transparent proxy of string_view as key for associative containers.

[Jonathan Wakely]

On 16/11/20 08:26 +0100, Daniel Krügler via Libstdc++ wrote:
>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.

Correct.