2978. Hash support for pmr::string and friends

Section: 27.4.6 [basic.string.hash], 27.4.2 [string.syn] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25

Priority: 0

View all other issues in [basic.string.hash].

View all issues with C++20 status.

Discussion:

In most cases, std::pmr::meow is a drop-in replacement for std::meow. The exception is std::pmr::{,w,u16,u32}string, because unlike their std:: counterparts, they don't come with enabled std::hash specializations.

The P/R below simply adds std::hash specializations for those four typedefs. An alternative approach, for which wording can be produced if desired, is to make the hash specializations for basic_string allocator-agnostic, similar to the partial specialization of hash for vector<bool>.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 27.4.2 [string.syn], header <string> synopsis, as indicated:

     namespace pmr {
       template <class charT, class traits = char_traits<charT>>
         using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>;
   
       using string    = basic_string<char>;
       using u16string = basic_string<char16_t>;
       using u32string = basic_string<char32_t>;
       using wstring   = basic_string<wchar_t>;
     }
     
     // 27.4.6 [basic.string.hash], hash support
     template<class T> struct hash;
     template<> struct hash<string>;
     template<> struct hash<u16string>;
     template<> struct hash<u32string>;
     template<> struct hash<wstring>;
     template<> struct hash<pmr::string>;
     template<> struct hash<pmr::u16string>;
     template<> struct hash<pmr::u32string>;
     template<> struct hash<pmr::wstring>;
   
     namespace pmr {
       template <class charT, class traits = char_traits<charT>>
         using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>;
   
       using string    = basic_string<char>;
       using u16string = basic_string<char16_t>;
       using u32string = basic_string<char32_t>;
       using wstring   = basic_string<wchar_t>;
     }
   

2. Edit 27.4.6 [basic.string.hash] as indicated:

     template<> struct hash<string>;
     template<> struct hash<u16string>;
     template<> struct hash<u32string>;
     template<> struct hash<wstring>;
     template<> struct hash<pmr::string>;
     template<> struct hash<pmr::u16string>;
     template<> struct hash<pmr::u32string>;
     template<> struct hash<pmr::wstring>;
   

   -1- If S is one of these string types, SV is the corresponding string view type, and s is an object of type S, then hash<S>()(s) == hash<SV>()(SV(s)).