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

 =========================================
 Libc++ 17.0.0 (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++ 17 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 17.0.0. 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++ 17.0.0?
 ============================

 Implemented Papers
 ------------------
 - P2520R0 - ``move_iterator<T*>`` should be a random access iterator
 - P1328R1 - ``constexpr type_info::operator==()``
 - P1413R3 - Formatting ``thread::id`` (the ``stacktrace`` is not done yet)
 - P2675R1 - ``format``'s width estimation is too approximate and not forward compatible
 - P2505R5 - Monadic operations for ``std::expected``
 - P2711R1 - Making Multi-Param Constructors Of views explicit (``join_with_view`` is not done yet)
 - P2572R1 - ``std::format`` fill character allowances

 Improvements and New Features
 -----------------------------
 - ``std::equal`` and ``std::ranges::equal`` are now forwarding to ``std::memcmp`` for integral types and pointers,
   which can lead up to 40x performance improvements.

 - ``std::string_view`` now provides iterators that check for out-of-bounds accesses when the safe
   libc++ mode is enabled.

 - The performance of ``dynamic_cast`` on its hot paths is greatly improved and is as efficient as the
   ``libsupc++`` implementation. Note that the performance improvements are shipped in ``libcxxabi``.

 - `D122780 <https://reviews.llvm.org/D122780>`_ Improved the performance of ``std::sort`` and ``std::ranges::sort``
   by up to 50% for arithmetic types and by approximately 10% for other types.

 - The ``<format>`` header is no longer considered experimental. Some
   ``std::formatter`` specializations are not yet available since the class used
   in the specialization has not been implemented in libc++. This prevents the
   feature-test macro to be set.

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

 - The ``<experimental/coroutine>`` header has been removed in this release. The ``<coroutine>`` header
   has been shipping since LLVM 14, so the Coroutines TS implementation is being removed per our policy
   for removing TSes.

 - Several incidental transitive includes have been removed from libc++. Those
   includes are removed based on the language version used. Incidental transitive
   inclusions of the following headers have been removed:

   - C++23: ``atomic``, ``bit``, ``cstdint``, ``cstdlib``, ``cstring``, ``initializer_list``, ``limits``, ``new``,
            ``stdexcept``, ``system_error``, ``type_traits``, ``typeinfo``

 - ``<algorithm>`` no longer includes ``<chrono>`` in any C++ version (it was previously included in C++17 and earlier).

 - ``<string>`` no longer includes ``<vector>`` in any C++ version (it was previously included in C++20 and earlier).

 - ``<string>``, ``<string_view>``, and ``<mutex>`` no longer include ``<functional>``
   in any C++ version (it was previously included in C++20 and earlier).

 - The headers ``<experimental/algorithm>`` and ``<experimental/functional>`` have been removed, since all the contents
   have been implemented in namespace ``std`` for at least two releases.

 - The formatter specialization ``template<size_t N> struct formatter<const charT[N], charT>``
   has been removed. Since libc++'s format library was marked experimental there
   is no backwards compatibility option. This specialization has been removed
   from the Standard since it was never used, the proper specialization to use
   instead is ``template<size_t N> struct formatter<charT[N], charT>``.

 - Libc++ used to provide some C++11 tag type global variables in C++03 as an extension, which are removed in
   this release. Those variables were ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``,
   ``std::adopt_lock``, and ``std::piecewise_construct``. Note that the types associated to those variables are
   still provided in C++03 as an extension (e.g. ``std::piecewise_construct_t``). Providing those variables in
   C++03 mode made it impossible to define them properly -- C++11 mandated that they be ``constexpr`` variables,
   which is impossible in C++03 mode. Furthermore, C++17 mandated that they be ``inline constexpr`` variables,
   which led to ODR violations when mixed with the C++03 definition. Cleaning this up is required for libc++ to
   make progress on support for C++20 modules.

 - The ``_LIBCPP_ABI_OLD_LOGNORMAL_DISTRIBUTION`` macro has been removed.

 - The classes ``strstreambuf`` , ``istrstream``, ``ostrstream``, and ``strstream`` have been deprecated.
   They have been deprecated in the Standard since C++98, but were never marked as deprecated in libc++.

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

 LLVM 18
 ~~~~~~~

 - The base template for ``std::char_traits`` has been marked as deprecated and
   will be removed in LLVM 18. If you are using ``std::char_traits`` with types
   other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t`` or
   a custom character type for which you specialized ``std::char_traits``, your code
   will stop working when we remove the base template. The Standard does not
   mandate that a base template is provided, and such a base template is bound
   to be incorrect for some types, which could currently cause unexpected
   behavior while going undetected.

 - The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro will not be honored anymore in LLVM 18.
   Please see the updated documentation about the safe libc++ mode and in particular the ``_LIBCPP_VERBOSE_ABORT``
   macro for details.

 - The headers ``<experimental/deque>``, ``<experimental/forward_list>``, ``<experimental/list>``,
   ``<experimental/map>``, ``<experimental/memory_resource>``, ``<experimental/regex>``, ``<experimental/set>``,
   ``<experimental/string>``, ``<experimental/unordered_map>``, ``<experimental/unordered_set>``,
   and ``<experimental/vector>`` will be removed in LLVM 18, as all their contents will have been implemented in
   namespace ``std`` for at least two releases.

 API Changes
 -----------

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

 - Symbols for ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``, ``std::adopt_lock``, and
   ``std::piecewise_construct`` have been removed from the built library. Under most circumstances, user code
   should not have been relying on those symbols anyway since those are empty classes and the compiler does
   not generate an undefined reference unless the address of the object is taken. However, this is an ABI break
   if the address of one of these objects has been taken in code compiled as C++03, since in those cases the
   objects were marked as defined in the shared library. In other Standard modes, this should never be a problem
   since those objects were defined in the headers as ``constexpr``.

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

 - Building libc++ and libc++abi for Apple platforms now requires targeting macOS 10.13 and later.
   This is relevant for vendors building the libc++ shared library and for folks statically linking
   libc++ into an application that has back-deployment requirements on Apple platforms.

 - ``LIBCXX_ENABLE_FILESYSTEM`` now represents whether a filesystem is supported on the platform instead
   of representing merely whether ``<filesystem>`` should be provided. This means that vendors building
   with ``LIBCXX_ENABLE_FILESYSTEM=OFF`` will now also get ``<fstream>`` excluded from their configuration
   of the library.

 - ``LIBCXX_ENABLE_FSTREAM`` is not supported anymore, please use ``LIBCXX_ENABLE_FILESYSTEM=OFF`` if your
   platform does not have support for a filesystem.
	=========================================
	Libc++ 17.0.0 (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++ 17 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 17.0.0. 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++ 17.0.0?
	============================

	Implemented Papers
	------------------
	- P2520R0 - ``move_iterator<T*>`` should be a random access iterator
	- P1328R1 - ``constexpr type_info::operator==()``
	- P1413R3 - Formatting ``thread::id`` (the ``stacktrace`` is not done yet)
	- P2675R1 - ``format``'s width estimation is too approximate and not forward compatible
	- P2505R5 - Monadic operations for ``std::expected``
	- P2711R1 - Making Multi-Param Constructors Of views explicit (``join_with_view`` is not done yet)
	- P2572R1 - ``std::format`` fill character allowances

	Improvements and New Features
	-----------------------------
	- ``std::equal`` and ``std::ranges::equal`` are now forwarding to ``std::memcmp`` for integral types and pointers,
	which can lead up to 40x performance improvements.

	- ``std::string_view`` now provides iterators that check for out-of-bounds accesses when the safe
	libc++ mode is enabled.

	- The performance of ``dynamic_cast`` on its hot paths is greatly improved and is as efficient as the
	``libsupc++`` implementation. Note that the performance improvements are shipped in ``libcxxabi``.

	- `D122780 <https://reviews.llvm.org/D122780>`_ Improved the performance of ``std::sort`` and ``std::ranges::sort``
	by up to 50% for arithmetic types and by approximately 10% for other types.

	- The ``<format>`` header is no longer considered experimental. Some
	``std::formatter`` specializations are not yet available since the class used
	in the specialization has not been implemented in libc++. This prevents the
	feature-test macro to be set.

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

	- The ``<experimental/coroutine>`` header has been removed in this release. The ``<coroutine>`` header
	has been shipping since LLVM 14, so the Coroutines TS implementation is being removed per our policy
	for removing TSes.

	- Several incidental transitive includes have been removed from libc++. Those
	includes are removed based on the language version used. Incidental transitive
	inclusions of the following headers have been removed:

	- C++23: ``atomic``, ``bit``, ``cstdint``, ``cstdlib``, ``cstring``, ``initializer_list``, ``limits``, ``new``,
	``stdexcept``, ``system_error``, ``type_traits``, ``typeinfo``

	- ``<algorithm>`` no longer includes ``<chrono>`` in any C++ version (it was previously included in C++17 and earlier).

	- ``<string>`` no longer includes ``<vector>`` in any C++ version (it was previously included in C++20 and earlier).

	- ``<string>``, ``<string_view>``, and ``<mutex>`` no longer include ``<functional>``
	in any C++ version (it was previously included in C++20 and earlier).

	- The headers ``<experimental/algorithm>`` and ``<experimental/functional>`` have been removed, since all the contents
	have been implemented in namespace ``std`` for at least two releases.

	- The formatter specialization ``template<size_t N> struct formatter<const charT[N], charT>``
	has been removed. Since libc++'s format library was marked experimental there
	is no backwards compatibility option. This specialization has been removed
	from the Standard since it was never used, the proper specialization to use
	instead is ``template<size_t N> struct formatter<charT[N], charT>``.

	- Libc++ used to provide some C++11 tag type global variables in C++03 as an extension, which are removed in
	this release. Those variables were ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``,
	``std::adopt_lock``, and ``std::piecewise_construct``. Note that the types associated to those variables are
	still provided in C++03 as an extension (e.g. ``std::piecewise_construct_t``). Providing those variables in
	C++03 mode made it impossible to define them properly -- C++11 mandated that they be ``constexpr`` variables,
	which is impossible in C++03 mode. Furthermore, C++17 mandated that they be ``inline constexpr`` variables,
	which led to ODR violations when mixed with the C++03 definition. Cleaning this up is required for libc++ to
	make progress on support for C++20 modules.

	- The ``_LIBCPP_ABI_OLD_LOGNORMAL_DISTRIBUTION`` macro has been removed.

	- The classes ``strstreambuf`` , ``istrstream``, ``ostrstream``, and ``strstream`` have been deprecated.
	They have been deprecated in the Standard since C++98, but were never marked as deprecated in libc++.

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

	LLVM 18
	~~~~~~~

	- The base template for ``std::char_traits`` has been marked as deprecated and
	will be removed in LLVM 18. If you are using ``std::char_traits`` with types
	other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t`` or
	a custom character type for which you specialized ``std::char_traits``, your code
	will stop working when we remove the base template. The Standard does not
	mandate that a base template is provided, and such a base template is bound
	to be incorrect for some types, which could currently cause unexpected
	behavior while going undetected.

	- The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro will not be honored anymore in LLVM 18.
	Please see the updated documentation about the safe libc++ mode and in particular the ``_LIBCPP_VERBOSE_ABORT``
	macro for details.

	- The headers ``<experimental/deque>``, ``<experimental/forward_list>``, ``<experimental/list>``,
	``<experimental/map>``, ``<experimental/memory_resource>``, ``<experimental/regex>``, ``<experimental/set>``,
	``<experimental/string>``, ``<experimental/unordered_map>``, ``<experimental/unordered_set>``,
	and ``<experimental/vector>`` will be removed in LLVM 18, as all their contents will have been implemented in
	namespace ``std`` for at least two releases.

	API Changes
	-----------

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

	- Symbols for ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``, ``std::adopt_lock``, and
	``std::piecewise_construct`` have been removed from the built library. Under most circumstances, user code
	should not have been relying on those symbols anyway since those are empty classes and the compiler does
	not generate an undefined reference unless the address of the object is taken. However, this is an ABI break
	if the address of one of these objects has been taken in code compiled as C++03, since in those cases the
	objects were marked as defined in the shared library. In other Standard modes, this should never be a problem
	since those objects were defined in the headers as ``constexpr``.

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

	- Building libc++ and libc++abi for Apple platforms now requires targeting macOS 10.13 and later.
	This is relevant for vendors building the libc++ shared library and for folks statically linking
	libc++ into an application that has back-deployment requirements on Apple platforms.

	- ``LIBCXX_ENABLE_FILESYSTEM`` now represents whether a filesystem is supported on the platform instead
	of representing merely whether ``<filesystem>`` should be provided. This means that vendors building
	with ``LIBCXX_ENABLE_FILESYSTEM=OFF`` will now also get ``<fstream>`` excluded from their configuration
	of the library.

	- ``LIBCXX_ENABLE_FSTREAM`` is not supported anymore, please use ``LIBCXX_ENABLE_FILESYSTEM=OFF`` if your
	platform does not have support for a filesystem.