Libc++ 15.0.0 Release Notes

Written by the Libc++ Team


This document contains the release notes for the libc++ C++ Standard Library, part of the LLVM Compiler Infrastructure, release 15.0.0. Here we describe the status of libc++ in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about libc++, please see the Libc++ Web Site or the LLVM Web Site.

Note that if you are reading this file from a Git checkout or the main Libc++ web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

What’s New in Libc++ 15.0.0?

The main focus of the libc++ team has been to implement new C++20 and C++23 features.

The C++20 format library is feature complete, but not all Standard LWG issues have been addressed. Since it is expected that at least one of these issues will cause an ABI break the format library is considered experimental.

The C++20 ranges library has progressed a lot since the last release and is almost complete. The ranges library is considered experimental.

Implemented Papers

  • P1004R2 - Making std::vector constexpr

  • P0627R6 - Function to mark unreachable code

  • P1165R1 - Make stateful allocator propagation more consistent for operator+(basic_string)

  • P0674R1 - Support arrays in make_shared and allocate_shared

  • P0980R1 - Making std::string constexpr

  • P2216R3 - std::format improvements

  • P0174R2 - Deprecating Vestigial Library Parts in C++17

  • N4190 - Removing auto_ptr, random_shuffle(), And Old <functional> Stuff

  • P0154R1 - Hardware inference size

  • P0618R0 - Deprecating <codecvt>

  • P2418R2 - Add support for std::generator-like types to std::format

  • LWG3659 - Consider ATOMIC_FLAG_INIT undeprecation

  • P1423R3 - char8_t backward compatibility remediation

  • P2508R1 - Exposing std::basic-format-string

  • Marked the following papers as “Complete” (note that some of those might have been implemented in a previous release but not marked as such):

    • P1207R4 - Movability of Single-pass Iterators

    • P1474R1 - Helpful pointers for ContiguousIterator

    • P1522R1 - Iterator Difference Type and Integer Overflow

    • P1523R1 - Views and Size Types

    • P1456R1 - Move-only views

    • P1870R1 - forwarding-range is too subtle

    • P1878R1 - Constraining Readable Types

    • P1970R2 - Consistency for size() functions: Add ranges::ssize

    • P1983R0 - Wording for GB301, US296, US292, US291, and US283

Improvements and New Features

  • std::pop_heap now uses an algorithm known as “bottom-up heapsort” or “heapsort with bounce” to reduce the number of comparisons, and rearranges elements using move-assignment instead of std::swap.

  • Libc++ now supports a variety of assertions that can be turned on to help catch undefined behavior in user code. This new support is now separate from the old (and incomplete) Debug Mode. Vendors can select whether the library they ship should include assertions or not by default. For details, see the documentation about this new feature.

  • Clang now implements the -fexperimental-library flag. This flag can be used to enable experimental library features such as TSes and other in-progress work like <ranges> and <format>. Using this flag makes it unnecessary to manually link against libc++experimental.a – just use -fexperimental-library and the compiler will do what’s needed to make experimental features work. However, be aware that experimental features are not stable!

  • The implementation of the function std::to_chars for integral types using base 10 has moved from the dylib to the header. This means the function no longer has a minimum deployment target.

  • The performance for std::to_chars, for integral types using base 2, 8, 10, or 16 has been improved.

  • The functions std::from_chars and std::to_chars now have 128-bit integral support.

  • The format functions (std::format, std::format_to, std::format_to_n, and std::formatted_size) now validate the format string at compile time. When the format string is invalid this will make the code ill-formed instead of throwing an exception at run-time. (This does not affect the v functions.)

  • All format functions in <format> allow the usage of non-copyable types as argument for the formatting functions. This change causes bit fields to become invalid arguments for the formatting functions.

  • The _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION macro has been added to allow re-enabling the allocator<void> specialization. When used in conjunction with _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS, this ensures that the members of allocator<void> removed in C++20 can be accessed.

  • boyer_moore_searcher and boyer_moore_horspool_searcher have been implemented.

  • vector<bool>::const_reference, vector<bool>::const_iterator::reference and bitset::const_reference are now aliases for bool in the unstable ABI, which improves libc++’s conformance to the Standard.

Deprecations and Removals

  • The header <experimental/filesystem> has been removed. Instead, use <filesystem> header. The associated macro _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM has been removed too.

  • The C++14 function std::quoted(const char*) is no longer supported in C++03 or C++11 modes.

  • Setting a custom debug handler with std::__libcpp_debug_function is not supported anymore. Please migrate to using the new support for assertions instead.

  • std::function has been removed in C++03. If you are using it, please remove usages or upgrade to C++11 or later. It is possible to re-enable std::function in C++03 by defining _LIBCPP_ENABLE_CXX03_FUNCTION. This option will be removed in LLVM 16.

  • unary_function and binary_function are no longer available in C++17 and C++20. They can be re-enabled by defining _LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION. They are also marked as [[deprecated]] in C++11 and later. To disable deprecation warnings you have to define _LIBCPP_DISABLE_DEPRECATION_WARNINGS. Note that this disables all deprecation warnings.

  • The contents of <codecvt>, wstring_convert and wbuffer_convert have been marked as deprecated. To disable deprecation warnings you have to define _LIBCPP_DISABLE_DEPRECATION_WARNINGS. Note that this disables all deprecation warnings.

  • The _LIBCPP_DISABLE_EXTERN_TEMPLATE macro is not honored anymore when defined by users of libc++. Instead, users not wishing to take a dependency on libc++ should link against the static version of libc++, which will result in no dependency being taken against the shared library.

  • The _LIBCPP_ABI_UNSTABLE macro has been removed in favour of setting _LIBCPP_ABI_VERSION=2. This should not have any impact on users because they were not supposed to set _LIBCPP_ABI_UNSTABLE manually, however we still feel that it is worth mentioning in the release notes in case some users had been doing it.

  • The integer distributions binomial_distribution, discrete_distribution, geometric_distribution, negative_binomial_distribution, poisson_distribution, and uniform_int_distribution now conform to the Standard by rejecting template parameter types other than short, int, long, long long, and the unsigned versions thereof. As an extension, int8_t, __int128_t and their unsigned versions are supported too. In particular, instantiating these distributions with non-integer types like bool and char will not compile anymore.

Upcoming Deprecations and Removals

  • The _LIBCPP_DEBUG macro is not supported anymore. It will be honoured until LLVM 16, and then it will be an error to define that macro. To enable basic assertions (previously _LIBCPP_DEBUG=0), please use _LIBCPP_ENABLE_ASSERTIONS=1. To enable the debug mode (previously _LIBCPP_DEBUG=1|2), please ensure that the library has been built with support for the debug mode, and it will be enabled automatically (no need to define _LIBCPP_DEBUG).

  • The experimental versions of boyer_moore_searcher and boyer_moore_horspool_searcher will be removed in LLVM 17. You can disable the deprecation warnings by defining _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_SEARCHERS.

  • The implementation of the Coroutines TS in std::experimental will be removed in LLVM 16.

  • Libc++ is getting ready to remove unnecessary transitive inclusions. This may break your code in the future. To future-proof your code to these removals, please compile your code with _LIBCPP_REMOVE_TRANSITIVE_INCLUDES defined and fix any compilation error resulting from missing includes.

ABI Affecting Changes

  • The _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION macro controlling whether we use an emulation for std::nullptr_t in C++03 mode has been removed. After this change, _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION will not be honoured anymore and there will be no way to opt back into the C++03 emulation of std::nullptr_t.

  • On FreeBSD, NetBSD, DragonFlyBSD and Solaris, std::random_device is now implemented on top of arc4random() instead of reading from /dev/urandom. Any implementation-defined token used when constructing a std::random_device will now be ignored instead of interpreted as a file to read entropy from.

  • std::valarray’s unary operators !, +, ~ and - now return an expression object instead of a valarray. This was done to fix an issue where any expression involving other valarray operators and one of these unary operators would end up with a dangling reference. This is a potential ABI break for code that exposes std::valarray on an ABI boundary, specifically if the return type of an ABI-boundary function is auto-deduced from an expression involving unary operators on valarray. If you are concerned by this, you can audit whether your executable or library exports any function that returns a valarray, and if so ensure that any such function uses std::valarray directly as a return type instead of relying on the type of valarray-expressions, which is not guaranteed by the Standard anyway.

  • By default, the legacy debug mode symbols are not provided with the library anymore. If you are a vendor and need to re-enable them, please use the LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS CMake flag, and contact the libc++ developers as this will be removed in LLVM 16. Furthermore, please note that LIBCXX_ENABLE_DEBUG_MODE_SUPPORT is not honored anymore.

Build System Changes

  • Support for standalone builds have been entirely removed from libc++, libc++abi and libunwind. Please use these instructions for building libc++, libc++abi and/or libunwind.


  • Previously, the C++ ABI library headers would be installed inside <prefix>/include/c++/v1 alongside the libc++ headers as part of building libc++. This is not the case anymore – the ABI library is expected to install its headers where it wants them as part of its own build. Note that no action is required for most users, who build libc++ against libc++abi, since libc++abi already installs its headers in the right location. However, vendors building libc++ against alternate ABI libraries should make sure that their ABI library installs its own headers.

  • The legacy testing configuration is now deprecated and will be removed in LLVM 16. For most users, this should not have any impact. However, if you are testing libc++, libc++abi, or libunwind in a configuration or on a platform that used to be supported by the legacy testing configuration and isn’t supported by one of the configurations in libcxx/test/configs, libcxxabi/test/configs, or libunwind/test/configs, please move to one of those configurations or define your own.

  • MinGW DLL builds of libc++ no longer use dllimport in their headers, which means that the same set of installed headers works for both DLL and static linkage. This means that distributors finally can build both library versions with a single CMake invocation.

  • The LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT configuration option has been removed. Indeed, the risk of ODR violations from mixing different versions of libc++ in the same program has been mitigated with a different technique that is simpler and does not have the drawbacks of using internal linkage.