std::__is_fast_hash - Man Page

typedef integral_constant< _Tp, __v > type
typedef _Tp value_type

constexpr operator value_type () const noexcept
constexpr value_type operator() () const noexcept

static constexpr _Tp value

struct std::__is_fast_hash< _Hash >"Hint about performance of hash functions.

If a given hash function object is not fast, the hash-based containers will cache the hash code. The default behavior is to consider that hashers are fast unless specified otherwise.

Users can specialize this for their own hash functions in order to force caching of hash codes in unordered containers. Specializing this trait affects the ABI of the unordered containers, so use it carefully.