llvm-project/libcxx/docs/ReleaseNotes/19.rst

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Libc++ 19.0.0 (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++ 19.0.0 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 19.0.0. 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++ 19.0.0?
==============================

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

- P2637R3 - Member ``visit``
- P2652R2 - Disallow User Specialization of ``allocator_traits``
- P2819R2 - Add ``tuple`` protocol to ``complex``
- P2495R3 - Interfacing ``stringstream``\s with ``string_view``
- P2302R4 - ``std::ranges::contains``
- P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``

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

- The performance of growing ``std::vector`` has been improved for trivially relocatable types.
- The performance of ``ranges::fill`` and ``ranges::fill_n`` has been improved for ``vector<bool>::iterator``\s,
  resulting in a performance increase of up to 1400x.
- The ``std::mismatch`` algorithm has been optimized for integral types, which can lead up to 40x performance
  improvements.

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

- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
  it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
  the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in
  the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
  <using-hardening-modes>` for more details.

- The base template for ``std::char_traits`` has been removed in LLVM 19. 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. 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_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` macro that changed the behavior for narrowing conversions
  in ``std::variant`` has been removed in LLVM 19.

- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` and ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION``
  macros have been removed in LLVM 19.

- TODO: The ``_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES`` and ``_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`` macros have
  been removed in LLVM 19. C++17 and C++20 removed features can still be re-enabled individually.

- The ``_LIBCPP_INLINE_VISIBILITY`` and ``_VSTD`` macros have been removed in LLVM 19.

- The ``_LIBCPP_ATOMIC_ONLY_USE_BUILTINS`` configuration option has been removed in LLVM 19. This should not affect
  many users, except perhaps users using the library with ``-ffreestanding`` with a toolchain where compiler-rt or
  libatomic is not available. If you are one such user, please reach out to the libc++ developers so we can collaborate
  on a path for supporting atomics properly on freestanding platforms.


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

LLVM 20
~~~~~~~

- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
  the safe mode will be removed in LLVM 20.

LLVM 21
~~~~~~~
TODO


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


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

- The ``LIBCXX_EXECUTOR`` and ``LIBCXXABI_EXECUTOR`` CMake variables have been removed. Please
  set ``LIBCXX_TEST_PARAMS`` to ``executor=<...>`` instead.