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Re: map[string_view] not compiling

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On 2022-08-01 at 13:52, Paavo Helde wrote:
> 
> Hi all,
> 
> Let's say I have a std::map with std::string keys and want to access it 
> with std::string_view keys. For lookup there is a trick to add 
> std::less<> to the map definition, but it appears this does not help 
> with operator[] by some reason. Maybe some expert can clarify this?
> 
> Here is an example:
> 
> #include <map>
> #include <string>
> using namespace std::string_view_literals;
> 
> 
> int main() {
> 
>      std::string_view key = "foo"sv;
> 
>      std::map<std::string, int> a;
>      //auto it = a.find(key); // not compiling
>      // cannot convert argument 1 from 'std::string_view' to 'const 
> std::basic_string ...
> 
>      std::map<std::string, int, std::less<>> b;
>      auto it = b.find(key); // OK now
> 
>      b[key] = 3; // still not compiling
>      //  binary '[': no operator found which takes a right-hand operand 
> of type 'std::string_view'
> 
>      b[std::string(key)] = 3; // OK now, but may construct a std::string 
> without reason
> 
> }

Not an expert, but operator[] takes a Key parameter, which is 
std::string here. And string_view is not implicitly convertible to 
std::string.

https://en.cppreference.com/w/cpp/container/map/operator_at


map::find is different in that it has extra overloads for types 
comparable to Key.

template< class K > iterator find( const K& x );

https://en.cppreference.com/w/cpp/container/map/find

You enable those by using the generic std::less<> comparator, that 
accepts different types for the two objects we want to compare.


Finally

b[std::string(key)] = 3;

works because it explicitly constructs a std::string matching the 
key-type. We specifically don't want this by default, as it has the cost 
of an extra construction. So that constructor is marked "explicit".

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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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