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.
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.
- There's support for the C++20 header
<format>
. Some parts are still missing, most notably the compile-time format string validation. Some functions are known to be inefficient, both in memory usage and performance. The implementation isn't API- or ABI-stable and therefore considered experimental. (Some not-yet-implemented papers require an API-break.) Vendors can still disable this header by turning the CMake option LIBCXX_ENABLE_INCOMPLETE_FEATURES off. - 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 to1
enables the randomization of unspecified behavior of standard algorithms (e.g. equal elements instd::sort
or randomization of both sides of partition forstd::nth_element
)- Floating-point support for
std::to_chars
support has been added. Thanks to Stephan T. Lavavej and Microsoft for providing their implementation to libc++.
- The functions
std::atomic<T*>::fetch_(add|sub)
andstd::atomic_fetch_(add|sub)
no longer accept a function pointer. While this is technically an API break, the invalid syntax isn't supported by libstdc++ 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 argumentT
are now ill-formed. While this is technically an API break, the invalid syntax isn't supported by libstdc++ 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
andstd::experimental::pmr::polymorphic_allocator<T>::allocate
now throw an exception of typestd::bad_array_new_length
when the requested size exceeds the maximum supported size, as required by the C++ standard. Previously the typestd::length_error
was used. - Removed the nonstandard methods
std::chrono::file_clock::to_time_t
andstd::chrono::file_clock::from_time_t
; neither libstdc++ nor MSVC STL had such methods. Instead, in C++20, you can usestd::chrono::file_clock::from_sys
andstd::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
, andget_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. std::filesystem::path::iterator
, which (in our implementation) stashes apath
value inside itself similar toistream_iterator
, now sets itsreference
type topath
and itsiterator_category
toinput_iterator_tag
, so that it is a conforming input iterator in C++17 and a conformingstd::bidirectional_iterator
in C++20. Before this release, it had set itsreference
type toconst path&
and itsiterator_category
tobidirectional_iterator_tag
, making it a non-conforming bidirectional iterator. After this change,for
loops of the formfor (auto& c : path)
must be rewritten as eitherfor (auto&& c : path)
orfor (const auto& c : path)
.std::reverse_iterator<path::iterator>
is no longer rejected.- Removed the nonstandard default constructor from
std::chrono::month_weekday
. You must now explicitly initialize with achrono::month
andchrono::weekday_indexed
instead of "meh, whenever".
- The C++17 variable templates
is_error_code_enum_v
andis_error_condition_enum_v
are now of typebool
instead ofsize_t
. - The C++03 emulation type for
std::nullptr_t
has been removed in favor of usingdecltype(nullptr)
in all standard modes. This is an ABI break for anyone compiling in C++03 mode and who hasstd::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 involvingstd::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 on D109459 if you are broken by this change and need to define the macro. - On Apple platforms,
std::random_device
is now implemented on top ofarc4random()
instead of reading from/dev/urandom
. Any implementation-defined token used when constructing astd::random_device
will now be ignored instead of interpreted as a file to read entropy from. std::lognormal_distribution::param_type
used to store a data member of typestd::normal_distribution
; now this member is stored in thelognormal_distribution
class itself, and theparam_type
stores only the mean and standard deviation, as required by the Standard. This changessizeof(std::lognormal_distribution::param_type)
. You can define the_LIBCPP_ABI_OLD_LOGNORMAL_DISTRIBUTION
macro to return to the previous behavior. That macro will be removed in LLVM 15. Please comment on PR52906 if you are broken by this change and need to define the macro.
- 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 targeting 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>
- If you were rooting your CMake invocation at
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
- Libc++, libc++abi and libunwind will not be built with
-fPIC
by default anymore. If you want to build those runtimes with position independent code, please specify-DCMAKE_POSITION_INDEPENDENT_CODE=ON
explicitly when configuring the build, or-DRUNTIMES_<target-name>_CMAKE_POSITION_INDEPENDENT_CODE=ON
if using the bootstrapping build.