libcxx/docs/ReleaseNotes/22.rst

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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
------------------

- P2321R2: ``zip`` (`Github <https://github.com/llvm/llvm-project/issues/105169>`__) (The paper is partially
  implemented. ``zip_transform_view`` is implemented in this release)
- P3168R2: Give ``std::optional`` Range Support (`Github <https://github.com/llvm/llvm-project/issues/105430>`__)

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

- The performance of ``map::map(const map&)`` has been improved up to 2.3x
- The performance of ``map::operator=(const map&)`` has been improved by up to 11x
- The performance of ``unordered_set::unordered_set(const unordered_set&)`` has been improved by up to 3.3x.
- The performance of ``unordered_set::operator=(const unordered_set&)`` has been improved by up to 5x.
- The performance of ``map::erase`` and ``set::erase`` has been improved by up to 2x
- The performance of ``find(key)`` in ``map``, ``set``, ``multimap`` and ``multiset`` has been improved by up to 2.3x
- Some reallocations are now avoided in `std::filesystem::path::lexically_relative`, resulting in a performance
  improvement of up to 1.7x.
- The performance of the ``(iterator, iterator)`` constructors of ``map``, ``set``, ``multimap`` and ``multiset``
  has been improved by up to 3x
- The performance of ``insert(iterator, iterator)`` of ``map``, ``set``, ``multimap`` and ``multiset`` has been improved
  by up to 2.5x
- The performance of ``erase(iterator, iterator)`` in the unordered containers has been improved by up to 1.9x
- The performance of ``map::insert_or_assign`` has been improved by up to 2x

- ``ofstream::write`` has been optimized to pass through large strings to system calls directly instead of copying them
  in chunks into a buffer.

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

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 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.

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