# C++ named requirements: BitmaskType

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< cpp‎ | named req

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.

### 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 is nonzero and Ci & Cj is zero. 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 |= 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.

### 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 (since C++11)
- std::regex_constants::syntax_option_type (since C++11)
- std::regex_constants::match_flag_type (since C++11)
- std::launch (since C++11)
- std::filesystem::perms (since C++17)
- std::filesystem::copy_options (since C++17)
- std::filesystem::directory_options (since C++17)

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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