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Re: map[string_view] not compiling

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01.08.2022 15:57 Stefan Ram kirjutas:
> Bo Persson <bo@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).

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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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