Libc++ 16.0.0 (In-Progress) Release Notes¶
Written by the Libc++ Team
These are in-progress notes for the upcoming libc++ 16 release. Release notes for previous releases can be found on the Download Page.
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. 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++ 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 (with the exception of ranges::join_view which is still marked as experimental because it is about to undergo an ABI-breaking change in the Standard due to D2770). Work on C++23 ranges has started.
The C++20 spaceship operator has been added to more types, the work is still ongoing.
algorithms and their variations (
copy_backward, etc.) apply optimizations
for trivial types more often. This has the potential to expose bugs in code
using these algorithms that currently relies on undefined behavior (this
includes indirect usage – for example, these algorithms are used in the
implementation of some standard containers). This change also made the
algorithms check the given iterator types for conformance more strictly.
string_viewrange constructor should be
P2417R2 - A more constexpr bitset
P2273R3 - Making
P0591R4 - Utility functions to implement uses-allocator construction
P2291R3 - Add Constexpr Modifiers to Functions
from_charsfor Integral Types in
P0220R1 - Adopt Library Fundamentals V1 TS Components for C++17
P0482R6 - char8_t: A type for UTF-8 characters and strings
nodiscardin the library
polymorphic_allocator<>as a vocabulary type
P1035R7 - Input Range Adaptors
P2325R3 - Views should not be required to be default constructible
P1020R1 - Smart pointer creation with default initialization
P2210R2 - Superior String Splitting
P2286R8 - Formatting Ranges
Improvements and New Features¶
std::mbrtoc8()from P0482R6 are now provided when implementations in the global namespace are provided by the C library.
<memory_resource>header from C++17
D122780 Improved the performance of std::sort
move_backwardare now also optimized for
std::deque<>::iterator, which can lead to up to 20x performance improvements on certain algorithms.
move_backwardare now also optimized for
join_view::iterator, which can lead to up to 20x performance improvements on certain combinations of iterators and algorithms.
Deprecations and Removals¶
binary_functionare no longer provided in C++17 and newer Standard modes. They can be re-enabled with
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:
Users can also remove all incidental transitive includes by defining
_LIBCPP_REMOVE_TRANSITIVE_INCLUDESregardless 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.
from_charsfor 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.
_LIBCPP_ENABLE_CXX03_FUNCTIONmacro that allowed re-enabling the now-deprecated C++03 implementation of
std::functionhas been removed. Users who need to use
std::functionshould 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.
_LIBCPP_DEBUGmacro is not honored anymore, and it is an error to try to use it. Please migrate to
Upcoming Deprecations and Removals¶
The base template for
std::char_traitshas been marked as deprecated and will be removed in LLVM 18. If you are using
std::char_traitswith types other than
char32_tor 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.
_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDEDmacro will not be honored anymore in LLVM 18. Please see the updated documentation about the safe libc++ mode and in particular the
_LIBCPP_VERBOSE_ABORTmacro for details.
The comparison operators on
thread::idare now defined as free-standing functions instead of as hidden friends, in conformance with the C++ standard. Also see issue 56187.
_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17are 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_VERSIONwas previously defined to e.g.
15001to represent LLVM 15.0.01, but this value had been left undocumented. Starting with LLVM 16,
_LIBCPP_VERSIONwill contain the version of LLVM represented as
XXYYZZ. In other words,
_LIBCPP_VERSIONis gaining a digit. This should not be an issue for existing code, since using e.g.
_LIBCPP_VERSION > 15000will still give the right answer now that
_LIBCPP_VERSIONis 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
Tis 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_typewas changed to
uint16_tsince all values of
ctype_base::maskare taken. This is technically an ABI break, but including
<regex> `` has triggered a ``static_assertfailure since libc++ 14, so it is unlikely that this causes problems for existing users.
Build System Changes¶
LLVM_ENABLE_PROJECTShas officially been removed. Instead, please build according to these instructions.