Feature Test Macros¶
Overview¶
Libc++ implements the C++ feature test macros as specified in the C++20 standard, and before that in non-normative guiding documents (See cppreference)
Design¶
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 it 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/utils/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.
Usage¶
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,
the documentation and the <version> header.