I recently examined P4313 R1, a standards proposal adding bitmask operations for enums via a C++26 annotation. The core concept uses `[[=std::bitmask_type]]` to automatically generate bitwise operators, eliminating manual implementation. While C++ compilers happily compute bitwise operations on integral types, a critical detail involves integral promotion.
Types smaller than `int`, such as `short`, undergo promotion to `int` before the operation occurs. This means the result type of operations on smaller types defaults to `int`, requiring a `static_cast` back to the original type to avoid unexpected behavior. Furthermore, `bool` presents unique hazards.
Boolean conversion does not truncate bit patterns; instead, it maps any non-zero value to `true` (integer value 1) and zero to `false`. Combining these rules reveals a quirk: the complement of `true` (`~true`) promotes `true` to `int` (value 1), calculates the two's complement result as `-2`, and since `-2` is non-zero, boolean conversion returns `true`. Therefore, `static_cast(~true) == true`.
This behavior extends to enums based on `bool`, such as those defined with `[[=std::bitmask_type]]`, highlighting the importance of understanding these implicit conversions for robust C++ development.
Source: Hacker News · Summarized by HeadlinesBriefing