bitflag

A bitflag is a representation technique in computing that uses individual bits within a binary value to represent multiple boolean options. Each flag corresponds to a specific bit position; when the bit is 1, the flag is enabled, and when it is 0, the flag is disabled. Flags are typically defined as powers of two (1, 2, 4, 8, 16, …), allowing a collection of flags to fit in a single integer or a bitset.

Operations on bitflags are performed with bitwise operators. To combine flags, a bitwise OR is used. To test whether a flag is set, a bitwise AND is used. To enable or disable specific flags, a combination of OR, AND, or XOR can be employed, and a bitwise NOT can invert a set of flags. This enables compact storage of many boolean options and efficient checks.

Common usage areas include permission systems (read, write, execute), configuration options, feature toggles, protocol capabilities, and device or file flags. In programming, languages provide various patterns for bitflags: C and C++ often use enums with values as powers of two; C# commonly uses a Flags attribute to indicate combinable values; Java can use bit masks; Python offers IntFlag in the enum module for similar behavior.

Example: define READ = 1, WRITE = 2, EXECUTE = 4. A permissions value of READ | WRITE means read and write are enabled. To check for write permission, test if (permissions & WRITE) != 0. To disable write, set permissions = permissions & ~WRITE.

Advantages include compact memory usage, fast bitwise operations, and straightforward testing of flag subsets. Pitfalls involve exceeding the available bit width, handling unknown or reserved bits, and potential readability issues with many flags without clear naming.