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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 12 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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