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Re: map[string_view] not compiling

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On 2022-08-01 at 22:30, Öö Tiib wrote:
> On Monday, 1 August 2022 at 17:11:16 UTC+3, Paavo Helde wrote:
>> 01.08.2022 15:57 Stefan Ram kirjutas:
>>> Bo Persson <b...@bo-persson.se> writes:
>>>> Not an expert, but operator[] takes a Key parameter, which is
>>>> std::string here. And string_view is not implicitly convertible to
>>>> std::string.
>>>
>>> Even more not an expert, but I take it from the Web that
>>> even convertibililty alone would not suffice. The Web says:
>>>
>>> |Three things are required for something like this to happen:
>>> |
>>> |1. The map's comparator must be a transparent comparator
>>> |
>>> |2. The at() method must have an overload that participates in
>>> | overload resolution when the container has a transparent
>>> | comparator.
>>> |
>>> |3. the parameter must be convertible to the map's key_type.
>>> |
>>> |Neither of these are true in your example. The default
>>> |std::less comparator is not a transparent comparator, there
>>> |is no such overload for at(), and std::string does not have
>>> |an implicit conversion from std::string_view.
>>> |
>>> The World-Wide Web (2021).
>>>
>>
>> Thanks, this is very clear for me now *how* it is not working. But I'm
>> more interested in *why* is it not working. This would be a quite
>> reasonable operation in my mind:
>>
>> 1. Look up the map element via a string_view.
>> 2. If not found, only then create a string from string_view and insert
>> it as a new element.
>>
>> As for now, it looks like I need to use a pessimization by creating a
>> string object always (together with the copy and dynamic allocation
>> penalties), or I need to devise some non-trivial workaround via
>> lower_bound and insert(hint).
> 
> Yes, the heterogeneous lookup of ordered containers was added by C++14,
> but it was for things like find and lower_bound not for operator [].
> C++17 added non-heterogeneous insert_or_assign().
> C++20 added heterogeneous lookup to unordered containers but also did
> not touch those access functions.
> Perhaps no one proposed it.
> 
> The string_view is optimization and so it is livable that its usage does
> not bring most elegant code. Also there have been controversy about
> map operator[] before, maybe they did not want to risk with heterogenous
> overload to it.

One complication is that for find it is enough that a type is comparable 
to the key. For operator[] it would also have to be convertible to the key.

For string and string_view this might be the same thing, but in general 
it is not.

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map[string_view] not compiling Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-01 14:52 +0300
  Re: map[string_view] not compiling Bo Persson <bo@bo-persson.se> - 2022-08-01 14:32 +0200
    Re: map[string_view] not compiling Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-01 17:11 +0300
      Re: map[string_view] not compiling Öö Tiib <ootiib@hot.ee> - 2022-08-01 13:30 -0700
        Re: map[string_view] not compiling Bo Persson <bo@bo-persson.se> - 2022-08-01 23:23 +0200
          Re: map[string_view] not compiling Öö Tiib <ootiib@hot.ee> - 2022-08-01 22:52 -0700
    Re: map[string_view] not compiling Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-08-04 08:31 -0700
  Re: map[string_view] not compiling "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-08-03 11:34 +0200
    Re: map[string_view] not compiling Juha Nieminen <nospam@thanks.invalid> - 2022-08-03 10:49 +0000
      Re: map[string_view] not compiling "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-08-03 13:38 +0200
        Re: map[string_view] not compiling Juha Nieminen <nospam@thanks.invalid> - 2022-08-04 06:16 +0000
    Re: map[string_view] not compiling Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-03 14:43 +0300
      Re: map[string_view] not compiling "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-08-03 13:56 +0200

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