Libc++ 16.0.0 (In-Progress) Release Notes

Written by the Libc++ Team


These are in-progress notes for the upcoming libc++ 16 release. Release notes for previous releases can be found on the Download Page.


This document contains the release notes for the libc++ C++ Standard Library, part of the LLVM Compiler Infrastructure, release 16.0.0. 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++ 16.0.0?

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

The C++20 format library has improved but it not yet considered stable. The main improvements are additional formatters for the chrono calendar types. Work on formatting ranges has started.

The C++20 ranges library has been completed and is no longer experimental (with the exception of ranges::join_view which is still marked as experimental because it is about to undergo an ABI-breaking change in the Standard due to D2770). Work on C++23 ranges has started.

The C++20 spaceship operator has been added to more types, the work is still ongoing.

D139235 made copy and move algorithms and their variations (copy_backward, etc.) apply optimizations for trivial types more often. This has the potential to expose bugs in code using these algorithms that currently relies on undefined behavior (this includes indirect usage – for example, these algorithms are used in the implementation of some standard containers). This change also made the algorithms check the given iterator types for conformance more strictly.

Implemented Papers

  • P2499R0 - string_view range constructor should be explicit

  • P2417R2 - A more constexpr bitset

  • P2445R1 - std::forward_like

  • P2273R3 - Making std::unique_ptr constexpr

  • P0591R4 - Utility functions to implement uses-allocator construction

  • P2291R3 - Add Constexpr Modifiers to Functions to_chars and from_chars for Integral Types in <charconv> Header

  • P0220R1 - Adopt Library Fundamentals V1 TS Components for C++17

  • P0482R6 - char8_t: A type for UTF-8 characters and strings

  • P2438R2 - std::string::substr() &&

  • P0600R1 - nodiscard in the library

  • P0339R6 - polymorphic_allocator<> as a vocabulary type

  • P1169R4 - static operator()

  • P0415R1 - constexpr for std::complex

  • P1208R6 - std::source_location

  • P0323R12 - std::expected

  • P1035R7 - Input Range Adaptors

  • P2325R3 - Views should not be required to be default constructible

  • P2446R2 - views::as_rvalue

  • P1020R1 - Smart pointer creation with default initialization

  • P2210R2 - Superior String Splitting

  • P2286R8 - Formatting Ranges

Improvements and New Features

  • Declarations of std::c8rtomb() and std::mbrtoc8() from P0482R6 are now provided when implementations in the global namespace are provided by the C library.

  • Implemented <memory_resource> header from C++17

  • D122780 Improved the performance of std::sort

  • The ranges versions of copy, move, copy_backward and move_backward are now also optimized for std::deque<>::iterator, which can lead to up to 20x performance improvements on certain algorithms.

  • The std and ranges versions of copy, move, copy_backward and move_backward are now also optimized for join_view::iterator, which can lead to up to 20x performance improvements on certain combinations of iterators and algorithms.

Deprecations and Removals

  • unary_function and binary_function are no longer provided in C++17 and newer Standard modes. They can be re-enabled with _LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION.

  • Several incidental transitive includes have been removed from libc++. Those includes are removed based on the language version used. Incidental transitive inclusions of the following headers have been removed:

    • C++11, C++14, C++17, and C++20: chrono

    • C++2b: algorithm, array, atomic, bit, chrono, climits, cmath, compare, concepts, cstdarg, cstddef, cstdint, cstdlib, cstring, ctime, exception, functional, initializer_list, iosfwd, iterator, limits, memory, new, numeric, optional, ratio, stdexcept, string, tuple, type_traits, typeinfo, unordered_map, utility, variant, vector.

    Users can also remove all incidental transitive includes by defining _LIBCPP_REMOVE_TRANSITIVE_INCLUDES regardless of the language version in use. Note that in the future, libc++ reserves the right to remove incidental transitive includes more aggressively, in particular regardless of the language version in use.

  • The legacy testing system for libc++, libc++abi, and libunwind has been removed. All known clients have been migrated to the new configuration system, but please reach out to the libc++ developers if you find something missing in the new configuration system.

  • The functions to_chars and from_chars for integral types are available starting with C++17. Libc++ offered these functions in C++11 and C++14 as an undocumented extension. This extension makes it hard to implement the C++23 paper that makes these functions constexpr, therefore the extension has been removed.

  • The _LIBCPP_ENABLE_CXX03_FUNCTION macro that allowed re-enabling the now-deprecated C++03 implementation of std::function has been removed. Users who need to use std::function should switch to C++11 and above.

  • The contents of <experimental/memory_resource> are now deprecated since libc++ ships <memory_resource> now. Please migrate to <memory_resource> instead. Per libc++’s TS deprecation policy, <experimental/memory_resource> will be removed in LLVM 18.

  • The _LIBCPP_DEBUG macro is not honored anymore, and it is an error to try to use it. Please migrate to _LIBCPP_ENABLE_DEBUG_MODE instead.

Upcoming Deprecations and Removals

  • The base template for std::char_traits has been marked as deprecated and will be removed in LLVM 18. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or a custom character type for which you specialized std::char_traits, your code will stop working when we remove the base template. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior while going undetected.

  • The _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED macro will not be honored anymore in LLVM 18. Please see the updated documentation about the safe libc++ mode and in particular the _LIBCPP_VERBOSE_ABORT macro for details.

API Changes

  • The comparison operators on thread::id are now defined as free-standing functions instead of as hidden friends, in conformance with the C++ standard. Also see issue 56187.

  • _LIBCPP_ENABLE_NODISCARD and _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17 are no longer respected. Any standards-required [[nodiscard]] applications in C++20 are now always enabled. Any extended applications are now enabled by default and can be disabled by defining _LIBCPP_DISABLE_NODISCARD_EXT.

  • _LIBCPP_VERSION was previously defined to e.g. 15001 to represent LLVM 15.0.01, but this value had been left undocumented. Starting with LLVM 16, _LIBCPP_VERSION will contain the version of LLVM represented as XXYYZZ. In other words, _LIBCPP_VERSION is gaining a digit. This should not be an issue for existing code, since using e.g. _LIBCPP_VERSION > 15000 will still give the right answer now that _LIBCPP_VERSION is defined as e.g. 160000 (with one more digit).

ABI Affecting Changes

  • In freestanding mode, atomic<small enum class> does not contain a lock byte anymore if the platform can implement lockfree atomics for that size. More specifically, in LLVM <= 11.0.1, an atomic<small enum class> would not contain a lock byte. This was broken in LLVM >= 12.0.0, where it started including a lock byte despite the platform supporting lockfree atomics for that size. Starting in LLVM 15.0.1, the ABI for these types has been restored to what it used to be (no lock byte), which is the most efficient implementation.

    This ABI break only affects users that compile with -ffreestanding, and only for atomic<T> where T is a non-builtin type that could be lockfree on the platform. See for more details.

  • When building libc++ against newlib/picolibc, the type of regex_type_traits::char_class_type was changed to uint16_t since all values of ctype_base::mask are taken. This is technically an ABI break, but including <regex> `` has triggered a ``static_assert failure since libc++ 14, so it is unlikely that this causes problems for existing users.

Build System Changes

  • Support for libcxx, libcxxabi and libunwind in LLVM_ENABLE_PROJECTS has officially been removed. Instead, please build according to these instructions.