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