=========================================== Libc++ 19.0.0 (In-Progress) Release Notes =========================================== .. contents:: :local: :depth: 2 Written by the `Libc++ Team `_ .. 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 `_. 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 `_. All LLVM releases may be downloaded from the `LLVM releases web site `_. For more information about libc++, please see the `Libc++ Web Site `_ or the `LLVM Web Site `_. 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 `_. 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 `__ 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::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 ```` available. - The ``_LIBCPP_ENABLE_CXX26_REMOVED_WSTRING_CONVERT`` macro has been added to make the declarations in ```` 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``. - 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 (````, ````, ``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 `_ and `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 ` 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`` and containers of ``const``-qualified element type, such as ``std::vector`` and ``std::list``. This used to be supported as an undocumented extension. If you were using ``std::vector``, replace it with ``std::vector`` 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 `_ enabled. - When configuring libc++ with localization or threads disabled, the library no longer emits an error when trying to ``#include `` 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.