# "libc++" C++ Standard Library

libc++ is a new implementation of the C++ standard library, targeting C++0X.

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 (currently draft) C++0X 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++'0x?

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++'0x, 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++'0x, 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++'0x support. Our experience (and the experience of libstdc++ developers) is that adding support for C++0x (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++0X language support disables some functionality).

- Mac OS X i386
- Mac OS X x86_64

## Current Status

libc++ is still under development. It has about 98% of N3126 implemented/tested. C++'98 support is fully featured, and most of C++'0x support is as well. The only major missing piece of C++'0x support is `<atomic>`.

Here is a by-chapter breakdown of what is passing tests and what isn't. This chart is currently based on testing against g++-4.4.0 with -std=c++0x.

A Windows port is underway. Here are recent test results.

## Get it and get involved!

First please review our Developer's Policy.

To check out the code, use:

- `svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx`

To build on Mac OS X 10.6, you need a helper library and header found here. cp cxxabi.h to /usr/include, and cp libc++abi.dylib to /usr/lib.

Next:

- `cd libcxx/lib`
- `export TRIPLE=-apple-`
- `export MACOSX_DEPLOYMENT_TARGET=10.6`
- `./buildit`

That should result in a libc++.1.dylib. To install it I like to use links instead of copying, but either should work:

- `cd /usr/lib`
- `sudo ln -sf path-to-libcxx/lib/libc++.1.dylib libc++.1.dylib`
- `sudo ln -sf libc++.1.dylib libc++.dylib`
- `cd /usr/include/c++`
- `sudo ln -sf path-to-libcxx/include v1`

To use with clang you can:

- `clang++ -stdlib=libc++ test.cpp`
- `clang++ -std=c++0x -stdlib=libc++ test.cpp`

To run the libc++ test suit (recommended):

- `cd libcxx/test`
- `./testit`

- On Mac OS 10.6, to work around bugs in libc headers like math.h and inttypes.h, add "-U__STRICT_ANSI__" and "-D__STDC_FORMAT_MACROS" to the command line with:

  >     export OPTIONS="-std=c++0x -stdlib=libc++ -U__STRICT_ANSI__ -D__STDC_FORMAT_MACROS"

- People porting libc++ to other OSes will likely have to define similar macros.

Send discussions to the (clang mailing list).

## Design Documents

- `<atomic>`
- `<type_traits>`