Re: What a bug

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From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c++
Subject	Re: What a bug
Date	Tue, 05 Sep 2023 03:04:44 -0700
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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.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

Thread

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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