<xstd/bits/bit_set/bit_flag_mapping.hpp>

bit_flag_mapping

template<class Key, size_t N = see below>
  requires see below
struct bit_flag_mapping {
  using block_type = see below;
  static constexpr size_t size = N;
  static constexpr auto is_key(const Key& key) noexcept -> bool;
  static constexpr auto to_index(const Key& key) noexcept -> size_t;
  static constexpr auto from_index(size_t index) noexcept -> Key;
  static constexpr auto to_block(const Key& mask) noexcept -> block_type;
  static constexpr auto from_block(const block_type& block) noexcept -> Key;
};

A bitmask type keyed on its own one-bit values.

• Key is an enumeration, an integer type other than bool and the character types, or a std::bitset<M> with M > 0. block_type is the unsigned word a mask is read and written as: an enumeration's or an integer's unsigned counterpart, or for a bitset the least block holding M, or past the widest block an array of them.
• N defaults to every bit of that word but a sign bit, and for a bitset to M. An enumeration with no fixed underlying type has no default, its values stopping at the bit-field its enumerators need. Mandates: N is no more than that default width.
• is_key(k) holds where k has exactly one bit set, below N; zero and a value of several bits are no key.
• to_index(k) is the position of k's one bit. Preconditions: k has exactly one bit set.
• from_index(i) is the value with bit i alone. Preconditions: i < N.
• to_block and from_block read any value of Key, one-bit or not, as its block and back, which makes the mapping model bit_mask_mapping<bit_flag_mapping<Key, N>, Key>.