Libc++ 17.0.0 (In-Progress) Release Notes

Written by the Libc++ Team

Warning

These are in-progress notes for the upcoming libc++ 17 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 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==()

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.

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++2b: atomic, bit, cstdint, cstdlib, cstring, initializer_list, new, stdexcept,
      type_traits, typeinfo
  • 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.

Upcoming Deprecations and Removals

  • 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.

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.