.. _index:

=============================
"libc++" C++ Standard Library
=============================

Overview
========

libc++ is a new implementation of the C++ standard library, targeting C++11 and
above.

* Features and Goals

  * Correctness as defined by the C++11 standard.
  * Fast execution.
  * Minimal memory use.
  * Fast compile times.
  * ABI compatibility with gcc's libstdc++ for some low-level features
    such as exception objects, rtti and memory allocation.
  * Extensive unit tests.

* Design and Implementation:

  * Extensive unit tests
  * Internal linker model can be dumped/read to textual format
  * Additional linking features can be plugged in as "passes"
  * OS specific and CPU specific code factored out


Getting Started with libc++
===========================

.. toctree::
   :maxdepth: 1

   ReleaseNotes
   UserDocumentation
   VendorDocumentation
   ABIGuarantees
   Contributing
   CodingGuidelines
   TestingLibcxx
   ImplementationDefinedBehavior
   Modules
   Hardening
   ReleaseProcedure
   Status/Cxx17
   Status/Cxx20
   Status/Cxx23
   Status/Cxx2c
   Status/Format
   Status/Parallelism


.. toctree::
    :hidden:

    AddingNewCIJobs
    FeatureTestMacroTable


Current Status
==============

libc++ has become the default C++ Standard Library implementation for many major platforms, including Apple's macOS,
iOS, watchOS, and tvOS, Google Search, the Android operating system, and FreeBSD. As a result, libc++ has an estimated
user base of over 1 billion daily active users.

Since its inception, libc++ has focused on delivering high performance, standards-conformance, and portability. It has
been extensively tested and optimized, making it robust and production ready. libc++ fully implements C++11 and C++14,
with C++17, C++20, C++23, and C++26 features being actively developed and making steady progress.

libc++ is continuously integrated and tested on a wide range of platforms and configurations, ensuring its reliability
and compatibility across various systems. The library's extensive test suite and rigorous quality assurance process have
made it a top choice for platform providers looking to offer their users a robust and efficient C++ Standard Library.

As an open-source project, libc++ benefits from a vibrant community of contributors who work together to improve the
library and add new features. This ongoing development and support ensure that libc++ remains at the forefront of
C++ standardization efforts and continues to meet the evolving needs of C++ developers worldwide.


History
-------
After its initial introduction, many people have asked "why start a new
library instead of contributing to an existing library?" (like Apache's
libstdcxx, GNU's libstdc++, STLport, etc).  There are many contributing
reasons, but some of the major ones are:

* From years of experience (including having implemented the standard
  library before), we've learned many things about implementing
  the standard containers which require ABI breakage and fundamental changes
  to how they are implemented.  For example, it is generally accepted that
  building std::string using the "short string optimization" instead of
  using Copy On Write (COW) is a superior approach for multicore
  machines (particularly in C++11, which has rvalue references).  Breaking
  ABI compatibility with old versions of the library was
  determined to be critical to achieving the performance goals of
  libc++.

* Mainline libstdc++ has switched to GPL3, a license which the developers
  of libc++ cannot use.  libstdc++ 4.2 (the last GPL2 version) could be
  independently extended to support C++11, but this would be a fork of the
  codebase (which is often seen as worse for a project than starting a new
  independent one).  Another problem with libstdc++ is that it is tightly
  integrated with G++ development, tending to be tied fairly closely to the
  matching version of G++.

* STLport and the Apache libstdcxx library are two other popular
  candidates, but both lack C++11 support.  Our experience (and the
  experience of libstdc++ developers) is that adding support for C++11 (in
  particular rvalue references and move-only types) requires changes to
  almost every class and function, essentially amounting to a rewrite.
  Faced with a rewrite, we decided to start from scratch and evaluate every
  design decision from first principles based on experience.
  Further, both projects are apparently abandoned: STLport 5.2.1 was
  released in Oct'08, and STDCXX 4.2.1 in May'08.

..
  LLVM RELEASE bump version

.. _SupportedPlatforms:

Platform and Compiler Support
=============================

Libc++ aims to support common compilers that implement the C++11 Standard. In order to strike a
good balance between stability for users and maintenance cost, testing coverage and development
velocity, libc++ drops support for older compilers as newer ones are released.

============ =================== ========================== =====================
Compiler     Versions            Restrictions               Support policy
============ =================== ========================== =====================
Clang        21, 22, 23-git                                 latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_ and the development version
AppleClang   26.4                                           latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
Open XL      17.1.4 (AIX)                                   latest stable release per `Open XL's documentation page <https://www.ibm.com/docs/en/openxl-c-and-cpp-aix>`_
GCC          15                  In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
============ =================== ========================== =====================

Libc++ also supports common platforms and architectures:

===================== ========================= ============================
Target platform       Target architecture       Notes
===================== ========================= ============================
macOS 11.0+           i386, x86_64, arm64       Deployment target lower bound is the minimum between what Chrome resp. the last stable `Xcode release <https://developer.apple.com/support/xcode>`_ support
FreeBSD 12+           i386, x86_64, arm
Linux                 i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
Android 5.0+          i386, x86_64, arm, arm64
Windows 7+            i386, x86_64, arm64       Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
AIX 7.2TL5+           powerpc, powerpc64
Embedded (picolibc)   arm
===================== ========================= ============================

Generally speaking, libc++ should work on any platform that provides a fairly complete
C Standard Library. It is also possible to turn off parts of the library for use on
systems that provide incomplete support.

However, libc++ aims to provide a high-quality implementation of the C++ Standard
Library, especially when it comes to correctness. As such, we aim to have test coverage
for all the platforms and compilers that we claim to support. If a platform or compiler
is not listed here, it is not officially supported. It may happen to work, and
in practice the library is known to work on some platforms not listed here, but
we don't make any guarantees. If you would like your compiler and/or platform
to be formally supported and listed here, please work with the libc++ team to set
up testing for your configuration.

.. _HeaderSupportPolicy:

Libc++ maintains backwards compatibility with programs compiled against older
versions of the headers. The library can currently be configured by vendors to
support headers from LLVM 2.8 or any later major release.

C++ Standards Conformance
=========================

Libc++ provides full support for C++11 and C++14, and provides most of newer standards
with a few omissions. The conformance status of the library's tip is tracked in real-time
using `this page <https://github.com/orgs/llvm/projects/31>`_. The conformance status of
this release is described in the pages below:

* C++11 - Complete
* C++14 - Complete
* :ref:`C++17 - In Progress <cxx17-status>`
* :ref:`C++20 - In Progress <cxx20-status>`
* :ref:`C++23 - In Progress <cxx23-status>`
* :ref:`C++2c - In Progress <cxx2c-status>`
* :ref:`C++ Feature Test Macro Status <feature-status>`


Getting Involved
================

First please review our `Developer's Policy <https://llvm.org/docs/DeveloperPolicy.html>`__
and `Getting started with LLVM <https://llvm.org/docs/GettingStarted.html>`__.

**Bug Reports**

If you think you've found a bug in libc++, please report it using the `LLVM bug tracker`_.
If you're not sure, you can ask for support on the `libc++ forum`_ or in the `libc++ chat`_.

**Patches**

If you want to contribute a patch to libc++, please start by reviewing our
:ref:`documentation about contributing <ContributingToLibcxx>`.

**Discussion and Questions**

Send discussions and questions to the `libc++ forum`_.


Design Documents
================

.. toctree::
   :maxdepth: 1

   DesignDocs/ABIVersioning
   DesignDocs/AtomicDesign
   DesignDocs/CapturingConfigInfo
   DesignDocs/ExperimentalFeatures
   DesignDocs/ExtendedCXX03Support
   DesignDocs/FeatureTestMacros
   DesignDocs/FileTimeType
   DesignDocs/HeaderRemovalPolicy
   DesignDocs/NoexceptPolicy
   DesignDocs/PSTLIntegration
   DesignDocs/ThreadingSupportAPI
   DesignDocs/UniquePtrTrivialAbi
   DesignDocs/UnspecifiedBehaviorRandomization
   DesignDocs/VisibilityMacros
   DesignDocs/TimeZone


Build Bots and Test Coverage
============================

* `Github Actions CI pipeline <https://github.com/llvm/llvm-project/actions/workflows/libcxx-build-and-test.yaml>`_
* `Buildkite CI pipeline <https://buildkite.com/llvm-project/libcxx-ci>`_
* `LLVM Buildbot Builders <https://lab.llvm.org/buildbot>`_
* :ref:`Adding New CI Jobs <AddingNewCIJobs>`


Quick Links
===========
* `LLVM Homepage <https://llvm.org/>`_
* `libc++abi Homepage <http://libcxxabi.llvm.org/>`_
* `LLVM Bug Tracker <https://github.com/llvm/llvm-project/labels/libc++/>`_
* `libcxx-commits Mailing List <http://lists.llvm.org/mailman/listinfo/libcxx-commits>`_
* `libc++ forum <https://discourse.llvm.org/c/runtimes/libcxx/>`_
* `libc++ chat <https://discord.com/channels/636084430946959380/636732894974312448>`_ (`invite link <https://discord.gg/xS7Z362>`_)
* `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/main/libcxx/>`_
