16 Library introduction [library]

16.4 Method of description [description]

16.4.2 Other conventions [conventions]

16.4.2.2 Type descriptions [type.descriptions]

16.4.2.2.4 Bitmask types [bitmask.types]

Several types defined in [support] through [thread] and [depr] are bitmask types.

Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset .

The bitmask type bitmask can be written:

// For exposition only.
// int_type is an integral type capable of representing all values of the bitmask type.
enum bitmask : int_type {
   $V_{0}$  = 1 << 0,  $V_{1}$  = 1 << 1,  $V_{2}$  = 1 << 2,  $V_{3}$  = 1 << 3, …
};

inline constexpr  ${bitmask C}_{0}$ ( $V_{0}$ );
inline constexpr  ${bitmask C}_{1}$ ( $V_{1}$ );
inline constexpr  ${bitmask C}_{2}$ ( $V_{2}$ );
inline constexpr  ${bitmask C}_{3}$ ( $V_{3}$ );
  ⋮

constexpr bitmask operator&(bitmask X, bitmask Y) {
  return static_cast<bitmask>(
    static_cast<int_type>(X) & static_cast<int_type>(Y));
}
constexpr bitmask operator|(bitmask X, bitmask Y) {
  return static_cast<bitmask>(
    static_cast<int_type>(X) | static_cast<int_type>(Y));
}
constexpr bitmask operator^(bitmask X, bitmask Y){
  return static_cast<bitmask>(
    static_cast<int_type>(X) ^ static_cast<int_type>(Y));
}
constexpr bitmask operator~(bitmask X){
  return static_cast<bitmask>(~static_cast<int_type>(X));
}
bitmask& operator&=(bitmask& X, bitmask Y){
  X = X & Y; return X;
}
bitmask& operator|=(bitmask& X, bitmask Y) {
  X = X | Y; return X;
}
bitmask& operator^=(bitmask& X, bitmask Y) {
  X = X ^ Y; return X;
}

Here, the names

C_{0}

C_{1}

, etc. represent bitmask elements for this particular bitmask type.

All such elements have distinct, nonzero values such that, for any pair

C_{i}

and

C_{j}

where

i \neq j

C_{i}

C_{i}

is nonzero and

C_{i}

C_{j}

is zero.

Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.

The following terms apply to objects and values of bitmask types:

(4.1)
To set a value Y in an object X is to evaluate the expression X |= Y.
(4.2)
To clear a value Y in an object X is to evaluate the expression X &= ~Y.
(4.3)
The value Y is set in the object X if the expression X & Y is nonzero.