.. _module-pw_polyfill: =========== pw_polyfill =========== The ``pw_polyfill`` module backports new C++ features to C++14. ------------------------------------------------ Backport new C++ features to older C++ standards ------------------------------------------------ The main purpose of ``pw_polyfill`` is to bring new C++ library and language features to older C++ standards. No additional ``#include`` statements are required to use these features; simply write code assuming that the features are available. This implicit feature backporting is provided through the ``overrides`` library in the ``pw_polyfill`` module. GN automatically adds this library as a dependency in ``pw_source_set``. ``pw_polyfill`` backports C++ library features by wrapping the standard C++ and C headers. The wrapper headers include the original header using `#include_next `_, then add missing features. The backported features are only defined if they aren't provided by the standard header, so ``pw_polyfill`` is safe to use when compiling with any standard C++14 or newer. The wrapper headers are in ``public_overrides``. These are provided through the ``"$dir_pw_polyfill"`` libraries, which the GN build adds as a dependency for all targets: * ``$dir_pw_polyfill:bit`` * ``$dir_pw_polyfill:cstddef`` * ``$dir_pw_polyfill:iterator`` * ``$dir_pw_polyfill:span`` * ``$dir_pw_polyfill:type_traits`` To apply overrides in Bazel or CMake, depend on the targets you need such as ``//pw_polyfill:span`` or ``pw_polyfill.span`` as an example. Backported features =================== ================== ================================ =============================== ======================================== Header Feature Level of support Feature test macro ================== ================================ =============================== ======================================== std::endian full __cpp_lib_endian std::byte full __cpp_lib_byte std::data, std::size full __cpp_lib_nonmember_container_access std::bool_constant full __cpp_lib_bool_constant std::negation, etc. full __cpp_lib_logical_traits ================== ================================ =============================== ======================================== ---------------------------------------------------- Adapt code to compile with different versions of C++ ---------------------------------------------------- ``pw_polyfill`` provides features for adapting to different C++ standards when ``pw_polyfill:overrides``'s automatic backporting is insufficient: - ``pw_polyfill/standard.h`` -- provides a macro for checking the C++ standard - ``pw_polyfill/language_feature_macros.h`` -- provides macros for adapting code to work with or without newer language features Language feature macros ======================= ====================== ================================ ======================================== ========================== Macro Feature Description Feature test macro ====================== ================================ ======================================== ========================== PW_INLINE_VARIABLE inline variables inline if supported by the compiler __cpp_inline_variables PW_CONSTEXPR_CPP20 constexpr in C++20 constexpr if compiling for C++20 __cplusplus >= 202002L PW_CONSTEVAL consteval consteval if supported by the compiler __cpp_consteval PW_CONSTINIT constinit constinit in clang and GCC 10+ __cpp_constinit ====================== ================================ ======================================== ========================== In GN, Bazel, or CMake, depend on ``$dir_pw_polyfill``, ``//pw_polyfill``, or ``pw_polyfill``, respectively, to access these features. In other build systems, add ``pw_polyfill/standard_library_public`` and ``pw_polyfill/public_overrides`` as include paths. ------------- Compatibility ------------- C++14 Zephyr ====== To enable ``pw_polyfill`` for Zephyr add ``CONFIG_PIGWEED_POLYFILL=y`` to the project's configuration. Similarly, to enable ``pw_polyfill.overrides``, add ``CONFIG_PIGWEED_POLYFILL_OVERRIDES=y`` to the project's configuration.