<xstd/ints/concepts/bit_mask.hpp>

xstd::bit_mask

template<class T_cv, class T = remove_cv_t<T_cv>>
concept bit_mask =
  same_as<T, remove_cv_t<T_cv>> and
  (unsigned_integer<T> or (not signed_integer<T> and not integral<T>)) and
  regular<T> and
  requires(const T a) {
    { ~a } -> same_as<promoted_t<T>>;
  } and
  requires(T a, const T b) {
    a &= b;
    a ^= b;
    a |= b;
  } and
  requires(const T a, const T b) {
    { a & b } -> same_as<promoted_t<T>>;
    { a ^ b } -> same_as<promoted_t<T>>;
    { a | b } -> same_as<promoted_t<T>>;
  };

The bitmask type of [bitmask.types]: an unsigned integer, or any type that is no kind of integer and has the operators.

Models

• Every unsigned exact-width type models it under every cv-qualification, and so does every unsigned built-in, the narrow ones through promotion.
• No signed exact-width type models it, const or not, nor signed char, int or long long, whose ~ is negative.
• bool and the five character types do not, cv-qualified or not, although they answer every operator clause through promotion: integral<T> turns them away.
• bitset<N> models it, const too.
• The standard's own bitmask enumerations model it: filesystem::perms, perm_options, copy_options, directory_options, launch, chars_format, regex_constants::syntax_option_type and match_flag_type.
• An enumeration is one by its operators alone: byte, which the standard never calls a bitmask type, models it, and filesystem::file_type and regex_constants::error_type, enumerated types declared beside bitmask types, do not.
• The compound forms need only be valid, whatever they return, so libstdc++'s ios_base flag types, whose compound assignments return const X&, model it where they are not signed integers.

Notes

• Over the built-in types it admits exactly what popcount, countr_zero, rotl and bit_width accept.
• No ordering is asked for: bitset, which has no operator<, models it.
• No shifts are asked for: the standard's bitmask enumerations, which have none, model it.

See also

Entity	Relation
unsigned_integer	the integers it admits
signed_integer	the integers it turns away
promoted_t	the type its operators return
popcount, rotl, bit_width	take exactly its built-in models
BitmaskType	the standard's named requirement