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Libc++ 20.0.0 (In-Progress) Release Notes

Written by the Libc++ Team

Warning

These are in-progress notes for the upcoming libc++ 20.0.0 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 20.0.0. 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++ 20.0.0?

Implemented Papers

  • P2747R2: constexpr placement new (Github)
  • P2609R3: Relaxing Ranges Just A Smidge (Github)
  • P2985R0: A type trait for detecting virtual base classes (Github)
  • std::jthread and <stop_token> are not guarded behind -fexperimental-library anymore

Improvements and New Features

  • The lexicographical_compare and ranges::lexicographical_compare algorithms have been optimized for trivially equality comparable types, resulting in a performance improvement of up to 40x.
  • The _LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER macro has been added to make std::get_temporary_buffer and std::return_temporary_buffer available.
  • The _LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION macro has been added to make std::uncaught_exception available in C++20 and later modes.

Deprecations and Removals

  • TODO: 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.
  • Support for the C++20 synchronization library (<barrier>, <latch>, atomic::wait, etc.) has been removed in language modes prior to C++20. If you are using these features prior to C++20, you will need to update to -std=c++20.
  • TODO: 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 no longer has any effect. std::allocator<const T> is not supported as an extension anymore, please migrate any code that uses e.g. std::vector<const T> to be standards conforming.

Upcoming Deprecations and Removals

LLVM 20

  • TODO

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

  • TODO

Build System Changes

  • TODO