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Re: What a bug

From jak <nospam@please.ty>
Newsgroups comp.lang.c++
Subject Re: What a bug
Date 2023-09-07 01:18 +0200
Organization A noiseless patient Spider
Message-ID <udb1c2$2mc8a$1@dont-email.me> (permalink)
References (1 earlier) <ud08l2$i4aq$1@dont-email.me> <ud2t4q$12vk6$1@dont-email.me> <87pm2y6af4.fsf@nosuchdomain.example.com> <ud6qom$1tniq$1@dont-email.me> <87v8cp3phf.fsf@nosuchdomain.example.com>

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Keith Thompson ha scritto:
> jak <nospam@please.ty> writes:
> [...]
>> I probably expressed myself badly. I agree with what you answered and
>> everything is clear to me. What I am contesting is the part where you
>> say "...may or may not be stored at the same memory location" because
>> that would mean that a program like this:
>>
>> #include <iostream>
>>
>> int main()
>> {
>>      auto eif = [] (bool t) {
>>          std::cout << ( t ? "equal" : "not equal" ) << std::endl;
>>      };
>>
>>      char const *sp = "hello";
>>      char sl[] = "hello";
>>
>>      eif( sp == sp );
>>
>>      eif( "hello" == sp );
>>
>>      eif( "hello" == "hello" );
>>
>>      eif( "hello" == sl );
>>
>>      return 0;
>> }
>>
>> which has this as a result:
>>
>> equal
>> equal
>> equal
>> not equal
>>
>> could give different results depending on how the compiler acts when
>> deciding where to store strings (because here we are comparing
>> pointers).
> 
> Yes, that's exactly what it means.
> 
>>             I simply think that different objects shouldn't be lumped
>> together just because they contain the same things, otherwise I would
>> have declared a single string and would have used its pointer around the
>> code. Pointers are for that too. it is my opinion that the program I
>> posted should have this as a result:
>>
>> equal
>> not equal
>> not equal
>> not equal
> 
> I suggest the language standard is more relevant than either of our
> opinions.
> 
> C++17 5.13.5 [lex.string] paragraph 16:
> 
>      Evaluating a *string-literal* results in a string literal
>      object with static storage duration, initialized from the given
>      characters as specified above. Whether all string literals are
>      distinct (that is, are stored in nonoverlapping objects) and
>      whether successive evaluations of a *string-literal* yield the
>      same or a different object is unspecified. [ Note: The effect
>      of attempting to modify a string literal is undefined. — end
>      note ]
> 
> I believe that all versions of C and C++ have similar rules.
> 
> Which means that ("hello" == "hello") may be either true or false, and
> an implementation is not required to document which.
> 
> In your program above, this:
>      sp == sp
> is guaranteed to be true, these:
>      "hello" == sp
>      "hello" == "hello"
> may be either true or false, and this:
>      "hello" == sl
> is false because sl is distinct and explicitly defined array object.
> 
> If you compile your program with clang++, it prints warnings on all four
> comparisons.
> 
Thanks for sending the paragraph. Unfortunately I had only sought it
just before your post.
However, it is obvious that it prints the warnings, I extracted the test
intentionally from the function/method, because otherwise the compiler
does not recognize the UB. If I modified my program in this way:

$ cat ./pp.cpp
#include <iostream>

int main()
{
     auto eif = [] (char const *a, char const *b) {
         std::cout << ( ( a == b ) ? "equal" : "not equal" ) << std::endl;
     };
     char const *sp = "hello";
     char sl[] = "hello";

     eif( sp, sp );
     eif( "hello", sp );
     eif( "hello", "hello" );
     eif( "hello", sl );

     return 0;
}

$ g++ -Wall -Wpedantic -Werror -std=c++17 ./pp.cpp -o ./pp

$ ./pp
equal
equal
equal
not equal

$ _

The warnings disappear but the UB remain. For this reason I think that
making objects unique because they have the same content is a bad
practice. Furthermore, the fact that it may happen or not makes the fact
even worse.

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What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-01 19:35 +0200
  Re: What a bug "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2023-09-01 23:09 +0200
    Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-01 14:52 -0700
    Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 06:23 +0200
      Re: What a bug Muttley@dastardlyhq.com - 2023-09-02 10:25 +0000
        Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 12:56 +0200
          Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 14:35 +0200
            Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 14:41 +0200
          Re: What a bug Muttley@dastardlyhq.com - 2023-09-02 15:30 +0000
            Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 18:10 +0200
  Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-01 14:50 -0700
    Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-02 06:19 +0200
      Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-03 15:25 -0700
        Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-04 04:53 +0200
          Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-03 20:01 -0700
            Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-04 06:59 +0200
              Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-04 11:11 +0200
            Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-04 18:48 -0700
              Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-14 16:27 -0700
        Re: What a bug scott@slp53.sl.home (Scott Lurndal) - 2023-09-04 14:24 +0000
          Re: What a bug "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-09-18 16:20 -0700
      Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-04 18:43 -0400
        Re: What a bug red floyd <no.spam.here@its.invalid> - 2023-09-04 17:36 -0700
        Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-05 05:49 +0200
          Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-06 12:00 -0400
            Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-06 19:46 +0200
              Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-06 14:44 -0400
                Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-07 04:51 +0200
                Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-06 23:54 -0400
                Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-07 06:58 +0200
                Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-07 11:08 -0400
                Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-07 17:56 +0200
                Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-07 18:04 +0200
                Re: What a bug Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-09-07 19:35 -0400
                Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-07 17:46 +0200
  Re: What a bug jak <nospam@please.ty> - 2023-09-02 23:15 +0200
    Re: What a bug jak <nospam@please.ty> - 2023-09-03 23:17 +0200
      Re: What a bug Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-03 17:37 -0700
        Re: What a bug jak <nospam@please.ty> - 2023-09-05 11:01 +0200
          Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-05 11:12 +0200
          Re: What a bug Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-05 03:04 -0700
            Re: What a bug jak <nospam@please.ty> - 2023-09-07 01:18 +0200
              Re: What a bug James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-09-08 00:09 -0400
        Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-05 11:11 +0200
      Re: What a bug red floyd <no.spam.here@its.invalid> - 2023-09-05 20:53 -0700
        Re: What a bug Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-09-05 21:14 -0700
          Re: What a bug red floyd <no.spam.here@its.invalid> - 2023-09-06 17:51 -0700
        Re: What a bug Bonita Montero <Bonita.Montero@gmail.com> - 2023-09-06 06:49 +0200

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