Re: transparent proxy of string_view as key for associative containers.

["Daniel Krügler" <daniel.kruegler@gmail.com>]

Am Mo., 16. Nov. 2020 um 07:44 Uhr schrieb sotrdg sotrdg via Libstdc++
<libstdc++@gcc.gnu.org>:
>
> The code fails to compile for both unordered_map and map. Is this a bug of libstdc++ or a defect of WG21?
> #include<unordered_map>
> #include<string>
> #include<string_view>
> #include<map>
>
> int main()
> {
>                 std::unordered_map<std::string,std::string> umap;
>                 std::string_view key{"a key"};
>                 auto p{umap.find(key)};
>                 std::map<std::string,std::string> map;
>                 auto p1{map.find(key)};
> }

I don't see any WG21 defect involved here, but a mislead user expectation:

Let's start with the observation that this code isn't valid. To
activate transparent comparison, you need to provide function objects
that have the is_transparent tag defined, e.g. aomething like the
following should be more appropriate:

#include<unordered_map>
#include<string>
#include<string_view>
#include<map>

struct hash_transparent
{
    using is_transparent = void;

    template<class T>
    std::size_t operator()(const T& s) const
    {
        return std::hash<T>()(s);
    }
};

int main()
{
    std::unordered_map<std::string, std::string, hash_transparent,
std::equal_to<>> umap;
    std::string_view key{ "a key" };
    auto p{umap.find(key)};
    std::map<std::string, std::string, std::less<>> map;
    auto p1{map.find(key)};
}

Nonetheless, when you test this, you will still get a compile error
with gcc/clang for the unordered_map (Not for the map), so I'm
assuming that this feature has not been implemented yet. Visual Studio
2019 accepts that code, though.

- Daniel