“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

Getting Started with libc++

• Libc++ 19.0.0 (In-Progress) Release Notes
• Using libc++
• Building libc++
• Testing libc++
• Contributing to libc++
• Implementation-defined behavior
• Modules in libc++
• Hardening Modes
• Release procedure
• libc++ C++14 Status
• libc++ C++17 Status
• libc++ C++20 Status
• libc++ C++23 Status
• libc++ C++2c Status
• libc++ Format Status
• libc++ Parallelism TS Status (N4808)
• libc++ Parallel STL Status
• libc++ Ranges Status
• libc++ Spaceship Operator Status (operator<=>)
• libc++ Mathematical Special Functions Status (P0226R1)
• libc++ Zip Status (P2321R2)

Current Status

libc++ has become the default C++ Standard Library implementation for many major platforms, including Apple’s macOS, iOS, watchOS, and tvOS, Google Search, the Android operating system, and FreeBSD. As a result, libc++ has an estimated user base of over 1 billion daily active users.

Since its inception, libc++ has focused on delivering high performance, standards-conformance, and portability. It has been extensively tested and optimized, making it robust and production ready. libc++ fully implements C++11 and C++14, with C++17, C++20, C++23, and C++26 features being actively developed and making steady progress.

libc++ is continuously integrated and tested on a wide range of platforms and configurations, ensuring its reliability and compatibility across various systems. The library’s extensive test suite and rigorous quality assurance process have made it a top choice for platform providers looking to offer their users a robust and efficient C++ Standard Library.

As an open-source project, libc++ benefits from a vibrant community of contributors who work together to improve the library and add new features. This ongoing development and support ensure that libc++ remains at the forefront of C++ standardization efforts and continues to meet the evolving needs of C++ developers worldwide.

History

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	17, 18, 19-git		latest two stable releases per LLVM’s release page and the development version
AppleClang	15		latest stable release per Xcode’s release page
Open XL	17.1 (AIX)		latest stable release per Open XL’s documentation page
GCC	14	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 10.13+	i386, x86_64, arm64
FreeBSD 12+	i386, x86_64, arm
Linux	i386, x86_64, arm, arm64	Only glibc-2.24 and later and no other libc is officially supported
Android 5.0+	i386, x86_64, arm, arm64
Windows	i386, x86_64	Both MSVC and MinGW style environments, ABI in MSVC environments is unstable
AIX 7.2TL5+	powerpc, powerpc64
Embedded (picolibc)	arm

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.

C++ Dialect Support

• C++11 - Complete

• C++14 - Complete

• C++17 - In Progress

• C++20 - In Progress

• C++23 - In Progress

• C++2c - In Progress

• C++ Feature Test Macro Status

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.

Design Documents

• Libc++ ABI stability
• <atomic> Design
• Capturing configuration information in the headers
• Experimental Features
• Extended C++03 Support
• Feature Test Macros
• File Time Type
• Header Removal Policy
• Guidelines for applying [[nodiscard]] in libc++
• noexcept Policy
• PSTL integration
• Threading Support API
• Enable std::unique_ptr [[clang::trivial_abi]]
• Unspecified Behavior Randomization
• Symbol Visibility Macros
• Time Zone Support

Build Bots and Test Coverage

• Github Actions CI pipeline

• Buildkite CI pipeline

• LLVM Buildbot Builders

• Adding New CI Jobs

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 bug tracker. If you’re not sure, you can ask for support on the libcxx forum or on IRC.

Patches

If you want to contribute a patch to libc++, please start by reviewing our documentation about contributing.

Discussion and Questions

Send discussions and questions to the libcxx forum.

Quick Links

• LLVM Homepage

• libc++abi Homepage

• LLVM Bug Tracker

• libcxx-commits Mailing List

• libcxx Forum

• Browse libc++ Sources