Issue 3705: Hashability shouldn't depend on basic_string's allocator (original) (raw)

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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].

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Discussion:

27.4.6 [basic.string.hash] says:

template<> struct hash; template<> struct hash; template<> struct hash; template<> struct hash; template<> struct hash; template<> struct hashpmr::string; template<> struct hashpmr::u8string; template<> struct hashpmr::u16string; template<> struct hashpmr::u32string; template<> struct hashpmr::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, allocator>; template<> struct hash<basic_string<char8_t, char_traits, allocator>;

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

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

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

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

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

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

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

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

   […]

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

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