# "libc++" C++ Standard Library

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

All of the code in libc++ is dual licensed under the MIT license and the UIUC License (a BSD-like license).

## New Documentation Coming Soon!

Looking for documentation on how to use, build and test libc++? If so checkout the new libc++ documentation.

Click here for the new libc++ documentation.

## 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.

## Why a new C++ Standard Library for C++11?

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 Support

libc++ is known to work on the following platforms, using g++-4.2 and clang (lack of C++11 language support disables some functionality). Note that functionality provided by <atomic> is only functional with clang.

- Mac OS X i386
- Mac OS X x86_64
- FreeBSD 10+ i386
- FreeBSD 10+ x86_64
- FreeBSD 10+ ARM

## Current Status

libc++ is a 100% complete C++11 implementation on Apple's OS X.

LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux.

libc++ is also a 100% complete C++14 implementation. A list of new features and changes for C++14 can be found here.

libc++'s C++17 implementation is not yet complete. A list of features and changes for C++17 can be found here.

A list of features and changes for the next C++ standard, known here as "C++2a" (probably to be C++20) can be found here.

Implementation of the post-C++14 Technical Specifications is in progress. A list of features and the current status of these features can be found here.

As features get moved from the Technical Specifications into the main standard, we will (after a period for migration) remove them from the TS implementation. This process is detailed here.

## Build Bots

The latest libc++ build results can be found at the following locations.

- Buildbot libc++ builders
- Jenkins libc++ builders

## Get it and get involved!

First please review our Developer's Policy. The documentation for building and using libc++ can be found below.

- Using libc++ Documentation on using the library in your programs
- Building libc++ Documentation on building the library using CMake
- Testing libc++ Documentation for developers wishing to test the library

### Notes and Known Issues

- Building libc++ with `-fno-rtti` is not supported. However linking against it with `-fno-rtti` is supported.
- On OS X v10.8 and older the CMake option `-DLIBCXX_LIBCPPABI_VERSION=""` must be used during configuration.

Send discussions to the clang mailing list.

## Bug reports and patches

If you think you've found a bug in libc++, please report it using the LLVM Bugzilla. If you're not sure, you can post a message to the cfe-dev mailing list or on IRC. Please include "libc++" in your subject.

If you want to contribute a patch to libc++, the best place for that is Phabricator. Please include [libc++] in the subject and add cfe-commits as a subscriber.

## Design Documents

- `<atomic>`
- `<type_traits>`
- Excellent notes by Marshall Clow