22 General utilities library [utilities]

The unordered associative containers defined in [unord] use specializations of the class template hash ([functional.syn]) as the default hash function.

Each specialization of hash is either enabled or disabled, as described below.

Each header that declares the template hash provides enabled specializations of hash for nullptr_t and all cv-unqualified arithmetic, enumeration, and pointer types.

For any type Key for which neither the library nor the user provides an explicit or partial specialization of the class template hash, hash<Key> is disabled.

If the library provides an explicit or partial specialization of hash<Key>, that specialization is enabled except as noted otherwise, and its member functions are noexcept except as noted otherwise.

If H is a disabled specialization of hash, these values are false: is_default_constructible_v<H>, is_copy_constructible_v<H>, is_move_constructible_v<H>, is_copy_assignable_v<H>, and is_move_assignable_v<H>.

Disabled specializations of hash are not function object types ([function.objects]).

An enabled specialization hash<Key> will:

• (5.1)
  meet the Cpp17Hash requirements (Table 37), with Key as the function call argument type, the Cpp17DefaultConstructible requirements (Table 30), the Cpp17CopyAssignable requirements (Table 34), the Cpp17Swappable requirements ([swappable.requirements]),
• (5.2)
  meet 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;
• (5.3)
  meet 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 program-defined specialization.