Libc++ 22.0.0 (In-Progress) Release Notes

Written by the Libc++ Team

Warning

These are in-progress notes for the upcoming libc++ 22.0.0 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 22.0.0. 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++ 22.0.0?

Implemented Papers

  • P2321R2: zip (Github) (The paper is partially implemented. zip_transform_view is implemented in this release)

  • P3044R2: sub-string_view from string (Github)

  • P3223R2: Making std::istream::ignore less surprising (Github)

  • P3060R3: Add std::views::indices(n) (Github)

  • P2835R7: Expose std::atomic_ref’s object address (Github)

  • P2944R3: Comparisons for reference_wrapper (Github)

  • P3168R2: Give std::optional Range Support (Github)

Improvements and New Features

  • The performance of map::map(const map&) has been improved up to 2.3x

  • The performance of map::operator=(const map&) has been improved by up to 11x

  • The performance of unordered_set::unordered_set(const unordered_set&) has been improved by up to 3.3x.

  • The performance of unordered_set::operator=(const unordered_set&) has been improved by up to 5x.

  • The performance of map::erase and set::erase has been improved by up to 2x

  • The performance of find(key) in map, set, multimap and multiset has been improved by up to 2.3x

  • Some reallocations are now avoided in std::filesystem::path::lexically_relative, resulting in a performance improvement of up to 1.7x.

  • The performance of the (iterator, iterator) constructors of map, set, multimap and multiset has been improved by up to 3x

  • The performance of insert(iterator, iterator) of map, set, multimap and multiset has been improved by up to 2.5x

  • The performance of erase(iterator, iterator) in the unordered containers has been improved by up to 1.9x

  • The performance of map::insert_or_assign has been improved by up to 2x

  • ofstream::write has been optimized to pass through large strings to system calls directly instead of copying them in chunks into a buffer.

  • Multiple internal types have been refactored to use [[no_unique_address]], resulting in faster compile times and reduced debug information.

  • The performance of std::find has been improved by up to 2x for integral types

  • The std::distance and std::ranges::distance algorithms have been optimized for segmented iterators (e.g., std::join_view iterators), reducing the complexity from O(n) to O(n / segment_size). Benchmarks show performance improvements of over 1600x in favorable cases with large segment sizes (e.g., 1024).

  • The std::{fill, fill_n} and std::ranges::{fill, fill_n} algorithms have been optimized for segmented iterators, resulting in a performance improvement of at least 10x for std::deque<int> iterators and std::join_view<std::vector<std::vector<int>>> iterators.

  • The std::{generate, generate_n} and std::ranges::generate_n algorithms have been optimized for segmented iterators, resulting in a performance improvement for std::deque<short> and std::join_view<vector<vector<short>>> iterators.

Deprecations and Removals

Potentially breaking changes

  • The algorithm for multi{map,set}::find has been modified such that it doesn’t necessarily return an iterator to the first equal element in the container. This was never guaranteed by the Standard, but libc++ previously happened to always return the first equal element, like other implementations do. Starting with this release, code relying on the first element being returned from find will be broken, and lower_bound or equal_range should be used instead.

  • The ABI flag _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER has been split off from _LIBCPP_ABI_NO_ITERATOR_BASES. If you are using this flag and care about ABI stability, you should set _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER as well.

Announcements About Future Releases

ABI Affecting Changes

  • The ABI flag _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER has been split off from _LIBCPP_ABI_NO_ITERATOR_BASES. If you are using this flag and care about ABI stability, you should set _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER as well.

  • The internal types __map_value_compare, __unordered_map_hasher, __unordered_map_equal, __hash_map_hasher and __hash_map_equal have been refactored to use _LIBCPP_COMPRESSED_ELEMENT instead of potentially inheriting from the types they wrap. At this point in time we are not aware of any ABI changes caused by this.

Build System Changes