A unary function object type that acts a hash function for a `Key`. It takes a single argument of type `Key` and returns a value of type `std::size_t`.
`std::allocator_traits<Allocator>::pointer` and `std::allocator_traits<Allocator>::const_pointer` must be convertible to/from `value_type*` and `const value_type*`, respectively.
If `link:../../../../../container_hash/doc/html/hash.html#ref_hash_is_avalanchinghash[hash_is_avalanching]<Hash>::value` is `true`, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost.
Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].
Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^].