“libc++” C++ Standard Library

Overview

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

  • 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

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

OS Arch Compilers ABI Library
Mac OS X i386, x86_64 Clang, GCC libc++abi
FreeBSD 10+ i386, x86_64, ARM Clang, GCC libcxxrt, libc++abi
Linux i386, x86_64 Clang, GCC libc++abi

The following minimum compiler versions are strongly recommended.

  • Clang 3.5 and above
  • GCC 4.7 and above.

Anything older may work.

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.
  • On OS X v10.8 and older the CMake option -DLIBCXX_LIBCPPABI_VERSION="" must be used during configuration.

A full list of currently open libc++ bugs can be found here.

Getting Involved

First please review our Developer’s Policy and Getting started with LLVM.

Bug Reports

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.

Patches

If you want to contribute a patch to libc++, the best place for that is Phabricator. Please include [libcxx] in the subject and add cfe-commits as a subscriber. Also make sure you are subscribed to the cfe-commits mailing list.

Discussion and Questions

Send discussions and questions to the cfe-dev mailing list. Please include [libcxx] in the subject.