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?

The main focus of the libc++ team has been to implement new C++20, C++23, and C++26 features.

New hardened modes for the library have been added, replacing the legacy debug mode that was removed in the LLVM 17 release. Unlike the legacy debug mode, some of these hardening modes are also intended to be used in production. See Hardening Modes for more details.

Work on the ranges support has progressed. See libc++ Ranges Status for the current status.

Work on the experimental C++23 module support has progressed. The std.compat module is available and the feature is retroactively available in C++20. See Modules in libc++ for more information.

Work on the experimental C++17 Parallel STL has progressed. See libc++ Parallel STL Status for the current status.

Work on the experimental C++17 SIMD support has progressed. See libc++ Parallelism TS Status (N4808) for the current status.

Implemented Papers

  • P2093R14 - Formatted output

  • P2539R4 - Should the output of std::print to a terminal be synchronized with the underlying stream?

  • 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

  • P2905R2 - Runtime format strings

  • 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?)

  • P2821R5 -

  • P0521R0 - Proposed Resolution for CA 14 (shared_ptr use_count/unique)

  • P0543R3 - Saturation arithmetic

  • P1759R6 - Native handles and file streams

  • P2868R3 - Remove Deprecated std::allocator Typedef From C++26

  • P2517R1 - Add a conditional noexcept specification to std::apply

  • P2447R6 - span over initializer list

Improvements and New Features

  • std::ranges::count and std::ranges::find are now optimized for std::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.

  • The _LIBCPP_ENABLE_CXX26_REMOVED_ALLOCATOR_MEMBERS macro has been added to make the function allocator<T>::is_always_equal available.

  • The _LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE macro has been added to make the function std::shared_ptr<...>::unique() available.

  • The cmake option LIBCXX_ENABLE_STD_MODULES has been removed. The test infrastructure no longer depends on a modern CMake, it works with the minimal required LLVM version (3.20.0).

  • The .cppm files of experimental standard library modules can now be installed. By default, they are not installed. This can be enabled by configuring CMake with -DLIBCXX_INSTALL_MODULES=ON. The installation directory can be configured with the CMake option -DLIBCXX_INSTALL_MODULE_DIR=<path>. The default location is ${PREFIX}/share/libc++/v1.

  • AddressSanitizer annotations have been added to std::basic_string. These annotations are enabled for all allocators by default. It’s only enabled for long strings, strings using the small buffer optimization are not annotated.

  • The libc++ source code has been formatted with clang-format. This discourse thread contains information how to rebase downstream patches.

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.

  • Overriding __libcpp_verbose_abort no longer has any effect on library assertions. The only supported way to customize the assertion handler that gets invoked when a hardening assertion fails is now by setting the LIBCXX_ASSERTION_HANDLER_FILE CMake variable and providing a custom header. See the documentation on overriding the default assertion handler for details. The ability to override __libcpp_verbose_abort will be removed in an upcoming release in favor of the new overriding mechanism.

  • In safe mode (which is now equivalent to the extensive hardening mode), a failed assertion will now generate a trap rather than a call to verbose abort.

  • 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 on overriding the default assertion handler.

  • 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.

  • The macro _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS has been deprecated and will be removed in LLVM 19. This macro used to re-enable redundant members of std::allocator<T> like pointer, reference, rebind, address, max_size, construct, destroy, and the two-argument overload of allocate. However, this led to the library being non-conforming due to incorrect constexpr-ness.

  • The macros _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES and _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES have been deprecated and will be removed in LLVM 19. These macros used to re-enable all features that were removed in the C++17 and C++20 standards. Instead of using these macros, please use the macros to re-enable individual features.

  • The macro _LIBCPP_INLINE_VISIBILITY has been deprecated in LLVM 18 and will be removed entirely in LLVM 19. The macro _LIBCPP_HIDE_FROM_ABI is the drop-in replacement.

  • The macro _VSTD has been deprecated in LLVM 18 and will be removed entirely in LLVM 19. The code std is the drop-in replacement.

Upcoming Deprecations and Removals

  • The ability to override __libcpp_verbose_abort will be removed in an upcoming release.


  • The LIBCXX_EXECUTOR CMake variable has been deprecated. LLVM 19 will completely remove support for the *_EXECUTOR variables.

  • 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 in 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 _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS macro has been deprecated in LLVM 18 and will be removed entirely in LLVM 19.

  • The _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES and _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES macros have been deprecated in LLVM 18 and will be removed entirely in LLVM 19.

  • The macro _LIBCPP_INLINE_VISIBILITY has been deprecated in LLVM 18 and will be removed entirely in LLVM 19.

  • The macro _VSTD has been deprecated in LLVM 18 and will be removed entirely 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

  • When the shared/static library is built with -fno-exceptions, the behavior of operator new was changed to make it standards-conforming. In LLVM 17 and before, the throwing versions of operator new would return nullptr upon failure to allocate, when the shared/static library was built with exceptions disabled. This was non-conforming, since the throwing versions of operator new are never expected to return nullptr, and this non-conformance could actually lead to miscompiles in subtle cases.

    Starting in LLVM 18, the throwing versions of operator new will abort the program when they fail to allocate if the shared/static library has been built with -fno-exceptions. This is consistent with the behavior of all other potentially-throwing functions in the library, which abort the program instead of throwing when -fno-exceptions is used.

    Furthermore, when the shared/static library is built with -fno-exceptions, users who override the throwing version of operator new will now need to also override the std::nothrow_t version of operator new if they want to use it. Indeed, this is because there is no way to implement a conforming operator new(nothrow) from a conforming potentially-throwing operator new when compiled with -fno-exceptions. In that case, using operator new(nothrow) without overriding it explicitly but after overriding the throwing operator new will result in an error.

    Note that this change only impacts vendors/users that build the shared/static library themselves and pass -DLIBCXX_ENABLE_EXCEPTIONS=OFF, which is not the default configuration. If you are using the default configuration of the library, the libc++ shared/static library will be built with exceptions enabled, and there is no change between LLVM 17 and LLVM 18, even for users who build their own code using -fno-exceptions.

  • 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 instantiations 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.

  • 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).

  • The layout of some range adaptors that use the movable-box exposition-only type as an implementation detail has changed in order to fix a bug which could result in overwriting user data following the movable-box. This bug was caused by incorrect usage of the [[no_unique_address]] attribute inside the implementation of movable-box. This fix affects the layout of the following views: take_while_view, filter_view, single_view, drop_while_view, repeat_view, transform_view, chunk_by_view. In order to avoid silent breakage as a result of this fix, an ABI tag has been added to these views such that their mangled name will be different starting in this version of libc++. As a result, attempting to call a function that expects one of these views will fail to link until the code has been rebuilt against a matching version of libc++. In practice, we believe it is unusual for these views to appear at ABI boundaries so this should not be a major problem for most users. However it is probably worth auditing ranges-heavy code for ABI boundaries that would contain these views, or for types that contain these views as members and which are passed across ABI boundaries.

  • Some properties of libc++ may cause ODR-violations when mixing multiple libc++ instances. To avoid these, often benign, ODR-violations the ODR-affecting properties are now part of the ABI tag. The ODR-affecting properties are:

    • library version (This was part of the ABI tag prior to LLVM 18.)

    • exceptions vs no-exceptions

    • hardening mode

    This should not be ABI-affecting except that libc++ will be more robust against different configurations of it being used in different translation units.

  • The amount of padding bytes available for use at the end of certain std::expected instantiations has changed in this release. This is an ABI break for any code that held a std::expected member with [[no_unique_address]] in an ABI-facing type. In those cases, the layout of the enclosing type will change, breaking the ABI. However, the std::expected<T, E> member requires a few characteristics in order to be affected by this change:

    • A type equivalent to union {T ; E} needs to have more than one byte of padding available.

    • The std::expected<T, E> member must have been in a situation where its padding bytes were previously reused by another object, which can happen in a few cases (this is probably not exhaustive):

      • It is a member with [[no_unique_address]] applied to it, and it is followed by another data member, or

      • It is a member with [[no_unique_address]] applied to it, and it is the last member of the user-defined type, and that user-defined type is used in ways that its padding bytes can be reused, or

      • It is inherited from

    We expect that this will not be a very frequent occurrence. However, there is unfortunately no technique we can use in the library to catch such misuse. Indeed, even applying an ABI tag to std::expected would not help since ABI tags are not propagated to containing types. As a result, if you notice very difficult to explain bugs around the usage of a std::expected, you should consider checking whether you are hitting this ABI break. This change was done to fix #70494 and the vendor communication is handled in #70820.

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.

  • LIBCXXABI_USE_LLVM_UNWINDER and COMPILER_RT_USE_LLVM_UNWINDER switched defaults from OFF to ON. This means that by default, libc++abi and compiler-rt will link against the LLVM provided libunwind library instead of the system-provided unwinding library. If you are building the LLVM runtimes with the goal of shipping them so that they can interoperate with other system-provided libraries that might be using a different unwinding library (such as libgcc_s), you should pass LIBCXXABI_USE_LLVM_UNWINDER=OFF and COMPILER_RT_USE_LLVM_UNWINDER=OFF to make sure the system-provided unwinding library is used by the LLVM runtimes.