Libc++ 14.0.0 (In-Progress) Release Notes

Written by the Libc++ Team

Warning

These are in-progress notes for the upcoming libc++ 14 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 14.0.0. 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++ 14.0.0?

New Features

  • There’s initial support for the C++20 header <format>. The implementation is incomplete. Some functions are known to be inefficient; both in memory usage and performance. The implementation is considered experimental and isn’t considered ABI stable.
  • There’s a new CMake option LIBCXX_ENABLE_UNICODE to disable Unicode support in the <format> header. This only affects the estimation of the output width of the format functions.
  • Support for building libc++ on top of a C Standard Library that does not support wchar_t was added. This is useful for building libc++ in an embedded setting, and it adds itself to the various freestanding-friendly options provided by libc++.
  • _LIBCPP_DEBUG equals to 1 enables the randomization of unspecified behavior of standard algorithms (e.g. equal elements in std::sort or randomization of both sides of partition for std::nth_element)

API Changes

  • The functions std::atomic<T*>::fetch_(add|sub) and std::atomic_fetch_(add|sub) no longer accept a function pointer. While this is technically an API break, the invalid syntax isn’t supported by libstc++ and MSVC STL. See https://godbolt.org/z/49fvzz98d.

  • The call of the functions std::atomic_(add|sub)(std::atomic<T*>*, ...) with the explicit template argument T are now ill-formed. While this is technically an API break, the invalid syntax isn’t supported by libstc++ and MSVC STL. See https://godbolt.org/z/v9959re3v.

    Due to this change it’s now possible to call these functions with the explicit template argument T*. This allows using the same syntax on the major Standard library implementations. See https://godbolt.org/z/oEfzPhTTb.

    Calls to these functions where the template argument was deduced by the compiler are unaffected by this change.

  • The functions std::allocator<T>::allocate and std::experimental::pmr::polymorphic_allocator<T>::allocate now throw an exception of type std::bad_array_new_length when the requested size exceeds the maximum supported size, as required by the C++ standard. Previously the type std::length_error was used.

  • Removed the nonstandard methods std::chrono::file_clock::to_time_t and std::chrono::file_clock::from_time_t; neither libstdc++ nor MSVC STL had such methods. Instead, in C++20, you can use std::chrono::file_clock::from_sys and std::chrono::file_clock::to_sys, which are specified in the Standard. If you are not using C++20, you should move to it.

  • The declarations of functions declare_reachable, undeclare_reachable, declare_no_pointers, undeclare_no_pointers, and get_pointer_safety have been removed not only from C++2b but from all modes. Their symbols are still provided by the dynamic library for the benefit of existing compiled code. All of these functions have always behaved as no-ops.

ABI Changes

  • The C++17 variable templates is_error_code_enum_v and is_error_condition_enum_v are now of type bool instead of size_t.
  • The C++03 emulation type for std::nullptr_t has been removed in favor of using decltype(nullptr) in all standard modes. This is an ABI break for anyone compiling in C++03 mode and who has std::nullptr_t as part of their ABI. However, previously, these users’ ABI would be incompatible with any other binary or static archive compiled with C++11 or later. If you start seeing linker errors involving std::nullptr_t against previously compiled binaries, this may be the cause. You can define the _LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION macro to return to the previous behavior. That macro will be removed in LLVM 15. Please comment here if you are broken by this change and need to define the macro.

Build System Changes

  • Building the libc++ shared or static library requires a C++ 20 capable compiler. Consider using a Bootstrapping build to build libc++ with a fresh Clang if you can’t use the system compiler to build libc++ anymore.

  • Historically, there has been numerous ways of building libc++ and libc++abi. This has culminated in over 5 different ways to build the runtimes, which made it impossible to maintain with a good level of support. Starting with this release, the runtimes support exactly two ways of being built, which should cater to all use-cases. Furthermore, these builds are as lightweight as possible and will work consistently even when targetting embedded platforms, which used not to be the case. Please see the documentation on building libc++ to see those two ways of building and migrate over to the appropriate build instructions as soon as possible.

    All other ways to build are deprecated and will not be supported in the next release. We understand that making these changes can be daunting. For that reason, here’s a summary of how to migrate from the two most common ways to build:

    • If you were rooting your CMake invocation at <monorepo>/llvm and passing -DLLVM_ENABLE_PROJECTS=<...> (which was the previously advertised way to build the runtimes), please simply root your CMake invocation at <monorepo>/runtimes and pass -DLLVM_ENABLE_RUNTIMES=<...>.

    • If you were doing two CMake invocations, one rooted at <monorepo>/libcxx and one rooted at <monorepo>/libcxxabi (this used to be called a “Standalone build”), please move them to a single invocation like so:

      $ cmake -S <monorepo>/libcxx -B libcxx-build <LIBCXX-OPTIONS>
      $ cmake -S <monorepo>/libcxxabi -B libcxxabi-build <LIBCXXABI-OPTIONS>
      

      should become

      $ cmake -S <monorepo>/runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" <LIBCXX-OPTIONS> <LIBCXXABI-OPTIONS>
      
    • Support for building the runtimes using the GCC 32 bit multilib flag (-m32) has been removed. Support for this had been flaky for a while, and we didn’t know of anyone depending on this. Instead, please perform a normal cross-compilation of the runtimes using the appropriate target, such as passing the following to your bootstrapping build:

      -DLLVM_RUNTIME_TARGETS=i386-unknown-linux