This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++23 status.

3705. Hashability shouldn't depend on basic_string's allocator

Section: 27.4.6 [basic.string.hash] Status: C++23 Submitter: Casey Carter Opened: 2022-05-26 Last modified: 2023-11-22

Priority: Not Prioritized

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

View all issues with C++23 status.

Discussion:

27.4.6 [basic.string.hash] says:

template<> struct hash<string>;
template<> struct hash<u8string>;
template<> struct hash<u16string>;
template<> struct hash<u32string>;
template<> struct hash<wstring>;
template<> struct hash<pmr::string>;
template<> struct hash<pmr::u8string>;
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))

Despite that the hash value of a basic_string object is equivalent to the hash value of a corresponding basic_string_view object, which has no allocator, the capability to hash a basic_string depends on its allocator. All of the enabled specializations have specific allocators, which fact becomes more clear if we expand the type aliases:

template<> struct hash<basic_string<char, char_traits<char>, allocator<char>>;
template<> struct hash<basic_string<char8_t, char_traits<char8_t>, allocator<char8_t>>;

template<> struct hash<basic_string<char16_t, char_traits<char16_t>, allocator<char16_t>>;

template<> struct hash<basic_string<char32_t, char_traits<char32_t>, allocator<char32_t>>;

template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t>>;

template<> struct hash<basic_string<char, char_traits<char>, pmr::polymorphic_allocator<char>>;
template<> struct hash<basic_string<char8_t, char_traits<char8_t>, pmr::polymorphic_allocator<char8_t>>;

template<> struct hash<basic_string<char16_t, char_traits<char16_t>, pmr::polymorphic_allocator<char16_t>>;

template<> struct hash<basic_string<char32_t, char_traits<char32_t>, pmr::polymorphic_allocator<char32_t>>;

template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, pmr::polymorphic_allocator<wchar_t>>;

If the hash value doesn't depend on the allocator type, why should we care about the allocator type? I posit that we should not, and that these ten explicit specializations should be replaced by 5 partial specializations that enable hashing basic_string specializations using these combinations of character type and traits type with any allocator type.

[2022-06-21; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

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

   […]
   
   // 27.4.6 [basic.string.hash], hash support
   template<class T> struct hash;
   template<> struct hash<string>;
   template<> struct hash<u8string>;
   template<> struct hash<u16string>;
   template<> struct hash<u32string>;
   template<> struct hash<wstring>;
   template<> struct hash<pmr::string>;
   template<> struct hash<pmr::u8string>;
   template<> struct hash<pmr::u16string>;
   template<> struct hash<pmr::u32string>;
   template<> struct hash<pmr::wstring>;
   template<class A> struct hash<basic_string<char, char_traits<char>, A>>;
   template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>;
   template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>;
   template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>;
   template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>;
   
   […]
   

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

   template<> struct hash<string>;
   template<> struct hash<u8string>;
   template<> struct hash<u16string>;
   template<> struct hash<u32string>;
   template<> struct hash<wstring>;
   template<> struct hash<pmr::string>;
   template<> struct hash<pmr::u8string>;
   template<> struct hash<pmr::u16string>;
   template<> struct hash<pmr::u32string>;
   template<> struct hash<pmr::wstring>;
   template<class A> struct hash<basic_string<char, char_traits<char>, A>>;
   template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>;
   template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>;
   template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>;
   template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>;
   

   -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))