44 lines
1.5 KiB
ReStructuredText
44 lines
1.5 KiB
ReStructuredText
===================
|
|
Feature Test Macros
|
|
===================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
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 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,
|
|
the documentation and the `<version>` header.
|