16 Library introduction [library]

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 { ${\text{V}}_0$ = 1 << 0, ${\text{V}}_1$ = 1 << 1, ${\text{V}}_2$ = 1 << 2, ${\text{V}}_3$ = 1 << 3, … }; inline constexpr ${\text{bitmask C}}_0$(${\text{V}}_0$); inline constexpr ${\text{bitmask C}}_1$(${\text{V}}_1$); inline constexpr ${\text{bitmask C}}_2$(${\text{V}}_2$); inline constexpr ${\text{bitmask C}}_3$(${\text{V}}_3$); ⋮ 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 ${\text{C}}_0$, ${\text{C}}_1$, etc. represent bitmask elements for this particular bitmask type.

All such elements have distinct, nonzero values such that, for any pair ${\text{C}}_i$ and ${\text{C}}_j$ where i  ≠ j, ${\text{C}}_i$ & ${\text{C}}_i$ is nonzero and ${\text{C}}_i$ & ${\text{C}}_j$ 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.