Libc++ 21.0.0 (In-Progress) Release Notes¶
Written by the Libc++ Team
Warning
These are in-progress notes for the upcoming libc++ 20.0.0 release. Release notes for previous releases can be found on the Download Page.
Introduction¶
This document contains the release notes for the libc++ C++ Standard Library, part of the LLVM Compiler Infrastructure, release 20.0.0. 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++ 21.0.0?¶
Implemented Papers¶
N4258: Cleaning-up noexcept in the Library (Github)
P0767R1: Deprecate POD (Github)
P1361R2: Integration of chrono with text formatting (Github)
P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (Github)
P2562R1:
constexpr
Stable Sorting (Github)P0472R3: Put std::monostate in <utility> (Github)
P1222R4: A Standard
flat_set
(Github)P2897R7:
aligned_accessor
: An mdspan accessor expressing pointer over-alignment (Github)P3247R2: Deprecate the notion of trivial types (Github)
Improvements and New Features¶
The
std::ranges::{copy, copy_n, copy_backward, move, move_backward, rotate}
algorithms have been optimized forstd::vector<bool>::iterator
, resulting in a performance improvement of up to 2000x.The
std::ranges::equal
algorithm has been optimized forstd::vector<bool>::iterator
, resulting in a performance improvement of up to 188x.The
std::ranges::swap_ranges
algorithm has been optimized forstd::vector<bool>::iterator
, resulting in a performance improvement of up to 611x.Updated formatting library to Unicode 16.0.0.
The
num_put::do_put
integral overloads have been optimized, resulting in a performance improvement of up to 2.4x.The
std::stable_sort
algorithm uses radix sort for floating-point types now, which can improve the performance up to 10x, depending on type of sorted elements and the initial state of the sorted array.The segmented iterator optimization for
std::for_each
has been backported to C++11. Previously it was only available in C++23 and later.
Deprecations and Removals¶
std::is_pod
andstd::is_pod_v
are deprecated in C++20 and later.std::is_trivial
andstd::is_trivial_v
are deprecated in C++26 and later.The
_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT
has been removed, makingstd::__libcpp_verbose_abort
unconditionallynoexcept
.
Potentially breaking changes¶
The implementation of
num_put::do_put
has been replaced to improve the performance, which can lead to different output when printing pointers.
Upcoming Deprecations and Removals¶
LLVM 21¶
The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22, non-critical bug fixes may not be back-ported to C++03, including LWG issues. C++03 is a legacy platform, where most projects are no longer actively maintained. To reduce the amount of fixes required to keep such legacy projects compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid unintended breaking changes. See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.
If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.
Non-conforming extension
packaged_task::result_type
will be removed in LLVM 21.
LLVM 22¶
TODO
ABI Affecting Changes¶
When using GCC, the
std
namespace is now annotated with[[gnu::visibility("default")]]
. This may cause more symbols to be exported from shared libraries when building with-fvisibility=hidden
. This also fixes RTTI comparison between shared libraries, since all RTTI has the correct visibility now. There is no behaviour change on Clang.
Build System Changes¶
TODO