C++17 feature-test macros C++ standard compiler support portability

C++ feature-test macros conditional compilation portability __cplusplus compiler compatibility

C++11 feature-test macros C++ portability C++ features compiler compatibility

C++14 feature-test macros C++ features compile-time checks

C++20 feature-test macros programming C++ features C++ standards

C++23 feature-test macros C++ standard programming development