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Libc++ 21.0.0 (In-Progress) Release Notes
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Written by the `Libc++ Team <https://libcxx.llvm.org>`_

.. warning::

   These are in-progress notes for the upcoming libc++ 21.0.0 release.
   Release notes for previous releases can be found on
   `the Download Page <https://releases.llvm.org/download.html>`_.

Introduction
============

This document contains the release notes for the libc++ C++ Standard Library,
part of the LLVM Compiler Infrastructure, release 21.0.0. 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 <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.

For more information about libc++, please see the `Libc++ Web Site
<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.

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 <https://llvm.org/releases/>`_.

What's New in Libc++ 21.0.0?
==============================

Implemented Papers
------------------

- N4258: Cleaning-up noexcept in the Library (`Github <https://github.com/llvm/llvm-project/issues/99937>`__)
- P0767R1: Deprecate POD (`Github <https://github.com/llvm/llvm-project/issues/104013>`__)
- P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
- P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`__)
- P2372R3: Fixing locale handling in chrono formatters (`Github <https://github.com/llvm/llvm-project/issues/100043>`__)
- P2562R1: ``constexpr`` Stable Sorting (`Github <https://github.com/llvm/llvm-project/issues/105360>`__)
- P0472R3: Put std::monostate in <utility> (`Github <https://github.com/llvm/llvm-project/issues/127874>`__)
- P1222R4: A Standard ``flat_set`` (`Github <https://github.com/llvm/llvm-project/issues/105193>`__)
- P2897R7: ``aligned_accessor``: An mdspan accessor expressing pointer over-alignment (`Github <https://github.com/llvm/llvm-project/issues/118372>`__)
- P3247R2: Deprecate the notion of trivial types (`Github <https://github.com/llvm/llvm-project/issues/118387>`__)
- P3372R3: ``constexpr`` containers and adaptors (`Github <https://github.com/llvm/llvm-project/issues/127876>`__) (``forward_list``, ``list``, ``priority_queue``, ``flat_map``, and ``flat_set`` are implemented)
- P2441R2: ``views::join_with`` (`Github <https://github.com/llvm/llvm-project/issues/105185>`__)
- P2711R1: Making multi-param constructors of ``views`` ``explicit`` (`Github <https://github.com/llvm/llvm-project/issues/105252>`__)
- P2770R0: Stashing stashing ``iterators`` for proper flattening (`Github <https://github.com/llvm/llvm-project/issues/105250>`__)
- P2655R3: ``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type (`Github <https://github.com/llvm/llvm-project/issues/105260>`__)
- P2944R3: Comparisons for ``reference_wrapper`` (`Github <https://github.com/llvm/llvm-project/issues/105424>`__)
- P3379R0: Constrain ``std::expected equality`` operators (`Github <https://github.com/llvm/llvm-project/issues/118135>`__)

Improvements and New Features
-----------------------------

- The ``std::ranges::{copy, copy_n, copy_backward, move, move_backward, rotate}`` algorithms have been optimized for
  ``std::vector<bool>::iterator``, resulting in a performance improvement of up to 2000x.

- The ``std::ranges::equal`` algorithm has been optimized for ``std::vector<bool>::iterator``, resulting in a performance
  improvement of up to 188x.

- The ``std::ranges::swap_ranges`` algorithm has been optimized for ``std::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.

- The ``std::for_each_n``, ``std::ranges::for_each`` and ``std::ranges::for_each_n`` algorithms have been optimized for
  segmented iterators, resulting in a performance improvement of up to 17.7x for ``std::deque<short>`` iterators, and up
  to 13.9x for ``std::join_view<vector<vector<short>>>`` iterators.

- The ``bitset::to_string`` function has been optimized, resulting in a performance improvement of up to 8.3x for bitsets
  with uniformly distributed zeros and ones, and up to 13.5x and 16.1x for sparse and dense bitsets, respectively.

- The ``flat_map::insert`` and ``flat_set::insert_range`` have been optimized, resulting in a performance improvement of up
  to 10x for inserting elements into a ``flat_map`` when the input range is a ``flat_map`` or a ``zip_view``.

- ``ctype::tolower`` and ``ctype::toupper`` have been optimized, resulting in a 2x performance improvement.

- As an experimental feature, Hardening now supports assertion semantics that allow customizing how a hardening
  assertion failure is handled. The four available semantics, modeled on C++26 Contracts, are ``ignore``, ``observe``,
  ``quick-enforce`` and ``enforce``. The ``observe`` semantic is intended to make it easier to adopt Hardening in
  production but should not be used outside of this scenario. Please refer to the :ref:`Hardening documentation
  <hardening>` for details.

Deprecations and Removals
-------------------------

- ``std::is_pod`` and ``std::is_pod_v`` are deprecated in C++20 and later.

- ``std::is_trivial`` and ``std::is_trivial_v`` are deprecated in C++26 and later.

- The ``_LIBCPP_VERBOSE_ABORT_NOT_NOEXCEPT`` has been removed, making ``std::__libcpp_verbose_abort``
  unconditionally ``noexcept``.

- libc++ no longer adds ``constexpr`` to ``std::hash<std::vector<bool, A>>::operator()``, as the ``constexpr`` addition
  since C++20 was an unintended extension.

- The non-conforming extension ``packaged_task::result_type`` has been removed in LLVM 21.

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.

- User-defined specializations of ``std::common_reference`` are diagnosed now. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.

- ``std::function`` used to have allocator support, which was removed from the Standard by `http://wg21.link/p0302r1`
  due to issues with its design and inconsistent support from implementations. Previously, libc++ would provide
  allocator-aware APIs in ``std::function`` in C++11 and C++14, but ignores the allocator argument in all places but
  one. Starting in this release, the allocator argument is always ignored.

Announcements About Future Releases
-----------------------------------

LLVM 22
~~~~~~~

- 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++.


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
