Libc++ 23.0.0 (In-Progress) Release Notes¶

Warning

These are in-progress notes for the upcoming libc++ 23.0.0 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 23.0.0. 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++ 23.0.0?¶

Implemented Papers ¶

P2440R1: ranges::iota, ranges::shift_left and ranges::shift_right (Github)

Improvements and New Features ¶

The std::ranges::fold_left_with_iter algorithm has been optimized for segmented iterators, resulting in a performance improvement of up to 1.38x for std::deque<int> iterators.
std::copy(CharT*, CharT*, ostreambuf_iterator<CharT>) has been optimized, resulting in performance improvements of up to 25x.

Deprecations and Removals ¶

The std::launch::any enumerator that was accidentally provided as an extension is now deprecated. It will be removed in LLVM 25.
__wrap_iter’s (iterator type for array, span, string, string_view and vector) base() method has been removed as it was non-standard.

std::allocator is trivially default constructible since LLVM 22. 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[] returns bool since LLVM 22, 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.
vector::iterator and string::iterator no longer use __bounded_iter<__wrap_iter<Iter>>, but instead __bounded_iter<Iter> directly if _LIBCPP_ABI_BOUNDED_ITERATORS_IN_VECTOR and _LIBCPP_ABI_BOUNDED_ITERATORS_IN_STRING are defined.
The const_iterator member type of std::deque is now corrected to hold a (possibly fancy) pointer to the (possibly fancy) pointer allocated in the internal map. E.g. when the allocators use fancy pointers, the internal map stores FancyPtr<T> objects, and the previous strategy accessed these objects via const FancyPtr<const T> lvalues, causing undefined behavior. Now const_iterator stores FancyPtr<const FancyPtr<T>> instead of FancyPtr<const FancyPtr<const T>>, which is technically an ABI break when these two types have incompatible layouts. The iterator member type is also changed to store FancyPtr<const FancyPtr<T>> because the internal map is never modified via an iterator. This is necessary for reducing undefined behavior and for adding constexpr support for deque in C++26, so we do not provide any way to opt-out of that behavior.

libc++ documentation