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Libc++ 22.0.0 (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++ 22.0.0 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 22.0.0. 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++ 22.0.0?
==============================

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

- P2592R3: Hashing support for ``std::chrono`` value classes (`Github <https://llvm.org/PR105358>`__)
- P2321R2: ``zip`` (`Github <https://llvm.org/PR105169>`__)
- P2988R12: ``std::optional<T&>`` (`Github <https://llvm.org/PR148131>`__)
- P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
- P3223R2: Making ``std::istream::ignore`` less surprising (`Github <https://llvm.org/PR148178>`__)
- P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
- P2404R3: Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with``
  (`Github <https://llvm.org/PR105210>`__)
- P2641R4: Checking if a ``union`` alternative is active (``std::is_within_lifetime``)
  (`Github <https://llvm.org/PR105381>`__)
- P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
- P2944R3: Comparisons for ``reference_wrapper`` (`Github <https://llvm.org/PR105424>`__)
- P3168R2: Give ``std::optional`` Range Support
  (guarded by ``-fexperimental-library``, `Github <https://llvm.org/PR105430>`__)
- P3567R2: ``flat_meow`` Fixes (`Github <https://llvm.org/PR162022>`__)
- P3836R2: Make ``optional<T&>`` trivially copyable (`Github <https://llvm.org/PR171275>`__)
- P1789R3: Library Support for Expansion Statements (`Github <https://llvm.org/PR167184>`__)

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

- The performance of associative and unordered containers has been significantly improved. This is an overview of the
  different functions and by up to how much they have been improved:

  - ``map::map(const map&)``: 2.3x
  - ``map::operator=(const map&)``: 11x
  - ``unordered_set::unordered_set(const unordered_set&)``: 3.3x
  - ``unordered_set::operator=(const unordered_set&)``: 5x
  - ``erase`` of ``map`` and ``set``: 2x
  - ``find(key)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.3x
  - ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``: 3x
  - ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset``: 2.5x
  - ``erase(iterator, iterator)`` of the unordered containers: 1.9x
  - ``map::insert_or_assign``: 2x

- The performance of many algorithms has been improved. This is an overview of the different functions and by up to how
  much they have been improved:

  - ``std::find`` for integral types: 2x
  - ``std::for_each`` and ``ranges::for_each`` over the associative containers: 2x
  - ``std::rotate``: 3x
  - Some have been specifically optimized for segmented iterators:

    - ``std::distance`` and ``std::ranges::distance`` on non-random-access iterators: 1600x
    - ``{std,ranges}::{fill, fill_n}``: 10x
    - ``std::{generate, generate_n}`` and ``std::ranges::generate_n``: 1.8x

  - ``std::search_n`` for random access iterators now tries to skip elements.
    In contrived cases improvements of 70,000x have been observed.

- There have also been performance improvements in other library facilities:

  - ``vector<bool>::reserve()``: 2x
  - ``deque::append_range``: 3.4x
  - ``num_get::do_get`` integral overloads: 2.8x
  - Some reallocations are now avoided in ``std::filesystem::path::lexically_relative``: 1.7x
  - ``ofstream::write`` passes large strings to system calls directly instead of copying them in chunks into a buffer
  - ``std::align`` is now an inline function, which allows the compiler to better optimize calls to it

- ``std::atomic::wait`` has been refactored to accept more types to use platform native wait functions directly.
  This is guarded behind the ABI Macro ``_LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE``.

- Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
  reduced debug information.

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

- The minimally supported deployment target on macOS has been increased to 11.0.

Potentially breaking changes
----------------------------

- The algorithm for ``multi{map,set}::find`` has been modified such that it doesn't necessarily return an iterator to
  the first equal element in the container. This was never guaranteed by the Standard, but libc++ previously happened to
  always return the first equal element, like other implementations do. Starting with this release, code relying on the
  first element being returned from ``find`` will be broken, and ``lower_bound`` or ``equal_range`` should be used
  instead.

- The algorithms for ``std::{map,set}`` ``lower_bound`` and ``upper_bound`` operations were modified such that their
  result changed for comparators that are not a strict weak order. Being a strict weak order was always a requirement
  of the Standard and still is, however in this release libc++ changed the behavior of ``std::{map,set}`` for such
  comparators. Since this may be tricky to work around in some cases, an escape hatch is provided in this release:
  defining ``_LIBCPP_ENABLE_LEGACY_TREE_LOWER_UPPER_BOUND`` will revert to the historical implementation of
  these operations. That escape hatch will be removed in LLVM 23.

- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
  ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
  ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.

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

ABI Affecting Changes
---------------------

- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
  ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
  ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.

- The internal types ``__map_value_compare``, ``__unordered_map_hasher``, ``__unordered_map_equal``,
  ``__hash_map_hasher`` and ``__hash_map_equal`` have been refactored to use ``_LIBCPP_COMPRESSED_ELEMENT`` instead of
  potentially inheriting from the types they wrap. At this point in time we are not aware of any ABI changes caused by
  this.

- ``ranges::iota_view`` is now aware of ``__int128``. This causes ``iota_view::difference_type`` to change from
  ``long long`` to ``__int128`` in some cases.

- ``std::allocator`` is now trivially default constructible. The behaviour can be reverted by defining
  ``_LIBCPP_DEPRECATED_ABI_NON_TRIVIAL_ALLOCATOR``. Please inform the libc++ team if you need this flag, since it will
  be removed in LLVM 24 if there is no evidence that it's required.

- ``bitset::operator[]`` now returns ``bool``, making libc++ conforming. The behaviour can be reverted by defining
  ``_LIBCPP_DEPRECATED_ABI_BITSET_CONST_SUBSCRIPT_RETURN_REF``. Please inform the libc++ team if you need this flag,
  since it will be removed in LLVM 24 if there is no evidence that it's required.

Build System Changes
--------------------
