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.

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.

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, 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#

Header	Feature	Feature test macro	Module	Polyfill header	Polyfill name
`<array>`	`std::to_array`	`__cpp_lib_to_array`	pw_containers	`pw_containers/to_array.h`	`pw::containers::to_array`
`<bit>`	`std::endian`	`__cpp_lib_endian`	pw_bytes	`pw_bytes/bit.h`	`pw::endian`
`<cstdlib>`	`std::byte`	`__cpp_lib_byte`	pw_polyfill	`<cstdlib>`	`std::byte`
`<expected>`	`std::expected`	`__cpp_lib_expected`	pw_result	`pw_result/expected.h`	`pw::expected`
`<iterator>`	`std::data`, `std::size`	`__cpp_lib_nonmember_container_access`	pw_polyfill	`<iterator>`	`std::data`, `std::size`
`<span>`	`std::span`	`__cpp_lib_span`	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.