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30194d45f0
Adding a mingw based config is easy in the current CI environment (where we can just choose the different target by calling `i686-w64-mingw32-clang`), while adding a clang-cl based config would require setting up different environment variables pointing to the i386 library directory. Just adding one config (DLL) instead of exhaustively testing both (DLL and static) as very few tests would differ in practice, to keep the CI load reasonable. Differential Revision: https://reviews.llvm.org/D124991
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.. _index:
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=============================
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"libc++" C++ Standard Library
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=============================
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Overview
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========
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libc++ is a new implementation of the C++ standard library, targeting C++11 and
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above.
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* Features and Goals
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* Correctness as defined by the C++11 standard.
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* Fast execution.
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* Minimal memory use.
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* Fast compile times.
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* ABI compatibility with gcc's libstdc++ for some low-level features
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such as exception objects, rtti and memory allocation.
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* Extensive unit tests.
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* Design and Implementation:
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* Extensive unit tests
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* Internal linker model can be dumped/read to textual format
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* Additional linking features can be plugged in as "passes"
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* OS specific and CPU specific code factored out
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Getting Started with libc++
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===========================
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.. toctree::
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:maxdepth: 1
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ReleaseNotes
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UsingLibcxx
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BuildingLibcxx
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TestingLibcxx
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Contributing
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Status/Cxx14
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Status/Cxx17
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Status/Cxx20
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Status/Cxx2b
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Status/Format
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Status/Ranges
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Status/Spaceship
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Status/Zip
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.. toctree::
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:hidden:
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AddingNewCIJobs
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FeatureTestMacroTable
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Current Status
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==============
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After its initial introduction, many people have asked "why start a new
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library instead of contributing to an existing library?" (like Apache's
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libstdcxx, GNU's libstdc++, STLport, etc). There are many contributing
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reasons, but some of the major ones are:
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* From years of experience (including having implemented the standard
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library before), we've learned many things about implementing
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the standard containers which require ABI breakage and fundamental changes
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to how they are implemented. For example, it is generally accepted that
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building std::string using the "short string optimization" instead of
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using Copy On Write (COW) is a superior approach for multicore
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machines (particularly in C++11, which has rvalue references). Breaking
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ABI compatibility with old versions of the library was
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determined to be critical to achieving the performance goals of
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libc++.
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* Mainline libstdc++ has switched to GPL3, a license which the developers
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of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
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independently extended to support C++11, but this would be a fork of the
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codebase (which is often seen as worse for a project than starting a new
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independent one). Another problem with libstdc++ is that it is tightly
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integrated with G++ development, tending to be tied fairly closely to the
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matching version of G++.
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* STLport and the Apache libstdcxx library are two other popular
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candidates, but both lack C++11 support. Our experience (and the
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experience of libstdc++ developers) is that adding support for C++11 (in
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particular rvalue references and move-only types) requires changes to
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almost every class and function, essentially amounting to a rewrite.
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Faced with a rewrite, we decided to start from scratch and evaluate every
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design decision from first principles based on experience.
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Further, both projects are apparently abandoned: STLport 5.2.1 was
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released in Oct'08, and STDCXX 4.2.1 in May'08.
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Platform and Compiler Support
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=============================
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Libc++ aims to support common compilers that implement the C++11 Standard. In order to strike a
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good balance between stability for users and maintenance cost, testing coverage and development
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velocity, libc++ drops support for older compilers as newer ones are released.
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============ =============== ========================== =====================
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Compiler Versions Restrictions Support policy
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============ =============== ========================== =====================
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Clang 13, 14 latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_
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AppleClang 13 latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
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Open XL 17.1 (AIX) latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
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GCC 11 In C++11 or later only latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
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============ =============== ========================== =====================
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Libc++ also supports common platforms and architectures:
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=============== ========================= ============================
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Target platform Target architecture Notes
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=============== ========================= ============================
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macOS 10.9+ i386, x86_64, arm64 Building the shared library itself requires targetting macOS 10.11+
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FreeBSD 10+ i386, x86_64, arm
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Linux i386, x86_64, arm, arm64
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Windows i386, x86_64 Both MSVC and MinGW style environments
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AIX powerpc, powerpc64
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=============== ========================= ============================
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Generally speaking, libc++ should work on any platform that provides a fairly complete
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C Standard Library. It is also possible to turn off parts of the library for use on
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systems that provide incomplete support.
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However, libc++ aims to provide a high-quality implementation of the C++ Standard
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Library, especially when it comes to correctness. As such, we aim to have test coverage
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for all the platforms and compilers that we claim to support. If a platform or compiler
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is not listed here, it is not officially supported. It may happen to work, and
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in practice the library is known to work on some platforms not listed here, but
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we don't make any guarantees. If you would like your compiler and/or platform
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to be formally supported and listed here, please work with the libc++ team to set
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up testing for your configuration.
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C++ Dialect Support
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===================
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* C++11 - Complete
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* :ref:`C++14 - Complete <cxx14-status>`
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* :ref:`C++17 - In Progress <cxx17-status>`
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* :ref:`C++20 - In Progress <cxx20-status>`
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* :ref:`C++2b - In Progress <cxx2b-status>`
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* :ref:`C++ Feature Test Macro Status <feature-status>`
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Notes and Known Issues
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======================
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This list contains known issues with libc++
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* Building libc++ with ``-fno-rtti`` is not supported. However
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linking against it with ``-fno-rtti`` is supported.
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A full list of currently open libc++ bugs can be `found here`__.
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.. __: https://github.com/llvm/llvm-project/labels/libc%2B%2B
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Design Documents
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================
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.. toctree::
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:maxdepth: 1
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DesignDocs/ABIVersioning
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DesignDocs/AtomicDesign
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DesignDocs/CapturingConfigInfo
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DesignDocs/DebugMode
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DesignDocs/ExperimentalFeatures
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DesignDocs/ExtendedCXX03Support
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DesignDocs/FeatureTestMacros
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DesignDocs/FileTimeType
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DesignDocs/NoexceptPolicy
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DesignDocs/ThreadingSupportAPI
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DesignDocs/UniquePtrTrivialAbi
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DesignDocs/UnspecifiedBehaviorRandomization
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DesignDocs/VisibilityMacros
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Build Bots and Test Coverage
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============================
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* `Buildkite CI pipeline <https://buildkite.com/llvm-project/libcxx-ci>`_
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* `LLVM Buildbot Builders <http://lab.llvm.org:8011>`_
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* :ref:`Adding New CI Jobs <AddingNewCIJobs>`
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Getting Involved
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================
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First please review our `Developer's Policy <https://llvm.org/docs/DeveloperPolicy.html>`__
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and `Getting started with LLVM <https://llvm.org/docs/GettingStarted.html>`__.
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**Bug Reports**
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If you think you've found a bug in libc++, please report it using
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the `LLVM bug tracker`_. If you're not sure, you
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can ask for support on the `libcxx forum`_ or on IRC.
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**Patches**
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If you want to contribute a patch to libc++, the best place for that is
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`Phabricator <https://llvm.org/docs/Phabricator.html>`_. Please add `libcxx-commits` as a subscriber.
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Also make sure you are subscribed to the `libcxx-commits mailing list`_.
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**Discussion and Questions**
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Send discussions and questions to the `libcxx forum`_.
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Quick Links
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===========
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* `LLVM Homepage <https://llvm.org/>`_
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* `libc++abi Homepage <http://libcxxabi.llvm.org/>`_
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* `LLVM Bug Tracker <https://github.com/llvm/llvm-project/labels/libc++/>`_
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* `libcxx-commits Mailing List <http://lists.llvm.org/mailman/listinfo/libcxx-commits>`_
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* `libcxx Forum <https://discourse.llvm.org/c/runtimes/libcxx/>`_
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* `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/main/libcxx/>`_
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