0700
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-11 05:45 +0000
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-11 05:26 -0700
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-11 14:06 +0000
                Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-08-11 14:01 +0000
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-09 23:20 -0400
                Re: Is this really necessary MrSpud_m7k7yilq_8@u6w.biz - 2021-08-10 07:26 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-10 07:51 +0000
                Re: Is this really necessary MrSpud_ggBp1lq0@lxz.tv - 2021-08-10 07:59 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-10 12:24 +0000
                Re: Is this really necessary MrSpud_u7lm@bxuc3wy1g3cxiqu5j2x1_7iu.com - 2021-08-10 15:51 +0000
                Re: Is this really necessary Juha Nieminen <nospam@thanks.invalid> - 2021-08-11 05:47 +0000
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-11 08:46 +0000
                Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-08-10 11:20 +0200
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-10 10:27 -0400
                Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-08-10 17:44 +0200
                Re: Is this really necessary MrSpud_885p_dugji@l6zhsgj_v6t3dbj.tv - 2021-08-10 15:50 +0000
                Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-08-11 08:31 +0200
                Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-08-10 10:27 -0400
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-08-10 11:27 -0700
                Re: Is this really necessary mickspud@downthefarm.com - 2021-08-11 08:44 +0000
                Re: Is this really necessary Paavo Helde <myfirstname@osa.pri.ee> - 2021-08-11 16:05 +0300
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-10 06:41 -0700
          Re: Is this really necessary Paavo Helde <myfirstname@osa.pri.ee> - 2021-06-06 18:48 +0300
            Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-06 14:26 -0400
            Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-06 16:10 -0700
              Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-06 21:30 -0400
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-06 18:49 -0700
                Re: Is this really necessary Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-06 18:51 -0700
                Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-06 22:57 -0400
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-07 09:43 -0700
                Re: Is this really necessary Öö Tiib <ootiib@hot.ee> - 2021-06-07 09:51 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-07 10:22 -0700
                Re: Is this really necessary "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-06-07 10:13 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-07 10:22 -0700
                Re: Is this really necessary "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-08-07 12:40 -0700
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-07 12:54 -0700
              Re: Is this really necessary Öö Tiib <ootiib@hot.ee> - 2021-06-06 23:17 -0700
            Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-07 09:19 -0700
              Re: Is this really necessary Paavo Helde <myfirstname@osa.pri.ee> - 2021-06-07 21:58 +0300
                Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-10 11:29 -0700
    Re: Is this really necessary Öö Tiib <ootiib@hot.ee> - 2021-06-05 05:38 -0700
      Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 15:41 +0200
        Re: Is this really necessary Öö Tiib <ootiib@hot.ee> - 2021-06-05 08:18 -0700
          Re: Is this really necessary Richard Damon <Richard@Damon-Family.org> - 2021-06-05 11:58 -0400
    Re: Is this really necessary MrSpook_rs7x@4hhtozmpj299zx.tv - 2021-06-05 16:00 +0000
      Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-05 18:16 +0200
        Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-06 17:34 +0200
        Re: Is this really necessary MrSpook_ie@q67rq6_2ly9ut44j.org - 2021-06-06 13:55 +0000
      Re: Is this really necessary Bo Persson <bo@bo-persson.se> - 2021-06-05 19:50 +0200
        Re: Is this really necessary MrSpook_zn7@4tzq7j92.gov - 2021-06-06 13:59 +0000
    Re: Is this really necessary James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-06-05 17:42 -0400
      Re: Is this really necessary David Brown <david.brown@hesbynett.no> - 2021-06-06 11:05 +0200
      Re: Is this really necessary Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-07 00:27 -0700
  Re: Is this really necessary Vir Campestris <vir.campestris@invalid.invalid> - 2021-06-06 21:27 +0100
    Re: Is this really necessary "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-06-07 00:22 +0200
      Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-06-07 14:53 +0000
        Re: Is this really necessary Bonita Montero <Bonita.Montero@gmail.com> - 2021-06-07 18:02 +0200
      Re: Is this really necessary Vir Campestris <vir.campestris@invalid.invalid> - 2021-06-16 21:38 +0100
    Re: Is this really necessary scott@slp53.sl.home (Scott Lurndal) - 2021-06-07 14:52 +0000

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