20 General utilities library [utilities]

20.9 Function objects [function.objects]

20.9.12 Class template hash [unord.hash]

The unordered associative containers defined in [unord] use specializations of the class template hash as the default hash function. For all object types Key for which there exists a specialization hash<Key>, and for all enumeration types ([dcl.enum]) Key, the instantiation hash<Key> shall:

  • satisfy the Hash requirements ([hash.requirements]), with Key as the function call argument type, the DefaultConstructible requirements (Table [defaultconstructible]), the CopyAssignable requirements (Table [copyassignable]),

  • be swappable ([swappable.requirements]) for lvalues,

  • provide two nested types result_type and argument_type which shall be synonyms for size_t and Key, respectively,

  • satisfy the requirement that if k1 == k2 is true, h(k1) == h(k2) is also true, where h is an object of type hash<Key> and k1 and k2 are objects of type Key;

  • satisfy the requirement that the expression h(k), where h is an object of type hash<Key> and k is an object of type Key, shall not throw an exception unless hash<Key> is a user-defined specialization that depends on at least one user-defined type.

template <> struct hash<bool>; template <> struct hash<char>; template <> struct hash<signed char>; template <> struct hash<unsigned char>; template <> struct hash<char16_t>; template <> struct hash<char32_t>; template <> struct hash<wchar_t>; template <> struct hash<short>; template <> struct hash<unsigned short>; template <> struct hash<int>; template <> struct hash<unsigned int>; template <> struct hash<long>; template <> struct hash<unsigned long>; template <> struct hash<long long>; template <> struct hash<unsigned long long>; template <> struct hash<float>; template <> struct hash<double>; template <> struct hash<long double>; template <class T> struct hash<T*>;

The template specializations shall meet the requirements of class template hash ([unord.hash]).