“libc++” C++ Standard Library

Overview

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

  • 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++ aims to support common compilers that implement the C++11 Standard. In order to strike a good balance between stability for users and maintenance cost, testing coverage and development velocity, libc++ drops support for older compilers as newer ones are released.

Compiler Versions Restrictions Support policy
Clang 11, 12   latest two stable releases per LLVM’s release page
AppleClang 12   latest stable release per Xcode’s release page
GCC 11 In C++11 or later only latest stable release per GCC’s release page

Libc++ also supports common platforms and architectures:

Target platform Target architecture Notes
macOS 10.9+ i386, x86_64, arm64 Building the shared library itself requires targetting macOS 10.11+
FreeBSD 10+ i386, x86_64, arm  
Linux i386, x86_64, arm, arm64  
Windows x86_64  

Generally speaking, libc++ should work on any platform that provides a fairly complete C Standard Library. It is also possible to turn off parts of the library for use on systems that provide incomplete support.

However, libc++ aims to provide a high-quality implementation of the C++ Standard Library, especially when it comes to correctness. As such, we aim to have test coverage for all the platforms and compilers that we claim to support. If a platform or compiler is not listed here, it is not officially supported. It may happen to work, and in practice the library is known to work on some platforms not listed here, but we don’t make any guarantees. If you would like your compiler and/or platform to be formally supported and listed here, please work with the libc++ team to set up testing for your configuration.

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.

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 libcxx-dev mailing list or on IRC.

Patches

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

Discussion and Questions

Send discussions and questions to the libcxx-dev mailing list.