libcxx/docs/ReleaseNotes/22.rst - llvm-project.git - Git at Google

 ===========================================
 Libc++ 22.0.0 (In-Progress) Release Notes
 ===========================================

 .. contents::
    :local:
    :depth: 2

 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://llvm.org/PR105169>`__) (The paper is partially implemented. ``zip_transform_view``
   is implemented in this release)
 - 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>`__)
 - 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 (`Github <https://llvm.org/PR105430>`__)

 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`` and ``ifstream::read`` have been optimized to pass through large reads and writes to system calls
   directly instead of copying them in chunks.
 - Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
   reduced debug information.

 - The performance of ``std::find`` has been improved by up to 2x for integral types
 - The ``std::distance`` and ``std::ranges::distance`` algorithms have been optimized for segmented iterators (e.g.,
   ``std::join_view`` iterators), reducing the complexity from ``O(n)`` to ``O(n / segment_size)``. Benchmarks show
   performance improvements of over 1600x in favorable cases with large segment sizes (e.g., 1024).
 - The ``std::{fill, fill_n}`` and ``std::ranges::{fill, fill_n}`` algorithms have been optimized for segmented iterators,
   resulting in a performance improvement of at least 10x for ``std::deque<int>`` iterators and
   ``std::join_view<std::vector<std::vector<int>>>`` iterators.
 - The ``std::{generate, generate_n}`` and ``std::ranges::generate_n`` algorithms have been optimized for segmented
   iterators, resulting in a performance improvement for ``std::deque<short>`` and
   ``std::join_view<vector<vector<short>>>`` iterators.

 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.

 - 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.

 Build System Changes
 --------------------
	===========================================
	Libc++ 22.0.0 (In-Progress) Release Notes
	===========================================

	.. contents::
	:local:
	:depth: 2

	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://llvm.org/PR105169>`__) (The paper is partially implemented. ``zip_transform_view``
	is implemented in this release)
	- 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>`__)
	- 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 (`Github <https://llvm.org/PR105430>`__)

	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`` and ``ifstream::read`` have been optimized to pass through large reads and writes to system calls
	directly instead of copying them in chunks.
	- Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
	reduced debug information.

	- The performance of ``std::find`` has been improved by up to 2x for integral types
	- The ``std::distance`` and ``std::ranges::distance`` algorithms have been optimized for segmented iterators (e.g.,
	``std::join_view`` iterators), reducing the complexity from ``O(n)`` to ``O(n / segment_size)``. Benchmarks show
	performance improvements of over 1600x in favorable cases with large segment sizes (e.g., 1024).
	- The ``std::{fill, fill_n}`` and ``std::ranges::{fill, fill_n}`` algorithms have been optimized for segmented iterators,
	resulting in a performance improvement of at least 10x for ``std::deque<int>`` iterators and
	``std::join_view<std::vector<std::vector<int>>>`` iterators.
	- The ``std::{generate, generate_n}`` and ``std::ranges::generate_n`` algorithms have been optimized for segmented
	iterators, resulting in a performance improvement for ``std::deque<short>`` and
	``std::join_view<vector<vector<short>>>`` iterators.

	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.

	- 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.

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