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.. _module-pw_polyfill:
The ``pw_polyfill`` module backports new C++ features to older C++ standards.
When possible, features are adapted to work with in standards as old as C++11.
Pigweed does not support C++ standards older than C++11.
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++11 or newer.
Language features are backported or stubbed via the
``pw_polyfill/language_features.h`` header. This header is included in files
that depend on ``pw_polyfill`` with GCC's ``-include`` option so that no
``#include`` statement is required.
The wrapper headers are in ``public_overrides``. These are provided through the
``"$dir_pw_polyfill:overrides"`` library, which the GN build adds as a
dependency for all targets. To apply overrides in Bazel or CMake, depend on the
``//pw_polyfill:overrides`` or ``pw_polyfill.overrides`` targets. In other build
systems, add ``pw_polyfill/standard_library_public`` and
``pw_polyfill/public_overrides`` as include paths, and add a ``-include`` option
for the ``language_features.h`` header.
Backported features
================== ================================ ============================================ ========================================
Header Feature Level of support Feature test macro
================== ================================ ============================================ ========================================
<array> std::to_array full __cpp_lib_to_array
<bit> std::endian full __cpp_lib_endian
<cstdlib> std::byte full; some operators not constexpr in C++11 __cpp_lib_byte
<iterator> std::data, std::size full __cpp_lib_nonmember_container_access
<type_traits> \*_t trait aliases partial (can expand as needed) __cpp_lib_transformation_trait_aliases
<type_traits> std::is_null_pointer full __cpp_lib_is_null_pointer
<utilty> std::integer_sequence & helpers full __cpp_lib_integer_sequence
(language feature) consteval keyword ignored (equivalent to constexpr) __cpp_consteval
(language feature) constinit keyword supported in clang; ignored in GCC __cpp_constinit
(language feature) static_assert with no message full __cpp_static_assert
================== ================================ ============================================ ========================================
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
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.