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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?
==============================
The main focus of the libc++ team has been to implement new C++20, C++23,
and C++26 features.
Experimental support for the time zone database has progressed.
Work on the ranges support has progressed.
Work on the experimental C++17 Parallel STL has progressed. See
:ref:`pstl-status` for the current status.
Work on the C++17 mathematical special functions has started. See
`this issue <https://github.com/llvm/llvm-project/issues/99939>`__
for the current status.
Implemented Papers
------------------
- P1132R8 - ``out_ptr`` - a scalable output pointer abstraction
- P1614R2 - The Mothership has Landed
- P2637R3 - Member ``visit``
- P2652R2 - Disallow User Specialization of ``allocator_traits``
- P2819R2 - Add ``tuple`` protocol to ``complex``
- P2495R3 - Interfacing ``stringstream``\s with ``string_view``
- P2867R2 - Remove Deprecated ``strstream``\s From C++26
- P2872R3 - Remove ``wstring_convert`` From C++26
- P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
- P2944R3 - Comparisons for ``reference_wrapper`` (comparison operators for ``reference_wrapper`` only)
- P2591R5 - Concatenation of strings and string views
- P2968R2 - Make ``std::ignore`` a first-class object
- P2997R1 - Removing the common reference requirement from the indirectly invocable concepts (as DR against C++20)
- P2302R4 - ``std::ranges::contains``
- P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
- P3029R1 - Better ``mdspan``'s CTAD
- P2387R3 - Pipe support for user-defined range adaptors
- P2713R1 - Escaping improvements in ``std::format``
- P2231R1 - Missing ``constexpr`` in ``std::optional`` and ``std::variant``
- P0019R8 - ``std::atomic_ref``
- P2389R2 - Alias template ``dims`` for the ``extents`` of ``mdspan``
- P1223R5 - ``ranges::find_last()``, ``ranges::find_last_if()``, and ``ranges::find_last_if_not()``
- P2602R2 - Poison Pills are Too Toxic
- P1981R0 - Rename ``leap`` to ``leap_second``
- P1982R0 - Rename ``link`` to ``time_zone_link``
- P2602R2 - Poison Pills are Too Toxic (as DR against C++20)
Improvements and New Features
-----------------------------
- The performance of growing ``std::vector`` has been improved for trivially relocatable types.
- A lot of types are considered trivially relocatable now, including ``std::vector`` and ``std::string``.
- The performance of ``std::ranges::fill`` and ``std::ranges::fill_n`` has been improved for ``std::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.
- The ``std::ranges::minmax`` algorithm has been optimized for integral types, resulting in a performance increase of
up to 100x.
- The ``std::set_intersection`` and ``std::ranges::set_intersection`` algorithms have been optimized to fast-forward over
contiguous ranges of non-matching values, reducing the number of comparisons from linear to
logarithmic growth with the number of elements in best-case scenarios.
- The ``_LIBCPP_ENABLE_CXX26_REMOVED_STRSTREAM`` macro has been added to make the declarations in ``<strstream>`` available.
- The ``_LIBCPP_ENABLE_CXX26_REMOVED_WSTRING_CONVERT`` macro has been added to make the declarations in ``<locale>``
available.
- The formatting library is updated to Unicode 15.1.0.
- ``std::ignore``\s ``const __ignore_t& operator=(_Tp&&) const`` was changed to
``const __ignore_type& operator=(const _Tp&) const noexcept`` for all language versions.
- Vendors can now configure the ABI so that ``string`` and ``vector`` will use bounded iterators when hardening is
enabled. Note that checks for iterator invalidation are currently not supported -- any accesses made through an
invalidated bounded iterator will still result in undefined behavior (bounded iterators follow the normal invalidation
rules of the associated container). ``string`` bounded iterators use the logical size of the container (``index
< str.size()``) whereas ``vector`` bounded iterators use the "physical" size of the container (``index
< vec.capacity()``) which is a less strict check; refer to the implementation for further details.
Bounded iterators can be enabled via the ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STRING`` ABI macro for ``string`` and via
the ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_VECTOR`` ABI macro for ``vector``; note that checks will only be performed if
the hardening mode is set to ``fast`` or above (i.e., no checking is performed in the unchecked mode, even if bounded
iterators are enabled in the ABI configuration).
Note: bounded iterators currently are not supported for ``vector<bool>``.
- In C++23 and C++26 the number of transitive includes in several headers has been reduced, improving the compilation speed.
Deprecations and Removals
-------------------------
- The C++20 synchronization library (``<barrier>``, ``<latch>``, ``std::atomic::wait``, etc.) has been deprecated
in language modes prior to C++20. If you are using these features prior to C++20, please update to ``-std=c++20``.
In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
- ``_LIBCPP_DISABLE_NODISCARD_EXT`` has been removed. ``[[nodiscard]]`` applications are now unconditional.
This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>`_ and `P3162 <https://wg21.link/P3162>`_
to not use ``[[nodiscard]]`` in the standard.
- 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.
- 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.
- LWG3430 disallow implicit conversion of the source arguments to ``std::filesystem::path`` when
constructing ``std::basic_*fstream``. This effectively removes the possibility to directly construct
a ``std::basic_*fstream`` from a ``std::basic_string_view``, a input-iterator or a C-string, instead
you can construct a temporary ``std::basic_string``. This change has been applied to C++17 and later.
- The ``_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS`` macro has been removed and is not honored anymore. Additional
warnings provided by libc++ as a matter of QoI will now be provided unconditionally.
- libc++ no longer supports ``std::allocator<const T>`` and containers of ``const``-qualified element type, such
as ``std::vector<const T>`` and ``std::list<const T>``. This used to be supported as an undocumented extension.
If you were using ``std::vector<const T>``, replace it with ``std::vector<T>`` instead. The
``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro can be defined to temporarily re-enable this extension.
to temporarily re-enable this extension to make it easier to update user code
This macro will be honored for one released and ignored starting in LLVM 20.
To assist with the clean-up process, consider running your code through Clang Tidy, with
`std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`_
enabled.
- When configuring libc++ with localization or threads disabled, the library no longer emits an error when
trying to ``#include <locale>`` and other such headers. Instead, those headers have no content. This is
consistent with the behavior for all other libc++ carve-outs like filesystem, wide characters, a source
of randomness, and others. Users that were checking whether including a header would fail (e.g. via a script
or CMake's ``try_compile`` will experience a change in behavior).
- libc++ no longer supports relational comparison for ``std::chrono::weekday``. The relational comparison operators were
provided as an undocumented extension. If you were using relational comparison on ``std::chrono::weekday``, compare
the results of ``c_encoding()`` or ``iso_encoding()`` instead. The
``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro can be defined to temporarily re-enable this extension.
This macro will be honored for one release and ignored starting in LLVM 20.
- The operators in the ``rel_ops`` namespace have been deprecated. The deprecation is part of the paper
P0768R1 "Library Support for the Spaceship (Comparison) Operator".
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.
- The C++20 synchronization library will be removed entirely in language modes prior to C++20 in LLVM 20.
- The relational operators for ``std::chrono::weekday`` will be removed entirely, and the
``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be
ignored in LLVM 20.
- The ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro will no longer have an effect.
LLVM 21
~~~~~~~
- The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22, non-critical bug fixes may not be back-ported
to C++03, including LWG issues. C++03 is a legacy platform, where most projects are no longer actively maintained. To
reduce the amount of fixes required to keep such legacy projects compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid unintended breaking changes.
See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.
If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.
ABI Affecting Changes
---------------------
- The optional POSIX macro ``ENODATA`` has been deprecated in C++ and POSIX 2017. The
``random_device`` could throw a ``system_error`` with this value. It now
throws ``ENOMSG``.
Build System Changes
--------------------
- The ``LIBCXX_EXECUTOR`` and ``LIBCXXABI_EXECUTOR`` CMake variables have been removed. Please
set ``LIBCXX_TEST_PARAMS`` to ``executor=<...>`` instead.
- The CMake variable ``LIBCXX_ENABLE_CLANG_TIDY`` has been removed. The build system has been changed
to automatically detect the presence of ``clang-tidy`` and the required ``Clang`` libraries.
- The CMake options ``LIBCXX_INSTALL_MODULES`` now defaults to ``ON``.
- The CMake options ``LIBCXX_BENCHMARK_NATIVE_STDLIB`` and ``LIBCXX_BENCHMARK_NATIVE_GCC_TOOLCHAIN`` have
been removed. To benchmark the native standard library, configure the test suite against the native
standard library directly instead.
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