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.. _module-pw_polyfill:
===========
pw_polyfill
===========
The ``pw_polyfill`` module supports compiling code against different C++
standards. It also supports backporting a few C++17 features to C++14.
----------------------------------------------------
Adapt code to compile with different versions of C++
----------------------------------------------------
C++ standard macro
==================
``pw_polyfill/standard.h`` provides a macro for checking if a C++ standard is
supported.
.. c:macro:: PW_CXX_STANDARD_IS_SUPPORTED(standard)
Evaluates true if the provided C++ standard (98, 11, 14, 17, 20) is supported
by the compiler. This is a simpler, cleaner alternative to checking the value
of the ``__cplusplus`` macro.
Language feature macros
=======================
``pw_polyfill/language_feature_macros.h`` provides macros for adapting code to
work with or without C++ language features.
.. list-table::
:header-rows: 1
* - Macro
- Feature
- Description
- Feature test macro
* - ``PW_INLINE_VARIABLE``
- inline variables
- inline if supported by the compiler
- ``__cpp_inline_variables``
* - ``PW_CONSTEXPR_CPP20``
- ``constexpr``
- ``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/public`` as an include path.
------------------------------------------------
Backport new C++ features to older C++ standards
------------------------------------------------
Pigweed backports a few C++ features to older C++ standards. These features
are provided in the ``pw`` namespace. If the features are provided by the
toolchain, the ``pw`` versions are aliases of the ``std`` versions.
``pw_polyfill`` also backports a few C++17 library features to C++14 by wrapping
the standard C++ and C headers. The wrapper headers include the original header
using `#include_next
<https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html>`_, then add missing
features. The backported features are only defined if they aren't provided by
the standard header and can only be used when compiling with C++14 in GN.
Backported features
===================
.. list-table::
:header-rows: 1
* - Header
- Feature
- Feature test macro
- Module
- Polyfill header
- Polyfill name
* - ``<array>``
- ``std::to_array``
- ``__cpp_lib_to_array``
- :ref:`module-pw_containers`
- ``pw_containers/to_array.h``
- ``pw::containers::to_array``
* - ``<bit>``
- ``std::endian``
- ``__cpp_lib_endian``
- :ref:`module-pw_bytes`
- ``pw_bytes/bit.h``
- ``pw::endian``
* - ``<cstdlib>``
- ``std::byte``
- ``__cpp_lib_byte``
- pw_polyfill
- ``<cstdlib>``
- ``std::byte``
* - ``<iterator>``
- ``std::data``, ``std::size``
- ``__cpp_lib_nonmember_container_access``
- pw_polyfill
- ``<iterator>``
- ``std::data``, ``std::size``
* - ``<span>``
- ``std::span``
- ``__cpp_lib_span``
- :ref:`module-pw_span`
- ``pw_span/span.h``
- ``pw::span``
-------------
Compatibility
-------------
C++14
Zephyr
======
To enable ``pw_polyfill`` for Zephyr add ``CONFIG_PIGWEED_POLYFILL=y`` to the
project's configuration.
|
