"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).

Features and Goals

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:

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.

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.

A list of features and changes for the next C++ standard, known here as "C++1z" (probably to be C++17) 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.

Ports to other platforms are underway. Here are recent test results for Windows and Linux.

Build Bots

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

Get it and get involved!

First please review our Developer's Policy.

On Mac OS 10.7 (Lion) and later, the easiest way to get this library is to install Xcode 4.2 or later. However if you want to install tip-of-trunk from here (getting the bleeding edge), read on. However, be warned that Mac OS 10.7 will not boot without a valid copy of libc++.1.dylib in /usr/lib.

To check out the code, use:

Note that for an in-tree build, you should check out libcxx to llvm/projects.

The following instructions are for building libc++ on FreeBSD, Linux, or Mac using libc++abi as the C++ ABI library. On Linux, it is also possible to use libsupc++ or libcxxrt.

In-tree build:

Out-of-tree buildc:

To run the tests:

If you wish to run a subset of the test suite:

The above is currently quite inconvenient. Sorry! We're working on it!

More information on using LIT can be found here. For more general information about the LLVM testing infrastructure, see the LLVM Testing Infrastructure Guide

Shared libraries for libc++ should now be present in llvm/build/lib. Note that it is safest to use this from its current location rather than replacing your system's libc++ (if it has one, if not, go right ahead).

Mac users, remember to be careful when replacing the system's libc++. Your system will not be able to boot without a functioning libc++.

Notes and Known Issues

Send discussions to the clang mailing list.

Using libc++ in your programs

FreeBSD and Mac OS X

To use your system-installed libc++ with clang you can:

To use your tip-of-trunk libc++ on Mac OS with clang you can:

Linux

You will need to keep the source tree of libc++abi available on your build machine and your copy of the libc++abi shared library must be placed where your linker will find it.

Unfortunately you can't simply run clang with "-stdlib=libc++" at this point, as clang is set up to link for libc++ linked to libsupc++. To get around this you'll have to set up your linker yourself (or patch clang). For example:

Alternately, you could just add libc++abi to your libraries list, which in most situations will give the same result:

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.

Build on Linux using CMake and libsupc++.

You will need libstdc++ in order to provide libsupc++.

Figure out where the libsupc++ headers are on your system. On Ubuntu this is /usr/include/c++/<version> and /usr/include/c++/<version>/<target-triple>

You can also figure this out by running 

$ echo | g++ -Wp,-v -x c++ - -fsyntax-only
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.7
 /usr/include/c++/4.7/x86_64-linux-gnu
 /usr/include/c++/4.7/backward
 /usr/lib/gcc/x86_64-linux-gnu/4.7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
Note the first two entries happen to be what we are looking for. This may not be correct on other platforms.

We can now run CMake:

You can now run clang with -stdlib=libc++.

Build on Linux using CMake and libcxxrt.

You will need to keep the source tree of libcxxrt available on your build machine and your copy of the libcxxrt shared library must be placed where your linker will find it.

We can now run CMake:

Unfortunately you can't simply run clang with "-stdlib=libc++" at this point, as clang is set up to link for libc++ linked to libsupc++. To get around this you'll have to set up your linker yourself (or patch clang). For example,

Alternately, you could just add libcxxrt to your libraries list, which in most situations will give the same result:

Using a local ABI library

Note: This is not recommended in almost all cases.
Generally these instructions should only be used when you can't install your ABI library.

Normally you must link libc++ against a ABI shared library that the linker can find. If you want to build and test libc++ against an ABI library not in the linker's path you need to set -DLIBCXX_CXX_ABI_LIBRARY_PATH=/path/to/abi/lib when configuring CMake.

An example build using libc++abi would look like:

When testing libc++ LIT will automatically link against the proper ABI library.

Design Documents