1=========================================== 2Libc++ 19.0.0 (In-Progress) Release Notes 3=========================================== 4 5.. contents:: 6 :local: 7 :depth: 2 8 9Written by the `Libc++ Team <https://libcxx.llvm.org>`_ 10 11.. warning:: 12 13 These are in-progress notes for the upcoming libc++ 19.0.0 release. 14 Release notes for previous releases can be found on 15 `the Download Page <https://releases.llvm.org/download.html>`_. 16 17Introduction 18============ 19 20This document contains the release notes for the libc++ C++ Standard Library, 21part of the LLVM Compiler Infrastructure, release 19.0.0. Here we describe the 22status of libc++ in some detail, including major improvements from the previous 23release and new feature work. For the general LLVM release notes, see `the LLVM 24documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may 25be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_. 26 27For more information about libc++, please see the `Libc++ Web Site 28<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_. 29 30Note that if you are reading this file from a Git checkout or the 31main Libc++ web page, this document applies to the *next* release, not 32the current one. To see the release notes for a specific release, please 33see the `releases page <https://llvm.org/releases/>`_. 34 35What's New in Libc++ 19.0.0? 36============================== 37 38Implemented Papers 39------------------ 40 41- P2637R3 - Member ``visit`` 42- P2652R2 - Disallow User Specialization of ``allocator_traits`` 43- P2819R2 - Add ``tuple`` protocol to ``complex`` 44- P2495R3 - Interfacing ``stringstream``\s with ``string_view`` 45- P2302R4 - ``std::ranges::contains`` 46- P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with`` 47 48Improvements and New Features 49----------------------------- 50 51- The performance of growing ``std::vector`` has been improved for trivially relocatable types. 52- The performance of ``ranges::fill`` and ``ranges::fill_n`` has been improved for ``vector<bool>::iterator``\s, 53 resulting in a performance increase of up to 1400x. 54- The ``std::mismatch`` algorithm has been optimized for integral types, which can lead up to 40x performance 55 improvements. 56 57Deprecations and Removals 58------------------------- 59 60- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting 61 it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly, 62 the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in 63 the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation 64 <using-hardening-modes>` for more details. 65 66- The base template for ``std::char_traits`` has been removed in LLVM 19. If you are using ``std::char_traits`` with 67 types other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t`` or a custom character type for which you 68 specialized ``std::char_traits``, your code will stop working. The Standard does not mandate that a base template is 69 provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior 70 while going undetected. 71 72- The ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` macro that changed the behavior for narrowing conversions 73 in ``std::variant`` has been removed in LLVM 19. 74 75- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`` and ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION`` 76 macros have been removed in LLVM 19. 77 78- TODO: The ``_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES`` and ``_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES`` macros have 79 been removed in LLVM 19. C++17 and C++20 removed features can still be re-enabled individually. 80 81- The ``_LIBCPP_INLINE_VISIBILITY`` and ``_VSTD`` macros have been removed in LLVM 19. 82 83- The ``_LIBCPP_ATOMIC_ONLY_USE_BUILTINS`` configuration option has been removed in LLVM 19. This should not affect 84 many users, except perhaps users using the library with ``-ffreestanding`` with a toolchain where compiler-rt or 85 libatomic is not available. If you are one such user, please reach out to the libc++ developers so we can collaborate 86 on a path for supporting atomics properly on freestanding platforms. 87 88 89Upcoming Deprecations and Removals 90---------------------------------- 91 92LLVM 20 93~~~~~~~ 94 95- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable 96 the safe mode will be removed in LLVM 20. 97 98LLVM 21 99~~~~~~~ 100TODO 101 102 103ABI Affecting Changes 104--------------------- 105TODO 106 107 108Build System Changes 109-------------------- 110 111- The ``LIBCXX_EXECUTOR`` and ``LIBCXXABI_EXECUTOR`` CMake variables have been removed. Please 112 set ``LIBCXX_TEST_PARAMS`` to ``executor=<...>`` instead. 113