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46 lines
1.6 KiB
ReStructuredText
===================
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Feature Test Macros
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===================
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.. contents::
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:local:
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Overview
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========
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Libc++ implements the C++ feature test macros as specified in the C++2a standard,
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and before that in non-normative guiding documents
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(`See cppreference <https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros>`_)
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Design
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======
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Feature test macros are tricky to track, implement, test, and document correctly.
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They must be available from a list of headers, they may have different values in
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different dialects, and they may or may not be implemented by libc++. In order to
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track all of these conditions correctly and easily, we want a Single Source of
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Truth (SSoT) that defines each feature test macro, its values, the headers it
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lives in, and whether or not is is implemented by libc++. From this SSoA we
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have enough information to automatically generate the `<version>` header,
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the tests, and the documentation.
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Therefore we maintain a SSoA in `libcxx/utils/generate_feature_test_macro_components.py`
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which doubles as a script to generate the following components:
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* The `<version>` header.
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* The version tests under `support.limits.general`.
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* Documentation of libc++'s implementation of each macro.
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Usage
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=====
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The `generate_feature_test_macro_components.py` script is used to track and
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update feature test macros in libc++.
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Whenever a feature test macro is added or changed, the table should be updated
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and the script should be re-ran. The script will clobber the existing test files
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and the documentation and it will generate a new `<version>` header as a
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temporary file. The generated `<version>` header should be merged with the
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existing one.
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