"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
- 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.
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
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.
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
To check out the code, use:
svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx
Note that for an in-tree build, you should check out libcxx to llvm/projects.
- Check out libcxx and libcxxabi into llvm/projects
mkdir build && cd build
cmake path/to/llvm # Linux may require -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
- Check out libcxx and llvm
- If not on a Mac, also check out libcxxabi
mkdir build && cd build
cmake -DLLVM_PATH=path/to/llvm -DLIBCXX_CXX_ABI=libcxxabi -DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/libcxxabi/include -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ path/to/libcxx
To run the tests:
If you wish to run a subset of the test suite:
alias lit='python path/to/llvm/utils/lit/lit.py'
lit -sv test/re/ # or whichever subset of tests you're interested in
The above is currently quite inconvenient. Sorry! We're working on it!
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
Building libc++ with
-fno-rttiis not supported. However linking against it with
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.
Using libc++ in your programs
FreeBSD and Mac OS X
To use your system-installed libc++ with clang you can:
clang++ -stdlib=libc++ test.cpp
clang++ -std=c++11 -stdlib=libc++ test.cpp
To use your tip-of-trunk libc++ on Mac OS with clang you can:
clang++ -std=c++11 -stdlib=libc++ -nostdinc++ -I<path-to-libcxx>/include -L<path-to-libcxx>/lib test.cpp
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:
clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc
Alternately, you could just add libc++abi to your libraries list, which in most situations will give the same result:
clang++ -stdlib=libc++ helloworld.cpp -lc++abi
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
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:
CC=clang CXX=clang++ cmake -G "Unix Makefiles" -DLIBCXX_CXX_ABI=libstdc++ -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr <libc++-source-dir>
- You can also substitute
-DLIBCXX_CXX_ABI=libsupc++above, which will cause the library to be linked to libsupc++ instead of libstdc++, but this is only recommended if you know that you will never need to link against libstdc++ in the same executable as libc++. GCC ships libsupc++ separately but only as a static library. If a program also needs to link against libstdc++, it will provide its own copy of libsupc++ and this can lead to subtle problems.
sudo make install
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:
CC=clang CXX=clang++ cmake -G "Unix Makefiles" -DLIBCXX_CXX_ABI=libcxxrt -DLIBCXX_CXX_ABI_INCLUDE_PATHS="<libcxxrt-source-dir>/src" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr <libc++-source-dir>
sudo make install
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,
clang++ -stdlib=libc++ helloworld.cpp -nodefaultlibs -lc++ -lcxxrt -lm -lc -lgcc_s -lgcc
clang++ -stdlib=libc++ helloworld.cpp -lcxxrt
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
An example build using libc++abi would look like:
CC=clang CXX=clang++ cmake -DLIBCXX_CXX_ABI=libc++abi -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/path/to/libcxxabi/include" -DLIBCXX_CXX_ABI_LIBRARY_PATH="/path/to/libcxxabi-build/lib" path/to/libcxx
When testing libc++ LIT will automatically link against the proper ABI library.