| =================== |
| 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/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. |
| |
| 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. |
