Libc++ 19.0.0 (In-Progress) Release Notes¶

Warning

These are in-progress notes for the upcoming libc++ 19.0.0 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 19.0.0. 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++ 19.0.0?¶

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

Experimental support for the time zone database has progressed.

Work on the ranges support has progressed.

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

Work on the C++17 mathematical special functions has started. See this issue for the current status.

Implemented Papers ¶

P1132R8 - out_ptr - a scalable output pointer abstraction
P1614R2 - The Mothership has Landed
P2637R3 - Member visit
P2652R2 - Disallow User Specialization of allocator_traits
P2819R2 - Add tuple protocol to complex
P2495R3 - Interfacing stringstreams with string_view
P2867R2 - Remove Deprecated strstreams From C++26
P2872R3 - Remove wstring_convert From C++26
P3142R0 - Printing Blank Lines with println (as DR against C++23)
P2944R3 - Comparisons for reference_wrapper (comparison operators for reference_wrapper only)
P2591R5 - Concatenation of strings and string views
P2968R2 - Make std::ignore a first-class object
P2997R1 - Removing the common reference requirement from the indirectly invocable concepts (as DR against C++20)
P2302R4 - std::ranges::contains
P1659R3 - std::ranges::starts_with and std::ranges::ends_with
P3029R1 - Better mdspan’s CTAD
P2387R3 - Pipe support for user-defined range adaptors
P2713R1 - Escaping improvements in std::format
P2231R1 - Missing constexpr in std::optional and std::variant
P0019R8 - std::atomic_ref
P2389R2 - Alias template dims for the extents of mdspan
P1223R5 - ranges::find_last(), ranges::find_last_if(), and ranges::find_last_if_not()
P2602R2 - Poison Pills are Too Toxic
P1981R0 - Rename leap to leap_second
P1982R0 - Rename link to time_zone_link
P2602R2 - Poison Pills are Too Toxic (as DR against C++20)

Improvements and New Features ¶

The performance of growing std::vector has been improved for trivially relocatable types.
A lot of types are considered trivially relocatable now, including std::vector and std::string.
The performance of std::ranges::fill and std::ranges::fill_n has been improved for std::vector<bool>::iterators, resulting in a performance increase of up to 1400x.
The std::mismatch algorithm has been optimized for integral types, which can lead up to 40x performance improvements.
The std::ranges::minmax algorithm has been optimized for integral types, resulting in a performance increase of up to 100x.
The std::set_intersection and std::ranges::set_intersection algorithms have been optimized to fast-forward over contiguous ranges of non-matching values, reducing the number of comparisons from linear to logarithmic growth with the number of elements in best-case scenarios.
The _LIBCPP_ENABLE_CXX26_REMOVED_STRSTREAM macro has been added to make the declarations in <strstream> available.
The _LIBCPP_ENABLE_CXX26_REMOVED_WSTRING_CONVERT macro has been added to make the declarations in <locale> available.
The formatting library is updated to Unicode 15.1.0.
std::ignores const __ignore_t& operator=(_Tp&&) const was changed to const __ignore_type& operator=(const _Tp&) const noexcept for all language versions.
Vendors can now configure the ABI so that string and vector will use bounded iterators when hardening is enabled. Note that checks for iterator invalidation are currently not supported – any accesses made through an invalidated bounded iterator will still result in undefined behavior (bounded iterators follow the normal invalidation rules of the associated container). string bounded iterators use the logical size of the container (index < str.size()) whereas vector bounded iterators use the “physical” size of the container (index < vec.capacity()) which is a less strict check; refer to the implementation for further details.

Bounded iterators can be enabled via the _LIBCPP_ABI_BOUNDED_ITERATORS_IN_STRING ABI macro for string and via the _LIBCPP_ABI_BOUNDED_ITERATORS_IN_VECTOR ABI macro for vector; note that checks will only be performed if the hardening mode is set to fast or above (i.e., no checking is performed in the unchecked mode, even if bounded iterators are enabled in the ABI configuration).

Note: bounded iterators currently are not supported for vector<bool>.
In C++23 and C++26 the number of transitive includes in several headers has been reduced, improving the compilation speed.

Deprecations and Removals ¶

The C++20 synchronization library (<barrier>, <latch>, std::atomic::wait, etc.) has been deprecated in language modes prior to C++20. If you are using these features prior to C++20, please update to -std=c++20. In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
_LIBCPP_DISABLE_NODISCARD_EXT has been removed. [[nodiscard]] applications are now unconditional. This decision is based on LEWGs discussion on P3122 and P3162 to not use [[nodiscard]] in the standard.
The LIBCXX_ENABLE_ASSERTIONS CMake variable that was used to enable the safe mode has been deprecated and setting it triggers an error; use the LIBCXX_HARDENING_MODE CMake variable with the value extensive instead. Similarly, the _LIBCPP_ENABLE_ASSERTIONS macro has been 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 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. 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.
The _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT macro that changed the behavior for narrowing conversions in std::variant has been removed in LLVM 19.
The _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS and _LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION macros have been removed in LLVM 19.
The _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES and _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES macros have been removed in LLVM 19. C++17 and C++20 removed features can still be re-enabled individually.
The _LIBCPP_INLINE_VISIBILITY and _VSTD macros have been removed in LLVM 19.
The _LIBCPP_ATOMIC_ONLY_USE_BUILTINS configuration option has been removed in LLVM 19. This should not affect many users, except perhaps users using the library with -ffreestanding with a toolchain where compiler-rt or libatomic is not available. If you are one such user, please reach out to the libc++ developers so we can collaborate on a path for supporting atomics properly on freestanding platforms.
LWG3430 disallow implicit conversion of the source arguments to std::filesystem::path when constructing std::basic_*fstream. This effectively removes the possibility to directly construct a std::basic_*fstream from a std::basic_string_view, a input-iterator or a C-string, instead you can construct a temporary std::basic_string. This change has been applied to C++17 and later.
The _LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS macro has been removed and is not honored anymore. Additional warnings provided by libc++ as a matter of QoI will now be provided unconditionally.
libc++ no longer supports std::allocator<const T> and containers of const-qualified element type, such as std::vector<const T> and std::list<const T>. This used to be supported as an undocumented extension. If you were using std::vector<const T>, replace it with std::vector<T> instead. The _LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST macro can be defined to temporarily re-enable this extension. to temporarily re-enable this extension to make it easier to update user code This macro will be honored for one released and ignored starting in LLVM 20. To assist with the clean-up process, consider running your code through Clang Tidy, with std-allocator-const enabled.
When configuring libc++ with localization or threads disabled, the library no longer emits an error when trying to #include <locale> and other such headers. Instead, those headers have no content. This is consistent with the behavior for all other libc++ carve-outs like filesystem, wide characters, a source of randomness, and others. Users that were checking whether including a header would fail (e.g. via a script or CMake’s try_compile will experience a change in behavior).
libc++ no longer supports relational comparison for std::chrono::weekday. The relational comparison operators were provided as an undocumented extension. If you were using relational comparison on std::chrono::weekday, compare the results of c_encoding() or iso_encoding() instead. The _LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS macro can be defined to temporarily re-enable this extension. This macro will be honored for one release and ignored starting in LLVM 20.
The operators in the rel_ops namespace have been deprecated. The deprecation is part of the paper P0768R1 “Library Support for the Spaceship (Comparison) Operator”.

Upcoming Deprecations and Removals ¶

LLVM 20¶

The LIBCXX_ENABLE_ASSERTIONS CMake variable and the _LIBCPP_ENABLE_ASSERTIONS macro that were used to enable the safe mode will be removed in LLVM 20.
The C++20 synchronization library will be removed entirely in language modes prior to C++20 in LLVM 20.
The relational operators for std::chrono::weekday will be removed entirely, and the _LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS macro that was used to re-enable this extension will be ignored in LLVM 20.
The _LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST macro will no longer have an effect.

LLVM 21¶

The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22, non-critical bug fixes may not be back-ported to C++03, including LWG issues. C++03 is a legacy platform, where most projects are no longer actively maintained. To reduce the amount of fixes required to keep such legacy projects compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid unintended breaking changes. See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.

If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.

ABI Affecting Changes ¶

The optional POSIX macro ENODATA has been deprecated in C++ and POSIX 2017. The random_device could throw a system_error with this value. It now throws ENOMSG.

Build System Changes ¶

The LIBCXX_EXECUTOR and LIBCXXABI_EXECUTOR CMake variables have been removed. Please set LIBCXX_TEST_PARAMS to executor=<...> instead.
The CMake variable LIBCXX_ENABLE_CLANG_TIDY has been removed. The build system has been changed to automatically detect the presence of clang-tidy and the required Clang libraries.
The CMake options LIBCXX_INSTALL_MODULES now defaults to ON.
The CMake options LIBCXX_BENCHMARK_NATIVE_STDLIB and LIBCXX_BENCHMARK_NATIVE_GCC_TOOLCHAIN have been removed. To benchmark the native standard library, configure the test suite against the native standard library directly instead.