FeatureTestMacros.rst (1732B)
1 =================== 2 Feature Test Macros 3 =================== 4 5 .. contents:: 6 :local: 7 8 Overview 9 ======== 10 11 Libc++ implements the C++ feature test macros as specified in the C++2a standard, 12 and before that in non-normative guiding documents (`See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>`) 13 14 Design 15 ====== 16 17 Feature test macros are tricky to track, implement, test, and document correctly. 18 They must be available from a list of headers, they may have different values in 19 different dialects, and they may or may not be implemented by libc++. In order to 20 track all of these conditions correctly and easily, we want a Single Source of 21 Truth (SSoT) that defines each feature test macro, its values, the headers it 22 lives in, and whether or not is is implemented by libc++. From this SSoA we 23 have enough information to automatically generate the `<version>` header, 24 the tests, and the documentation. 25 26 Therefore we maintain a SSoA in 27 `libcxx/test/std/language.support/support.limits/support.limits.general/generate_feature_test_macro_components.py` 28 which doubles as a script to generate the following components: 29 30 * The `<version>` header. 31 * The version tests under `support.limits.general`. 32 * Documentation of libc++'s implementation of each macro. 33 34 Usage 35 ===== 36 37 The `generate_feature_test_macro_components.py` script is used to track and 38 update feature test macros in libc++. 39 40 Whenever a feature test macro is added or changed, the table should be updated 41 and the script should be re-ran. The script will clobber the existing test files 42 and the documentation and it will generate a new `<version>` header as a 43 temporary file. The generated `<version>` header should be merged with the 44 existing one.