“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++
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
For using the libc++ headers
The libc++ headers are known to work on the following platforms, using GCC and Clang. Note that functionality provided by <atomic> is only functional with Clang and GCC.
| OS | Arch | Compilers |
|---|---|---|
| macOS 10.9+ | i386, x86_64 | Clang, GCC |
| FreeBSD 10+ | i386, x86_64, ARM | Clang, GCC |
| Linux | i386, x86_64 | Clang, GCC |
The following minimum compiler versions are required:
- Clang 4.0 and above
- GCC 5.0 and above.
The C++03 dialect is only supported with Clang.
For building the libc++ library
Building the libc++ library (static or shared) requires some features from the operating system. As such, it has its own set of (slightly different) system requirements.
| OS | Arch | Compilers | ABI Library |
|---|---|---|---|
| macOS 10.12+ | 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 required:
- Clang 4.0 and above
- GCC 5.0 and above.
C++ Dialect Support
Notes and Known Issues
This list contains known issues with libc++
- Building libc++ with
-fno-rttiis not supported. However linking against it with-fno-rttiis supported.
A full list of currently open libc++ bugs can be found here.
Design Documents
Build Bots and Test Coverage
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.
