Libc++ 18.0.0 (In-Progress) Release Notes¶
Written by the Libc++ Team
Warning
These are in-progress notes for the upcoming libc++ 18.0.0 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 18.0.0. 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++ 18.0.0?¶
The “safe” mode is replaced by the hardened mode in this release. The
LIBCXX_ENABLE_ASSERTIONS
CMake variable is deprecated and setting it will trigger an error; useLIBCXX_HARDENING_MODE
instead. Similarly, the_LIBCPP_ENABLE_ASSERTIONS
macro is deprecated and setting it to1
now enables the hardened mode. Seelibcxx/docs/Hardening.rst
for more details.A new debug mode has been added, replacing the legacy debug mode that was removed in the LLVM 17 release. See
libcxx/docs/Hardening.rst
for more details.
Implemented Papers¶
P2497R0 - Testing for success or failure of
<charconv>
functionsP2697R1 - Interfacing
bitset
withstring_view
P2443R1 -
views::chunk_by
P2538R1 - ADL-proof
std::projected
Improvements and New Features¶
The library now provides a hardened mode under which common cases of library undefined behavior will be turned into a reliable program termination. Vendors can configure whether the hardened mode is enabled by default with the
LIBCXX_HARDENING_MODE
variable at CMake configuration time. Users can control whether the hardened mode is enabled on a per translation unit basis using the-D_LIBCPP_ENABLE_HARDENED_MODE=1
macro. See the hardening documentation for more details.The safe mode is now a part of hardening modes. Vendors can configure whether the safe mode is enabled by default with the
LIBCXX_HARDENING_MODE
variable at CMake configuration time. Users can control whether the safe mode is enabled on a per translation unit basis using the-D_LIBCPP_ENABLE_SAFE_MODE=1
macro. The_LIBCPP_ENABLE_ASSERTIONS
macro that was previously used to enable the safe mode is now deprecated. See the hardening documentation for more details.The library now provides a debug mode which is a superset of the safe mode, additionally enabling more expensive checks that are not suitable to be used in production. This replaces the legacy debug mode that was removed in this release. Unlike the legacy debug mode, this doesn’t affect the ABI and doesn’t require locking. Vendors can configure whether the debug mode is enabled by default with the
LIBCXX_HARDENING_MODE
variable at CMake configuration time. Users can control whether the debug mode is enabled on a per translation unit basis using the-D_LIBCPP_ENABLE_DEBUG_MODE=1
macro. See the hardening documentation for more details.
Deprecations and Removals¶
The “safe” mode is now controlled via the new generalized support for hardening. The
LIBCXX_ENABLE_ASSERTIONS
CMake variable that was used to enable the safe mode is now deprecated and setting it will trigger an error; use theLIBCXX_HARDENING_MODE
variable with the valuesafe
instead. Similarly, the_LIBCPP_ENABLE_ASSERTIONS
macro is deprecated (setting it to1
still enables the safe mode in this release while also issuing a deprecation warning)._LIBCPP_ENABLE_ASSERTIONS
will be removed entirely in the next release and setting it will become an error. See the hardening documentation for more details.
Upcoming Deprecations and Removals¶
LLVM 18¶
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 namespacestd
for at least two releases.
LLVM 19¶
The base template for
std::char_traits
has been marked as deprecated and will be removed in LLVM 19. If you are usingstd::char_traits
with types other thanchar
,wchar_t
,char8_t
,char16_t
,char32_t
or a custom character type for which you specializedstd::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. Note that the_LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION
macro can be defined in LLVM 18 to eagerly remove the specialization and prepare code bases for the unconditional removal in LLVM 19.
ABI Affecting Changes¶
The symbol of a non-visible function part of
std::system_error
was removed. This is not a breaking change as the private function__init
was never referenced internally outside of the dylibThis release of libc++ added missing visibility annotations on some types in the library. Users compiling with
-fvisbility=hidden
may notice that additional type infos from libc++ are being exported from their ABI. This is the correct behavior in almost all cases since exporting the RTTI is required for these types to work properly with dynamic_cast, exceptions and other mechanisms across binaries. However, if you intend to use libc++ purely as an internal implementation detail (i.e. you use libc++ as a static archive and never export libc++ symbols from your ABI) and you notice changes to your exported symbols list, then this means that you were not properly preventing libc++ symbols from being part of your ABI.The name mangling for intantiations of
std::projected
has changed in order to implement P2538R1. This technically results in an ABI break, however in practice we expect uses ofstd::projected
in ABI-sensitive places to be extremely rare. Any error resulting from this change should result in a link-time error.
Build System Changes¶
The
LIBCXX_EXECUTOR
CMake variable has been deprecated. If you are relying on this, the new replacement is passing-Dexecutor=...
tollvm-lit
. Alternatively, this flag can be made persistent in the generated test configuration file by passing-DLIBCXX_TEST_PARAMS=executor=...
. This also applies to theLIBUWIND_EXECTOR
andLIBCXXABI_EXECUTOR
CMake variables. LLVM 19 will completely remove support for the*_EXECUTOR
variables.