Libc++ 15.0.0 (In-Progress) Release Notes

Written by the Libc++ Team

Warning

These are in-progress notes for the upcoming libc++ 15 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 15.0.0. 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++ 15.0.0?

New Features

  • Implemented P0627R6 (Function to mark unreachable code)

  • Implemented P1165R1 (Make stateful allocator propagation more consistent for operator+(basic_string))

  • Implemented P0674R1 (Support arrays in make_shared and allocate_shared)

  • Implemented P0980R1 (Making std::string constexpr)

  • Marked the following papers as “Complete” (note that some of those might have been implemented in a previous release but not marked as such):

    • P1207R4 (Movability of Single-pass Iterators);
    • P1474R1 (Helpful pointers for ContiguousIterator);
    • P1522R1 (Iterator Difference Type and Integer Overflow);
    • P1523R1 (Views and Size Types);
    • P1456R1 (Move-only views);
    • P1870R1 (forwarding-range is too subtle);
    • P1878R1 (Constraining Readable Types);
    • P1970R2 (Consistency for size() functions: Add ranges::ssize);
    • P1983R0 (Wording for GB301, US296, US292, US291, and US283).
  • pop_heap now uses an algorithm known as “bottom-up heapsort” or “heapsort with bounce” to reduce the number of comparisons, and rearranges elements using move-assignment instead of swap.

  • Libc++ now supports a variety of assertions that can be turned on to help catch undefined behavior in user code. This new support is now separate from the old (and incomplete) Debug Mode. Vendors can select whether the library they ship should include assertions or not by default. For details, see the documentation about this new feature.

API Changes

  • The _LIBCPP_ABI_UNSTABLE macro has been removed in favour of setting _LIBCPP_ABI_VERSION=2. This should not have any impact on users because they were not supposed to set _LIBCPP_ABI_UNSTABLE manually, however we still feel that it is worth mentioning in the release notes in case some users had been doing it.

  • The header <experimental/filesystem> has been removed. Instead, use <filesystem> header. The associated macro _LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM has also been removed.

  • Some libc++ headers no longer transitively include all of:
    • <algorithm>
    • <chrono>
    • <functional>
    • <utility>

    If, after updating libc++, you see compiler errors related to missing declarations in namespace std, it might be because one of your source files now needs to include one or more of the headers listed above.

  • The integer distributions binomial_distribution, discrete_distribution, geometric_distribution, negative_binomial_distribution, poisson_distribution, and uniform_int_distribution now conform to the Standard by rejecting template parameter types other than short, int, long, long long, (as an extension) __int128_t, and the unsigned versions thereof. In particular, uniform_int_distribution<int8_t> is no longer supported.

  • The C++14 function std::quoted(const char*) is no longer supported in C++03 or C++11 modes.

  • Setting a custom debug handler with std::__libcpp_debug_function is not supported anymore. Please migrate to using the new support for assertions instead.

  • vector<bool>::const_reference, vector<bool>::const_iterator::reference and bitset::const_reference are now aliases for bool in the unstable ABI.

ABI Changes

  • The _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION macro controlling whether we use an emulation for std::nullptr_t in C++03 mode has been removed. After this change, _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION will not be honoured anymore and there will be no way to opt back into the C++03 emulation of std::nullptr_t.
  • On FreeBSD, NetBSD, DragonFlyBSD and Solaris, std::random_device is now implemented on top of arc4random() instead of reading from /dev/urandom. Any implementation-defined token used when constructing a std::random_device will now be ignored instead of interpreted as a file to read entropy from.

Build System Changes

  • Support for standalone builds have been entirely removed from libc++, libc++abi and libunwind. Please use these instructions for building libc++, libc++abi and/or libunwind.
  • The {LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE, {LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT and {LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN CMake variables have been removed. Instead, please use the CMAKE_CXX_COMPILER_TARGET, CMAKE_SYSROOT and CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN variables provided by CMake.
  • When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi library should set LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON when configuring CMake. The current behavior, which tries to guess the correct dll-export semantics based on whether we’re building the libc++ shared library, will be removed in LLVM 16.
  • Previously, the C++ ABI library headers would be installed inside <prefix>/include/c++/v1 alongside the libc++ headers as part of building libc++. This is not the case anymore – the ABI library is expected to install its headers where it wants them as part of its own build. Note that no action is required for most users, who build libc++ against libc++abi, since libc++abi already installs its headers in the right location. However, vendors building libc++ against alternate ABI libraries should make sure that their ABI library installs its own headers.
  • The legacy testing configuration is now deprecated and will be removed in the next release. For most users, this should not have any impact. However, if you are testing libc++ in a configuration or on a platform that used to be supported by the legacy testing configuration and isn’t supported by one of the configurations in libcxx/test/configs, please reach out to the libc++ developers to get your configuration supported officially.