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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 Sun, 03 Sep 2023 17:37:19 -0700
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jak <nospam@please.ty> writes:
> jak ha scritto:
>> Bonita Montero ha scritto:
>>> I checked for some commandline options and I thought it would be the
>>> best to map that through an unordered_map. I extracted this from my
>>> application in the below code.
>>>
>>> #include <Windows.h>
>>> #include <iostream>
>>> #include <variant>
>>> #include <unordered_map>
>>>
>>> using namespace std;
>>>
>>> int wmain( int argc, wchar_t **argv )
>>> {
>>>      static unordered_map<wchar_t const *, DWORD>
>>>          const opts =
>>>      {
>>>          { L"--idle", IDLE_PRIORITY_CLASS },
>>>          { L"--below", BELOW_NORMAL_PRIORITY_CLASS },
>>>          { L"--above", ABOVE_NORMAL_PRIORITY_CLASS },
>>>          { L"--high", HIGH_PRIORITY_CLASS },
>>>          { L"--realtime", REALTIME_PRIORITY_CLASS }
>>>      };
>>>      auto mappedPrio = opts.find( "--high" );
>>>      if( mappedPrio == opts.end() )
>>>          return -1;
>>>      return 0;
>>> }
>>>
>>> Why does my code have a bug and why doesn't mappedPrio
>>> point to end() at the end?
>> HI,
>> you just need to use wchar_t in your search string as well:
>> from:
>> auto mappedPrio = opts.find( "--high" );
>> to:
>> auto mappedPrio = opts.find( L"--high" );
>> 
>
> mmm. This only works because the compiler uses the same area of memory
> as the strings are equal and therefore compares a pointer to itself when
> it finds the string. In fact if I replace this:
>
> auto mappedPrio = opts.find( L"--high" );
>
> with this other:
>
> wchar_t what[] =  L"--high";
> auto mappedPrio = opts.find( what );
>
> the search fails.
>
> I find this sneaky. In my opinion the compiler shouldn't treat two
> objects as the same just because they have the same content.

The find() function of unordered_map<> compares keys for *equality*.

Since the code defines the key type as `wchar_t const *`, it does
pointer comparison, which has all the usual problems when you apply it
to string literals; L"foo" and L"foo" may or may not be stored at the
same memory location, and L"foo" and a string L"foo" derived from user
input will certainly not have the same address.

You can specify an equality function if you like.

A far simpler solution is to use std::wstring as the key type.

-- 
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 */

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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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