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# C++ concepts: BitmaskType

From cppreference.com
< cpp‎ | concept

Defines a type that can be used to represent a set of constant values or any combination of those values. This trait is typically implemented by integer types, std::bitset, or enumerations (scoped and unscoped) with additional operator overloads.

### [edit] Requirements

The bitmask type supports a finite number of bitmask elements, which are distinct non-zero values of the bitmask type, such that, for any pair Ci and Cj, Ci & Ci != 0 and Ci & Cj == 0. In addition, the value `0` is used to represent an empty bitmask, with no values set.

The bitwise operators operator&, operator|, operator^, operator~, operator&=, operator|=, and operator^= are defined for values of the bitmask type and have the same semantics as the corresponding built-in operators on unsigned integers would have if the bitmask elements were the distinct integer powers of two.

The following expressions are well-formed and have the following meaning for any BitmaskType X

X |= Y | sets the value Y in the object X
X &= ~Y | clears the value Y in the object X
(X&Y) != 0 | indicates that the value Y is set in the object X

Each representable bitmask element is defined as a inline (since C++17)constexpr value of the bitmask type.

### [edit] Usage

The following standard library types satisfy `BitmaskType`:

- std::ctype_base::mask
- std::ios_base::fmtflags
- std::ios_base::iostate
- std::ios_base::openmode
- std::regex_traits::char_class_type
- std::regex_constants::syntax_option_type
- std::regex_constants::match_flag_type
- std::launch
- `std::experimental::filesystem::perms` (filesystem TS)
- `std::experimental::filesystem::copy_options` (filesystem TS)
- `std::experimental::filesystem::directory_options` (filesystem TS)

Code that relies on some particular implementation option (e.g. int n = std::ios_base::hex), is nonportable because std::ios_base::fmtflags is not necessarily implicitly convertible to int.

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