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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 */