20 Library introduction [library]

20.4 Method of description (Informative) [description]

20.4.2 Other conventions [conventions]

20.4.2.1 Type descriptions [type.descriptions]

20.4.2.1.4 Bitmask types [bitmask.types]

Several types defined in Clauses [language.support] through [thread] and Annex [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 ([template.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.