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Summary: This patch implements all the feature test macros libc++ currently supports, as specified by the standard or cppreference prior to C++2a. The tests and `<version>` header are generated using a script. The script contains a table of each feature test macro, the headers it should be accessible from, and its values of each dialect of C++. When a new feature test macro is added or needed, the table should be updated and the script re-run. Reviewers: mclow.lists, jfb, serge-sans-paille Reviewed By: mclow.lists Subscribers: arphaman, jfb, ldionne, libcxx-commits Differential Revision: https://reviews.llvm.org/D56750 git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@351286 91177308-0d34-0410-b5e6-96231b3b80d8
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1.7 KiB
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44 lines
1.7 KiB
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===================
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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 (`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
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`libcxx/test/std/language.support/support.limits/support.limits.general/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. |