Libc++ 22.0.0 (In-Progress) Release Notes¶

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 ¶

P2592R3: Hashing support for std::chrono value classes (Github)
P2321R2: zip (Github)
P2988R12: std::optional<T&> (Github)
P3044R2: sub-string_view from string (Github)
P3223R2: Making std::istream::ignore less surprising (Github)
P3060R3: Add std::views::indices(n) (Github)
P2404R3: Move-only types for equality_comparable_with, totally_ordered_with, and three_way_comparable_with (Github)
P2641R4: Checking if a union alternative is active (std::is_within_lifetime) (Github)
P2835R7: Expose std::atomic_ref’s object address (Github)
P2944R3: Comparisons for reference_wrapper (Github)
P3168R2: Give std::optional Range Support (guarded by -fexperimental-library, Github)
P3567R2: flat_meow Fixes (Github)
P3836R2: Make optional<T&> trivially copyable (Github)
P1789R3: Library Support for Expansion Statements (Github)

Improvements and New Features ¶

The performance of associative and unordered containers has been significantly improved. This is an overview of the different functions and by up to how much they have been improved:
- map::map(const map&): 2.3x
- map::operator=(const map&): 11x
- unordered_set::unordered_set(const unordered_set&): 3.3x
- unordered_set::operator=(const unordered_set&): 5x
- erase of map and set: 2x
- find(key) of map, set, multimap and multiset: 2.3x
- (iterator, iterator) constructors of map, set, multimap and multiset: 3x
- insert(iterator, iterator) of map, set, multimap and multiset: 2.5x
- erase(iterator, iterator) of the unordered containers: 1.9x
- map::insert_or_assign: 2x
The performance of many algorithms has been improved. This is an overview of the different functions and by up to how much they have been improved:
- std::find for integral types: 2x
- std::for_each and ranges::for_each over the associative containers: 2x
- std::rotate: 3x
- Some have been specifically optimized for segmented iterators:
  - std::distance and std::ranges::distance on non-random-access iterators: 1600x
  - {std,ranges}::{fill, fill_n}: 10x
  - std::{generate, generate_n} and std::ranges::generate_n: 1.8x
- std::search_n for random access iterators now tries to skip elements. In contrived cases improvements of 70,000x have been observed.
There have also been performance improvements in other library facilities:
- vector<bool>::reserve(): 2x
- deque::append_range: 3.4x
- num_get::do_get integral overloads: 2.8x
- Some reallocations are now avoided in std::filesystem::path::lexically_relative: 1.7x
- ofstream::write passes large strings to system calls directly instead of copying them in chunks into a buffer
- std::align is now an inline function, which allows the compiler to better optimize calls to it
std::atomic::wait has been refactored to accept more types to use platform native wait functions directly. This is guarded behind the ABI Macro _LIBCPP_ABI_ATOMIC_WAIT_NATIVE_BY_SIZE.
Multiple internal types have been refactored to use [[no_unique_address]], resulting in faster compile times and reduced debug information.

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.
ranges::iota_view is now aware of __int128. This causes iota_view::difference_type to change from long long to __int128 in some cases.
std::allocator is now trivially default constructible. The behaviour can be reverted by defining _LIBCPP_DEPRECATED_ABI_NON_TRIVIAL_ALLOCATOR. Please inform the libc++ team if you need this flag, since it will be removed in LLVM 24 if there is no evidence that it’s required.
bitset::operator[] now returns bool, making libc++ conforming. The behaviour can be reverted by defining _LIBCPP_DEPRECATED_ABI_BITSET_CONST_SUBSCRIPT_RETURN_REF. Please inform the libc++ team if you need this flag, since it will be removed in LLVM 24 if there is no evidence that it’s required.

libc++ documentation