Feature Test Macros


Libc++ implements the C++ feature test macros as specified in the C++2a standard, and before that in non-normative guiding documents (See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>)


Feature test macros are tricky to track, implement, test, and document correctly. They must be available from a list of headers, they may have different values in different dialects, and they may or may not be implemented by libc++. In order to track all of these conditions correctly and easily, we want a Single Source of Truth (SSoT) that defines each feature test macro, its values, the headers it lives in, and whether or not is is implemented by libc++. From this SSoA we have enough information to automatically generate the <version> header, the tests, and the documentation.

Therefore we maintain a SSoA in libcxx/test/std/language.support/support.limits/support.limits.general/generate_feature_test_macro_components.py which doubles as a script to generate the following components:

  • The <version> header.
  • The version tests under support.limits.general.
  • Documentation of libc++’s implementation of each macro.


The generate_feature_test_macro_components.py script is used to track and update feature test macros in libc++.

Whenever a feature test macro is added or changed, the table should be updated and the script should be re-ran. The script will clobber the existing test files and the documentation and it will generate a new <version> header as a temporary file. The generated <version> header should be merged with the existing one.