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Groups > comp.lang.c++ > #82983
| From | "Alf P. Steinbach" <alf.p.steinbach@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: C++20 concepts rocks |
| Date | 2022-02-11 17:06 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <su61id$an$1@dont-email.me> (permalink) |
| References | (17 earlier) <86leymiga7.fsf@linuxsc.com> <877da3n2au.fsf@bsb.me.uk> <86czjvi97y.fsf@linuxsc.com> <87czjulvm9.fsf@bsb.me.uk> <868ruhikqf.fsf@linuxsc.com> |
On 11 Feb 2022 14:41, Tim Rentsch wrote:
> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>
>>> Rather than thinking about pointer values, it might help to start
>>> with objects. [..illustrative examples..]
>>>
>>> Casting a pointer to a different type doesn't change what objects
>>> it can access. At least, this rule holds in C, [...]
>
>> Really the only issue I had is resolved by the rule that "the array" is
>> always the smallest enclosing array that contains the thing pointed to
>> (before any conversions of course).
>
> Interesting. I hadn't thought of it that way before. Seems right.
>
> I should make a clarifying statement about casting not changing
> what objects can be accessed. If we have this code fragment
>
> int foo[10][20];
> extern void set_elements( int *, size_t, int )
>
> set_elements( (int*) &foo, 10*20, -1 );
>
> an argument could be made that set_elements() cannot use pointer
> arithmetic (including that implied by use of []) on its first
> argument other than to access between foo[0][0] and foo[0][19] (or
> to construct a pointer to foo[0][20]). The reasoning would be that
> there is no array of 200 elements, so the conditions for address
> arithmetic would not be met for index values other than between 0 and
> 20, and so would technically be undefined behavior.
First, for C++20 the above `reinterpret_cast`-expressed-as-C-cast
suffers from not involving "interconvertible" pointers, as noted in
§6.8.2/4.4:
"An array object and its first element are not pointer-interconvertible,
even though they have the same address."
Formally notes are not part of the formal language specification,
they're not "normative". But it's my impression that the C++ committee
has made their own more relaxed rules for ISO standards since (and maybe
including) C++11, and anyway the note surely reflects the committee's
intent.
Secondly, the `foo` object spans 200*sizeof(int) bytes, and an
implementation with range-checked pointers will necessarily allow that
byte range to be addressed. Even after a cast.
And as I see it that in-hypothetical-practice fat pointer view is a good
way to understand the otherwise baffling restrictions for this in the
standard. Fat pointer (slur: "phat pointer"): a pointer outfitted with
begin and end addresses for range checking. It's a good interpretation
because it's backed by a simple hypothetical implementation where one
can more easily reason with concrete examples about what goes on.
> It would be
> surprising if that putative undefined behavior would result in the
> code "doing the wrong thing", but I feel obliged to point out the
> possible alternative reading nonetheless.
>
> Even if the reading proposed above holds, set_elements() can be
> written in a way that avoids the putative undefined behavior:
>
> void
> set_elements( int *v, size_t n, int value ){
> while( n-- > 0 ){
> *(int*)( (char*)v + n * sizeof *v ) = value;
> }
> }
>
> This code has to work for the call that passes '(int*)&foo',
> because all objects have an implied character array that overlays
> the entire object, which is all of foo in this case.
Again, for C++ (though I understand you're discussing the C case) you're
colliding with a formal brick wall.
The general consensus is that only `memcpy` plus one more mechanism I
don't recall now (citing lack of coffee plus excessive blood sugar etc.)
is sufficiently formally supported to avoid formal UB for copying bytes
of objects.
In particular a `reinterpret_cast`, in the above code expressed as a C
style cast, is a good way to let loose the strict aliasing demons of the
g++ compiler. "Oh, formal UB? Well I'll optimize away all code paths
that lead to that, because surely they will never actually be executed,
and if they would it would be a bug. Don't thank me. I'm just doing this
in your best interest, giving you a really fastish program."
>> In the back of my mind I had thought that (at least for C) the standard
>> had been written to permit huge arrays to have some or all rows in
>> separate segments, while only requiring full pointer arithmetic for the
>> case of a char * accessing the object's representation. But that's not
>> the case, and (T *)&A can access all elements of any array A with base
>> type T.
>
> My understanding is that the underlying reasons have to do with
> possible program optimization. For example, if the array foo and
> the function set_elements() have been declared as shown above, and
> there is a call
>
> set_elements( foo[0], 20, 10 );
>
> we would like a compiler to be able to assume that foo[1], foo[2],
> ... , foo[9], are neither changed nor referenced by this call to
> set_elements().
Someone else mentioned (I believe in this thread) the possibility of
having a really large array with sub-arrays placed in two or more
distinct segments of memory, and pointer arithmetic on segment
selector+offset can't in general take you from one segment to another.
I hadn't thought of that; I always thought of this in terms fat pointers.
But as a rationale it makes sense. Just that it's now mostly irrelevant.
I can't imagine an embedded system with segmented memory, though I
assume that as everything that /can/ exist, some such gnarled beast must
exist. But surely C++ doesn't need to explicitly support it. Those who
do program it can surely note that any problem in their unit tests.
> AFAIAA all of the above statements apply to C++ as well as C
> (assuming of course there is no operator overloading or anything
> else along those lines). If anyone knows of any indication to the
> contrary in the C++ standard I would be interested to hear about
> that.
See above; I don't know if my C++ comments apply to C, but I suspect
that they do not, that C is now a far more practical language for stuff
like this.
So maybe one needs to combine C and C++ in order to do formally well
defined programaming.
C for union-based type conversions, multi-dimensional array traversing,
etc.; C++ for higher level control and the bulk of the application code;
C# for user interface (wait, wait, I'm joking, I'm just joking).
- Alf
Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-04 13:54 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-04 14:25 +0000
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-04 15:31 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-04 14:54 +0000
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-04 17:41 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-05 11:31 +0000
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-05 12:49 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-05 12:20 +0000
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-04 16:15 +0000
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-04 17:43 +0100
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-05 01:51 +0100
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-05 01:59 +0000
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-04 23:05 -0500
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-05 11:56 +0100
Re: C++20 concepts rocks David Brown <david.brown@hesbynett.no> - 2022-02-05 13:53 +0100
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-05 14:12 +0100
Re: C++20 concepts rocks Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2022-02-12 11:13 -0800
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-12 22:18 -0500
Re: C++20 concepts rocks Richard Damon <Richard@Damon-Family.org> - 2022-02-13 12:51 -0500
Re: C++20 concepts rocks Paavo Helde <eesnimi@osa.pri.ee> - 2022-02-13 20:16 +0200
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-13 14:12 -0500
Re: C++20 concepts rocks Richard Damon <Richard@Damon-Family.org> - 2022-02-13 16:22 -0500
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-13 17:18 -0500
Re: C++20 concepts rocks Öö Tiib <ootiib@hot.ee> - 2022-02-14 00:51 -0800
Re: C++20 concepts rocks "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-02-14 08:29 -0800
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-04 23:37 -0800
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-05 11:58 +0100
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 02:16 -0800
Re: C++20 concepts rocks "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-02-05 12:35 -0800
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 05:54 -0800
Re: C++20 concepts rocks Manfred <noname@add.invalid> - 2022-02-06 19:43 +0100
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-04 23:53 -0800
Re: C++20 concepts rocks red floyd <no.spam.here@its.invalid> - 2022-02-05 10:10 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-05 19:51 +0100
Re: C++20 concepts rocks red floyd <no.spam.here@its.invalid> - 2022-02-05 16:42 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-06 01:39 +0000
Re: C++20 concepts rocks red floyd <no.spam.here@its.invalid> - 2022-02-05 18:02 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-06 09:57 +0100
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-06 10:57 +0100
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-06 12:40 +0100
Re: C++20 concepts rocks Öö Tiib <ootiib@hot.ee> - 2022-02-06 03:53 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-06 14:35 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 11:20 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-06 21:53 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 19:03 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-07 11:50 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-07 06:26 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-07 15:48 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-07 12:16 -0800
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-07 22:27 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-09 21:43 +0000
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-09 21:07 -0500
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-09 21:25 -0800
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-10 13:05 +0000
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-10 11:19 -0500
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-11 05:41 -0800
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-11 17:06 +0100
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-11 11:50 -0500
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-11 21:13 +0100
Re: C++20 concepts rocks scott@slp53.sl.home (Scott Lurndal) - 2022-02-11 17:06 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-15 00:10 -0800
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-15 20:58 +0100
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-16 06:40 +0000
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-16 09:54 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-16 09:00 +0000
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-16 20:25 +0100
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-17 08:53 +0000
Re: C++20 concepts rocks Paavo Helde <eesnimi@osa.pri.ee> - 2022-02-17 16:11 +0200
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-17 10:58 -0500
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-17 07:32 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-19 02:21 -0700
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-04-19 11:56 +0200
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-25 03:02 -0700
Re: C++20 concepts rocks Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-02-12 00:01 +0000
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-14 23:55 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-07 18:31 +0100
Re: C++20 concepts rocks scott@slp53.sl.home (Scott Lurndal) - 2022-02-07 17:43 +0000
Re: C++20 concepts rocks James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-02-06 14:25 -0500
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 02:01 -0800
Re: C++20 concepts rocks red floyd <no.spam.here@its.invalid> - 2022-02-06 19:37 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-07 10:18 +0100
Re: C++20 concepts rocks Paavo Helde <eesnimi@osa.pri.ee> - 2022-02-07 11:32 +0200
Re: C++20 concepts rocks red floyd <no.spam.here@its.invalid> - 2022-02-07 07:55 -0800
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 02:11 -0800
Re: C++20 concepts rocks "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-02-06 13:13 +0100
Re: C++20 concepts rocks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-06 04:58 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-06 14:08 +0100
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-05 12:48 +0100
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-04 20:15 +0000
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-05 11:32 +0000
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-06 08:28 +0000
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-07 09:52 +0000
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-08 07:05 +0000
Re: C++20 concepts rocks Muttley@dastardlyhq.com - 2022-02-08 09:25 +0000
Re: C++20 concepts rocks Juha Nieminen <nospam@thanks.invalid> - 2022-02-08 10:09 +0000
Re: C++20 concepts rocks Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-02-04 12:10 -0800
Re: C++20 concepts rocks Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-05 13:41 +0100
Re: C++20 concepts rocks "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-02-05 19:28 -0800
A little benchmark: Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-05 13:22 +0100
A better benchmark Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-06 14:56 +0100
A improved routine Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-07 15:19 +0100
Re: A better benchmark Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-07 06:57 -0800
Re: A better benchmark Bonita Montero <Bonita.Montero@gmail.com> - 2022-02-07 18:30 +0100
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