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llvm-project/libcxx/docs/index.rst
Mark de Wever 8ff2d6af69 [libc++] Reduces the number of transitive includes. 
This defines a new policy for removal of transitive includes.
The goal of the policy it to make it relatively easy to remove
headers when needed, but avoid breaking developers using and
vendors shipping libc++.

The method used is to guard transitive includes based on the
C++ language version. For the upcoming C++23 we can remove
headers when we want, but for other language versions we try
to keep it to a minimum.

In this code the transitive include of `<chrono>` is removed
since D128577 introduces a header cycle between `<format>`
and `<chrono>`. This cycle is indirectly required by the
Standard. Our cycle dependency tool basically is a grep based
tool, so it needs some hints to ignore cycles. With the input
of our transitive include tests we can create a better tool.
However that's out of the scope of this patch.

Note the flag `_LIBCPP_REMOVE_TRANSITIVE_INCLUDES` remains
unchanged. So users can still opt-out of transitives includes
entirely.

Reviewed By: #libc, ldionne, philnik

Differential Revision: https://reviews.llvm.org/D132284
2022-08-31 19:50:03 +02:00

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.. _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
UsingLibcxx
BuildingLibcxx
TestingLibcxx
Contributing
Status/Cxx14
Status/Cxx17
Status/Cxx20
Status/Cxx2b
Status/Format
Status/Ranges
Status/Spaceship
Status/Zip
.. toctree::
:hidden:
AddingNewCIJobs
FeatureTestMacroTable
Current Status
==============
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.
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 13, 14 latest two stable releases per `LLVM's release page <https://releases.llvm.org>`_
AppleClang 13 latest stable release per `Xcode's release page <https://developer.apple.com/documentation/xcode-release-notes>`_
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>`_
GCC 12 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 10.9+ i386, x86_64, arm64 Building the shared library itself requires targetting macOS 10.11+
FreeBSD 10+ i386, x86_64, arm
Linux i386, x86_64, arm, arm64
Windows i386, x86_64 Both MSVC and MinGW style environments
AIX powerpc, powerpc64
=============== ========================= ============================
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.
C++ Dialect Support
===================
* C++11 - Complete
* :ref:`C++14 - Complete <cxx14-status>`
* :ref:`C++17 - In Progress <cxx17-status>`
* :ref:`C++20 - In Progress <cxx20-status>`
* :ref:`C++2b - In Progress <cxx2b-status>`
* :ref:`C++ Feature Test Macro Status <feature-status>`
Notes and Known Issues
======================
This list contains known issues with libc++
* Building libc++ with ``-fno-rtti`` is not supported. However
linking against it with ``-fno-rtti`` is supported.
A full list of currently open libc++ bugs can be `found here`__.
.. __: https://github.com/llvm/llvm-project/labels/libc%2B%2B
Design Documents
================
.. toctree::
:maxdepth: 1
DesignDocs/ABIVersioning
DesignDocs/AtomicDesign
DesignDocs/CapturingConfigInfo
DesignDocs/DebugMode
DesignDocs/ExperimentalFeatures
DesignDocs/ExtendedCXX03Support
DesignDocs/FeatureTestMacros
DesignDocs/FileTimeType
DesignDocs/HeaderRemovalPolicy
DesignDocs/NoexceptPolicy
DesignDocs/ThreadingSupportAPI
DesignDocs/UniquePtrTrivialAbi
DesignDocs/UnspecifiedBehaviorRandomization
DesignDocs/VisibilityMacros
Build Bots and Test Coverage
============================
* `Buildkite CI pipeline <https://buildkite.com/llvm-project/libcxx-ci>`_
* `LLVM Buildbot Builders <http://lab.llvm.org:8011>`_
* :ref:`Adding New CI Jobs <AddingNewCIJobs>`
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 `libcxx forum`_ or on IRC.
**Patches**
If you want to contribute a patch to libc++, the best place for that is
`Phabricator <https://llvm.org/docs/Phabricator.html>`_. Please add `libcxx-commits` as a subscriber.
Also make sure you are subscribed to the `libcxx-commits mailing list`_.
**Discussion and Questions**
Send discussions and questions to the `libcxx forum`_.
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>`_
* `libcxx Forum <https://discourse.llvm.org/c/runtimes/libcxx/>`_
* `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/main/libcxx/>`_
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