| ========================================= |
| Libc++ 16.0.0 (In-Progress) Release Notes |
| ========================================= |
| |
| .. contents:: |
| :local: |
| :depth: 2 |
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| Written by the `Libc++ Team <https://libcxx.llvm.org>`_ |
| |
| .. warning:: |
| |
| These are in-progress notes for the upcoming libc++ 16 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 16.0.0. 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++ 16.0.0? |
| ============================ |
| |
| The main focus of the libc++ team has been to implement new C++20 and C++23 |
| features. |
| |
| The C++20 format library has improved but it not yet considered stable. The |
| main improvements are additional formatters for the chrono calendar types. Work |
| on formatting ranges has started. |
| |
| The C++20 ranges library has been completed and is no longer experimental. Some |
| ``views`` have not been implemented yet. Work on C++23 ranges has started. |
| |
| The C++20 spaceship operator has been added to more types, the work is still |
| ongoing. |
| |
| Implemented Papers |
| ------------------ |
| - P2499R0 - ``string_view`` range constructor should be ``explicit`` |
| - P2417R2 - A more constexpr bitset |
| - P2445R1 - ``std::forward_like`` |
| - P2273R3 - Making ``std::unique_ptr`` constexpr |
| - P0591R4 - Utility functions to implement uses-allocator construction |
| - P2291R3 - Add Constexpr Modifiers to Functions ``to_chars`` and |
| ``from_chars`` for Integral Types in ``<charconv>`` Header |
| - P0220R1 - Adopt Library Fundamentals V1 TS Components for C++17 |
| - P0482R6 - char8_t: A type for UTF-8 characters and strings |
| - P2438R2 - ``std::string::substr() &&`` |
| - P0600R1 - ``nodiscard`` in the library |
| - P0339R6 - ``polymorphic_allocator<>`` as a vocabulary type |
| - P1169R4 - ``static operator()`` |
| - P0415R1 - ``constexpr`` for ``std::complex`` |
| - P1208R6 - ``std::source_location`` |
| - P0323R12 - ``std::expected`` |
| - P1035R7 - Input Range Adaptors |
| |
| Improvements and New Features |
| ----------------------------- |
| - Declarations of ``std::c8rtomb()`` and ``std::mbrtoc8()`` from P0482R6 are |
| now provided when implementations in the global namespace are provided by |
| the C library. |
| - Implemented ``<memory_resource>`` header from C++17 |
| - `D122780 <https://reviews.llvm.org/D122780>`_ Improved the performance of std::sort |
| |
| Deprecations and Removals |
| ------------------------- |
| - ``unary_function`` and ``binary_function`` are no longer provided in C++17 and newer Standard modes. |
| They can be re-enabled with ``_LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION``. |
| |
| - 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++11, C++14, C++17, and C++20: ``chrono`` |
| - C++2b: ``algorithm``, ``array``, ``atomic``, ``bit``, ``chrono``, |
| ``climits``, ``cmath``, ``compare``, ``concepts``, ``cstdarg``, ``cstddef``, |
| ``cstdint``, ``cstdlib``, ``cstring``, ``ctime``, ``exception``, |
| ``functional``, ``initializer_list``, ``iosfwd``, ``iterator``, ``limits``, |
| ``memory``, ``new``, ``numeric``, ``optional``, ``ratio``, ``stdexcept``, |
| ``string``, ``tuple``, ``type_traits``, ``typeinfo``, ``unordered_map``, |
| ``utility``, ``variant``, ``vector``. |
| |
| Users can also remove all incidental transitive includes by defining |
| ``_LIBCPP_REMOVE_TRANSITIVE_INCLUDES`` regardless of the language version |
| in use. Note that in the future, libc++ reserves the right to remove |
| incidental transitive includes more aggressively, in particular regardless |
| of the language version in use. |
| |
| - The legacy testing system for libc++, libc++abi, and libunwind has been removed. |
| All known clients have been migrated to the new configuration system, but please |
| reach out to the libc++ developers if you find something missing in the new |
| configuration system. |
| |
| - The functions ``to_chars`` and ``from_chars`` for integral types are |
| available starting with C++17. Libc++ offered these functions in C++11 and |
| C++14 as an undocumented extension. This extension makes it hard to implement |
| the C++23 paper that makes these functions ``constexpr``, therefore the |
| extension has been removed. |
| |
| - The ``_LIBCPP_ENABLE_CXX03_FUNCTION`` macro that allowed re-enabling the now-deprecated C++03 implementation of |
| ``std::function`` has been removed. Users who need to use ``std::function`` should switch to C++11 and above. |
| |
| - The contents of ``<experimental/memory_resource>`` are now deprecated since libc++ ships ``<memory_resource>`` now. |
| Please migrate to ``<memory_resource>`` instead. Per libc++'s TS deprecation policy, |
| ``<experimental/memory_resource>`` will be removed in LLVM 18. |
| |
| - The ``_LIBCPP_DEBUG`` macro is not honored anymore, and it is an error to try to use it. Please migrate to |
| ``_LIBCPP_ENABLE_DEBUG_MODE`` instead. |
| |
| Upcoming Deprecations and Removals |
| ---------------------------------- |
| - 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. |
| |
| API Changes |
| ----------- |
| - The comparison operators on ``thread::id`` are now defined as free-standing |
| functions instead of as hidden friends, in conformance with the C++ standard. |
| Also see `issue 56187 <https://github.com/llvm/llvm-project/issues/56187>`_. |
| |
| - ``_LIBCPP_ENABLE_NODISCARD`` and ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`` are no longer respected. |
| Any standards-required ``[[nodiscard]]`` applications in C++20 are now always enabled. Any extended applications |
| are now enabled by default and can be disabled by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``. |
| |
| - ``_LIBCPP_VERSION`` was previously defined to e.g. ``15001`` to represent LLVM 15.0.01, but this value had been |
| left undocumented. Starting with LLVM 16, ``_LIBCPP_VERSION`` will contain the version of LLVM represented as |
| ``XXYYZZ``. In other words, ``_LIBCPP_VERSION`` is gaining a digit. This should not be an issue for existing |
| code, since using e.g. ``_LIBCPP_VERSION > 15000`` will still give the right answer now that ``_LIBCPP_VERSION`` |
| is defined as e.g. ``160000`` (with one more digit). |
| |
| ABI Affecting Changes |
| --------------------- |
| - In freestanding mode, ``atomic<small enum class>`` does not contain a lock byte anymore if the platform |
| can implement lockfree atomics for that size. More specifically, in LLVM <= 11.0.1, an ``atomic<small enum class>`` |
| would not contain a lock byte. This was broken in LLVM >= 12.0.0, where it started including a lock byte despite |
| the platform supporting lockfree atomics for that size. Starting in LLVM 15.0.1, the ABI for these types has been |
| restored to what it used to be (no lock byte), which is the most efficient implementation. |
| |
| This ABI break only affects users that compile with ``-ffreestanding``, and only for ``atomic<T>`` where ``T`` |
| is a non-builtin type that could be lockfree on the platform. See https://llvm.org/D133377 for more details. |
| |
| - When building libc++ against newlib/picolibc, the type of ``regex_type_traits::char_class_type`` was changed to |
| ``uint16_t`` since all values of ``ctype_base::mask`` are taken. This is technically an ABI break, but including |
| ``<regex> `` has triggered a ``static_assert`` failure since libc++ 14, so it is unlikely that this causes |
| problems for existing users. |
| |
| Build System Changes |
| -------------------- |
| - Support for ``libcxx``, ``libcxxabi`` and ``libunwind`` in ``LLVM_ENABLE_PROJECTS`` has officially |
| been removed. Instead, please build according to :ref:`these instructions <build instructions>`. |