Libc++ 18.0.0 (In-Progress) Release Notes

Written by the Libc++ Team


These are in-progress notes for the upcoming libc++ 18.0.0 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 18.0.0. 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++ 18.0.0?

  • A new debug mode has been added, replacing the legacy debug mode that was removed in the LLVM 17 release. See libcxx/docs/Hardening.rst for more details.

Implemented Papers

  • P2497R0 - Testing for success or failure of <charconv> functions

  • P2697R1 - Interfacing bitset with string_view

  • P2443R1 - views::chunk_by

  • P2538R1 - ADL-proof std::projected

  • P2614R2 - Deprecate numeric_limits::has_denorm

  • P0053R7 - C++ Synchronized Buffered Ostream (in the experimental library)

  • P2467R1 - Support exclusive mode for fstreams

  • P0020R6 - Floating Point Atomic

  • P2918R2 - Runtime format strings II

  • P2871R3 - Remove Deprecated Unicode Conversion Facets from C++26

  • P2870R3 - Remove basic_string::reserve()

  • P2909R4 - Fix formatting of code units as integers (Dude, where’s my char?)

Improvements and New Features

  • std::ranges::count is now optimized for vector<bool>::iterator, which can lead up to 350x performance improvements.

  • std::for_each has been optimized for segmented iterators like std::deque::iterator in C++23 and later, which can lead up to 40x performance improvements.

  • The library now provides several hardening modes under which common cases of library undefined behavior will be turned into a reliable program termination. The fast hardening mode enables a set of security-critical checks with minimal runtime overhead; the extensive hardening mode additionally enables relatively cheap checks that catch common logic errors but aren’t necessarily security-critical; and the debug hardening mode enables all available checks, some of which might be very expensive. Vendors can configure which hardening mode is enabled by default with the LIBCXX_HARDENING_MODE variable at CMake configuration time. Users can control which hardening mode is enabled on a per translation unit basis using the _LIBCPP_HARDENING_MODE macro. See the hardening documentation for more details.

  • The _LIBCPP_ENABLE_CXX26_REMOVED_CODECVT macro has been added to make the declarations in <codecvt> available.

  • The _LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE macro has been added to make the function std::basic_string<...>::reserve() available.

Deprecations and Removals

  • Availability macros which will never trigger an error have been removed. This includes anything that has been introduced before macOS 10.13, iOS 12, tvOS 12 and watchOS 4. This shouldn’t affect anybody, since AppleClang 15 doesn’t support any older OSes. If you are a vendor and make use of these macros, please inform the libc++ team so we can re-introduce them and consider upstreaming support for your platform.

  • The non-conforming constructor std::future_error(std::error_code) has been removed. Please use the std::future_error(std::future_errc) constructor provided in C++17 instead.

  • P1957 <> has been implemented in Clang and libc++ removed a code path that led to narrowing conversions in std::variant behaving in a non-standard way. This may change how some uses of std::variant’s constructor behave in user code. The _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT macro is provided to restore the previous behavior, and it will be supported in the LLVM 18 release only. In LLVM 19 and beyond, _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT will not be honored anymore.

  • The _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED macro is not honored anymore in LLVM 18. Please see the updated documentation about the hardening modes in libc++ and in particular the _LIBCPP_VERBOSE_ABORT macro for details.

  • The headers <experimental/deque>, <experimental/forward_list>, <experimental/list>, <experimental/map>, <experimental/memory_resource>, <experimental/regex>, <experimental/set>, <experimental/string>, <experimental/unordered_map>, <experimental/unordered_set>, and <experimental/vector> have been removed in LLVM 18, as all their contents will have been implemented in namespace std for at least two releases.

Upcoming Deprecations and Removals


  • The LIBCXX_ENABLE_ASSERTIONS CMake variable that was used to enable the safe mode will be deprecated and setting it will trigger an error; use the LIBCXX_HARDENING_MODE variable with the value extensive instead. Similarly, the _LIBCPP_ENABLE_ASSERTIONS macro will be deprecated (setting it to 1 still enables the extensive mode the LLVM 19 release while also issuing a deprecation warning). See the hardening documentation for more details.

  • The base template for std::char_traits has been marked as deprecated and will be removed in LLVM 19. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or 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. Note that the _LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION macro can be defined in LLVM 18 to eagerly remove the specialization and prepare code bases for the unconditional removal in LLVM 19.

  • The _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT macro that changed the behavior for narrowing conversions in std::variant will be removed in LLVM 19.


  • The LIBCXX_ENABLE_ASSERTIONS CMake variable and the _LIBCPP_ENABLE_ASSERTIONS macro that were used to enable the safe mode will be removed.

ABI Affecting Changes

  • The symbol of a non-visible function part of std::system_error was removed. This is not a breaking change as the private function __init was never referenced internally outside of the dylib.

  • This release of libc++ added missing visibility annotations on some types in the library. Users compiling with -fvisbility=hidden may notice that additional type infos from libc++ are being exported from their ABI. This is the correct behavior in almost all cases since exporting the RTTI is required for these types to work properly with dynamic_cast, exceptions and other mechanisms across binaries. However, if you intend to use libc++ purely as an internal implementation detail (i.e. you use libc++ as a static archive and never export libc++ symbols from your ABI) and you notice changes to your exported symbols list, then this means that you were not properly preventing libc++ symbols from being part of your ABI.

  • The name mangling for intantiations of std::projected has changed in order to implement P2538R1. This technically results in an ABI break, however in practice we expect uses of std::projected in ABI-sensitive places to be extremely rare. Any error resulting from this change should result in a link-time error.

  • Under the unstable ABI, the internal alignment requirements for heap allocations inside std::string has decreased from 16 to 8. This saves memory since string requests fewer additional bytes than it did previously. However, this also changes the return value of std::string::max_size and can cause code compiled against older libc++ versions but linked at runtime to a new version to throw a different exception when attempting allocations that are too large (std::bad_alloc vs std::length_error).

Build System Changes

  • The LIBCXX_EXECUTOR CMake variable has been deprecated. If you are relying on this, the new replacement is passing -Dexecutor=... to llvm-lit. Alternatively, this flag can be made persistent in the generated test configuration file by passing -DLIBCXX_TEST_PARAMS=executor=.... This also applies to the LIBUWIND_EXECTOR and LIBCXXABI_EXECUTOR CMake variables. LLVM 19 will completely remove support for the *_EXECUTOR variables.