Libc++ 17.0.0 (In-Progress) Release Notes

Written by the Libc++ Team


These are in-progress notes for the upcoming libc++ 17 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 17.0.0. 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++ 17.0.0?

Implemented Papers

  • P2520R0 - move_iterator<T*> should be a random access iterator
  • P1328R1 - constexpr type_info::operator==()
  • P1413R3 - Formatting thread::id (the stacktrace is not done yet)
  • P2675R1 - format’s width estimation is too approximate and not forward compatible
  • P2505R5 - Monadic operations for std::expected
  • P2711R1 - Making Multi-Param Constructors Of views explicit (join_with_view is not done yet)
  • P2572R1 - std::format fill character allowances

Improvements and New Features

  • std::equal and std::ranges::equal are now forwarding to std::memcmp for integral types and pointers, which can lead up to 40x performance improvements.
  • std::string_view now provides iterators that check for out-of-bounds accesses when the safe libc++ mode is enabled.
  • The performance of dynamic_cast on its hot paths is greatly improved and is as efficient as the libsupc++ implementation. Note that the performance improvements are shipped in libcxxabi.
  • D122780 Improved the performance of std::sort and std::ranges::sort by up to 50% for arithmetic types and by approximately 10% for other types.
  • The <format> header is no longer considered experimental. Some std::formatter specializations are not yet available since the class used in the specialization has not been implemented in libc++. This prevents the feature-test macro to be set.

Deprecations and Removals

  • The <experimental/coroutine> header has been removed in this release. The <coroutine> header has been shipping since LLVM 14, so the Coroutines TS implementation is being removed per our policy for removing TSes.
  • 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++23: atomic, bit, cstdint, cstdlib, cstring, initializer_list, limits, new,
      stdexcept, system_error, type_traits, typeinfo
  • <algorithm> no longer includes <chrono> in any C++ version (it was previously included in C++17 and earlier).
  • <string> no longer includes <vector> in any C++ version (it was previously included in C++20 and earlier).
  • <string>, <string_view>, and <mutex> no longer include <functional> in any C++ version (it was previously included in C++20 and earlier).
  • The headers <experimental/algorithm> and <experimental/functional> have been removed, since all the contents have been implemented in namespace std for at least two releases.
  • The formatter specialization template<size_t N> struct formatter<const charT[N], charT> has been removed. Since libc++’s format library was marked experimental there is no backwards compatibility option. This specialization has been removed from the Standard since it was never used, the proper specialization to use instead is template<size_t N> struct formatter<charT[N], charT>.
  • Libc++ used to provide some C++11 tag type global variables in C++03 as an extension, which are removed in this release. Those variables were std::allocator_arg, std::defer_lock, std::try_to_lock, std::adopt_lock, and std::piecewise_construct. Note that the types associated to those variables are still provided in C++03 as an extension (e.g. std::piecewise_construct_t). Providing those variables in C++03 mode made it impossible to define them properly – C++11 mandated that they be constexpr variables, which is impossible in C++03 mode. Furthermore, C++17 mandated that they be inline constexpr variables, which led to ODR violations when mixed with the C++03 definition. Cleaning this up is required for libc++ to make progress on support for C++20 modules.
  • The _LIBCPP_ABI_OLD_LOGNORMAL_DISTRIBUTION macro has been removed.
  • The classes strstreambuf , istrstream, ostrstream, and strstream have been deprecated. They have been deprecated in the Standard since C++98, but were never marked as deprecated in libc++.

Upcoming Deprecations and Removals


  • The base template for std::char_traits has been marked as deprecated and will be removed in LLVM 18. 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.
  • The _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED macro will not be honored anymore in LLVM 18. Please see the updated documentation about the safe libc++ mode 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> will be removed in LLVM 18, as all their contents will have been implemented in namespace std for at least two releases.

ABI Affecting Changes

  • Symbols for std::allocator_arg, std::defer_lock, std::try_to_lock, std::adopt_lock, and std::piecewise_construct have been removed from the built library. Under most circumstances, user code should not have been relying on those symbols anyway since those are empty classes and the compiler does not generate an undefined reference unless the address of the object is taken. However, this is an ABI break if the address of one of these objects has been taken in code compiled as C++03, since in those cases the objects were marked as defined in the shared library. In other Standard modes, this should never be a problem since those objects were defined in the headers as constexpr.

Build System Changes

  • Building libc++ and libc++abi for Apple platforms now requires targeting macOS 10.13 and later. This is relevant for vendors building the libc++ shared library and for folks statically linking libc++ into an application that has back-deployment requirements on Apple platforms.
  • LIBCXX_ENABLE_FILESYSTEM now represents whether a filesystem is supported on the platform instead of representing merely whether <filesystem> should be provided. This means that vendors building with LIBCXX_ENABLE_FILESYSTEM=OFF will now also get <fstream> excluded from their configuration of the library.
  • LIBCXX_ENABLE_FSTREAM is not supported anymore, please use LIBCXX_ENABLE_FILESYSTEM=OFF if your platform does not have support for a filesystem.