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- ===================
- Feature Test Macros
- ===================
- .. contents::
- :local:
- Overview
- ========
- 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>`_)
- 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 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/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
- 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.
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