| # Feature Test Macros | |
| :::{contents} | |
| :local: true | |
| ::: | |
| ## 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](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 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. |