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Re: C++20 concepts rocks

From Paavo Helde <eesnimi@osa.pri.ee>
Newsgroups comp.lang.c++
Subject Re: C++20 concepts rocks
Date 2022-02-13 20:16 +0200
Organization A noiseless patient Spider
Message-ID <subhtm$42r$1@dont-email.me> (permalink)
References (5 earlier) <stkt1v$poe$1@dont-email.me> <stll5s$ao6$1@dont-email.me> <stls0p$jhf$1@dont-email.me> <35c223d5-0f11-4368-89d7-de986d7f5294n@googlegroups.com> <su9t9s$ig2$1@dont-email.me>

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13.02.2022 05:18 James Kuyper kirjutas:
> On 2/12/22 14:13, Anand Hariharan wrote:
>> On Saturday, February 5, 2022 at 12:54:29 PM UTC, David Brown wrote:
>>>
>> [...]
>>> char r[20];
>>>
>>> void foo1(void) {
>>> char *p = &r[20];
>> [...]
>>>
>>> "foo1" is clearly allowed - you can make a pointer that is one past an
>>> array
>> [...]
>>
>> This is clearly in "Language Lawyer" territory, and IANAL.
>>
>> To me x[y] is shorthand for *(x + y).  In other words, there is a dereference involved in x[y].
>>
>> So if &r[20] is not UB, it is only because there is implicit reliance on compiler 'optimization' -- something to the effect of &*(r + 20) being simplified ('optimized') to (r + 20)
> 
> If this were comp.lang.c, I'd have an easy response to that argument:
> it's not a mere "optimization", it's explicitly mandated by the
> standard. The following wording was first introduced in C99:
> 
> "The unary & operator yields the address of its operand. ... If the
> operand is the result of a unary * operator, neither that operator nor
> the & operator is evaluated and the result is as if both were omitted,
> except that the constraints on the operators still apply and the result
> is not an lvalue." (6.5.3.2p3).
> 
> Because of that clause, if there's no problem an expression E that has a
> pointer type, and if &*E violates no constraints, then there's also no
> problem with &*E, which has the same value, even if *E would have had
> undefined behavior (for instance, if the value of E is a null pointer or
> (as in this case) points one past the end of an array.
> 
> I don't know why the C++ committee never choose to add similar wording
> to the C++ standard.

One reason might be that this wording would prohibit debugging 
implementations of operator[] which check its arguments and fail if it
is >=N.

Such an instrumented operator[] would not have a way to know if its 
result is immediately passed to the & operator or not, so it cannot take 
this into an account. For a compiler implementer it would be easier to 
take into account, so e.g. the gcc bounds-checking regime would still be 
possible to implement, but an ordinary user-defined bounds-checking 
operator[] would not be possible.

So I would say the difference of C and C++ in this aspect is because in 
C++ one can redefine operator[], but not in C.

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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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