cxxopts is a lightweight, header-only C++11 library for parsing command line options in GNU style. It provides a fluent API to define options with short and long names, positional arguments, default/implicit values, and supports boolean, integer, floating-point, string, vector, optional, and filesystem types. The parser handles GNU-style syntax (e.g., --long, -a, --long=value, combined short options) and produces a ParseResult object for querying parsed values, unmatched arguments, and iteration. The library is self-contained in a single header file (include/cxxopts.hpp) and requires no external dependencies beyond the C++ standard library, with optional Unicode support via ICU. The code demonstrates careful attention to integer overflow detection, locale-independent parsing, and robust help text wrapping. It is widely used in the C++ ecosystem, with active maintenance, fuzzing, and comprehensive test coverage.