1=========================================
2Libc++ 15.0.0 (In-Progress) Release Notes
3=========================================
4
5.. contents::
6   :local:
7   :depth: 2
8
9Written by the `Libc++ Team <https://libcxx.llvm.org>`_
10
11.. warning::
12
13   These are in-progress notes for the upcoming libc++ 15 release.
14   Release notes for previous releases can be found on
15   `the Download Page <https://releases.llvm.org/download.html>`_.
16
17Introduction
18============
19
20This document contains the release notes for the libc++ C++ Standard Library,
21part of the LLVM Compiler Infrastructure, release 15.0.0. Here we describe the
22status of libc++ in some detail, including major improvements from the previous
23release and new feature work. For the general LLVM release notes, see `the LLVM
24documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM releases may
25be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
26
27For more information about libc++, please see the `Libc++ Web Site
28<https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
29
30Note that if you are reading this file from a Git checkout or the
31main Libc++ web page, this document applies to the *next* release, not
32the current one. To see the release notes for a specific release, please
33see the `releases page <https://llvm.org/releases/>`_.
34
35What's New in Libc++ 15.0.0?
36============================
37
38Implemented Papers
39------------------
40
41- P0627R6 (Function to mark unreachable code)
42- P1165R1 (Make stateful allocator propagation more consistent for ``operator+(basic_string)``)
43- P0674R1 (Support arrays in ``make_shared`` and ``allocate_shared``)
44- P0980R1 (Making ``std::string`` constexpr)
45- P2216R3 (std::format improvements)
46
47- Implemented P0174R2 (Deprecating Vestigial Library Parts in C++17)
48
49- Marked the following papers as "Complete" (note that some of those might have
50  been implemented in a previous release but not marked as such):
51
52    - P1207R4 (Movability of Single-pass Iterators);
53    - P1474R1 (Helpful pointers for ``ContiguousIterator``);
54    - P1522R1 (Iterator Difference Type and Integer Overflow);
55    - P1523R1 (Views and Size Types);
56    - P1456R1 (Move-only views);
57    - P1870R1 (``forwarding-range`` is too subtle);
58    - P1878R1 (Constraining Readable Types);
59    - P1970R2 (Consistency for ``size()`` functions: Add ``ranges::ssize``);
60    - P1983R0 (Wording for GB301, US296, US292, US291, and US283).
61
62New Features
63------------
64
65- `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
66  "heapsort with bounce" to reduce the number of comparisons, and rearranges
67  elements using move-assignment instead of `swap`.
68
69- Libc++ now supports a variety of assertions that can be turned on to help catch
70  undefined behavior in user code. This new support is now separate from the old
71  (and incomplete) Debug Mode. Vendors can select whether the library they ship
72  should include assertions or not by default. For details, see
73  :ref:`the documentation <assertions-mode>` about this new feature.
74
75- The implementation of the function ``std::to_chars`` for integral types has
76  moved from the dylib to the header. This means the function no longer has a
77  minimum deployment target.
78
79- The format functions (``std::format``, ``std::format_to``, ``std::format_to_n``, and
80  ``std::formatted_size``) now validate the format string at compile time.
81  When the format string is invalid this will make the code ill-formed instead
82  of throwing an exception at run-time.  (This does not affect the ``v``
83  functions.)
84
85
86API Changes
87-----------
88
89- The ``_LIBCPP_ABI_UNSTABLE`` macro has been removed in favour of setting
90  ``_LIBCPP_ABI_VERSION=2``. This should not have any impact on users because
91  they were not supposed to set ``_LIBCPP_ABI_UNSTABLE`` manually, however we
92  still feel that it is worth mentioning in the release notes in case some users
93  had been doing it.
94
95- The header ``<experimental/filesystem>`` has been removed. Instead, use
96  ``<filesystem>`` header. The associated macro
97  ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
98
99- Some libc++ headers no longer transitively include all of:
100    - ``<algorithm>``
101    - ``<chrono>``
102    - ``<exception>``
103    - ``<functional>``
104    - ``<iterator>``
105    - ``<new>``
106    - ``<typeinfo>``
107    - ``<utility>``
108
109  If, after updating libc++, you see compiler errors related to missing declarations
110  in namespace ``std``, it might be because one of your source files now needs to
111  include one or more of the headers listed above.
112
113- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
114  ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
115  and ``uniform_int_distribution`` now conform to the Standard by rejecting
116  template parameter types other than ``short``, ``int``, ``long``, ``long long``,
117  (as an extension) ``__int128_t``, and the unsigned versions thereof.
118  In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
119
120- The C++14 function ``std::quoted(const char*)`` is no longer supported in
121  C++03 or C++11 modes.
122
123- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
124  supported anymore. Please migrate to using the new support for
125  :ref:`assertions <assertions-mode>` instead.
126
127- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
128  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
129
130- The ``_LIBCPP_DEBUG`` macro is not supported anymore. It will be honoured until
131  LLVM 16, and then it will be an error to define that macro. To enable basic
132  assertions (previously ``_LIBCPP_DEBUG=0``), please use ``_LIBCPP_ENABLE_ASSERTIONS=1``.
133  To enable the debug mode (previously ``_LIBCPP_DEBUG=1|2``), please ensure that
134  the library has been built with support for the debug mode, and it will be
135  enabled automatically (no need to define ``_LIBCPP_DEBUG``).
136
137- The ``_LIBCPP_DISABLE_EXTERN_TEMPLATE`` macro is not honored anymore when defined by
138  users of libc++. Instead, users not wishing to take a dependency on libc++ should link
139  against the static version of libc++, which will result in no dependency being
140  taken against the shared library.
141
142- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION`` macro has been added to allow
143  re-enabling the ``allocator<void>`` specialization. When used in conjuction with
144  ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS``, this ensures that the members of
145  ``allocator<void>`` removed in C++20 can be accessed.
146
147- The experimental versions of ``boyer_moore_searcher`` and ``boyer_moore_horspool_searcher``
148  will be removed in LLVM 17. You can disable the deprecation warnings by defining
149  ``_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_SEARCHERS``.
150
151ABI Changes
152-----------
153
154- The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
155  emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
156  ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
157  will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
158
159- On FreeBSD, NetBSD, DragonFlyBSD and Solaris, ``std::random_device`` is now implemented on
160  top of ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
161  token used when constructing a ``std::random_device`` will now be ignored instead of
162  interpreted as a file to read entropy from.
163
164- ``std::valarray``'s unary operators ``!``, ``+``, ``~`` and ``-`` now return an expression
165  object instead of a ``valarray``. This was done to fix an issue where any expression involving
166  other ``valarray`` operators and one of these unary operators would end up with a dangling
167  reference. This is a potential ABI break for code that exposes ``std::valarray`` on an ABI
168  boundary, specifically if the return type of an ABI-boundary function is ``auto``-deduced
169  from an expression involving unary operators on ``valarray``. If you are concerned by this,
170  you can audit whether your executable or library exports any function that returns a
171  ``valarray``, and if so ensure that any such function uses ``std::valarray`` directly
172  as a return type instead of relying on the type of ``valarray``-expressions, which is
173  not guaranteed by the Standard anyway.
174
175Build System Changes
176--------------------
177
178- Support for standalone builds have been entirely removed from libc++, libc++abi and
179  libunwind. Please use :ref:`these instructions <build instructions>` for building
180  libc++, libc++abi and/or libunwind.
181
182- The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and
183  ``{LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN`` CMake variables have been removed. Instead, please
184  use the ``CMAKE_CXX_COMPILER_TARGET``, ``CMAKE_SYSROOT`` and ``CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN``
185  variables provided by CMake.
186
187- When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi
188  library should set ``LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON`` when configuring CMake. The current
189  behavior, which tries to guess the correct dll-export semantics based on whether we're building
190  the libc++ shared library, will be removed in LLVM 16.
191
192- Previously, the C++ ABI library headers would be installed inside ``<prefix>/include/c++/v1``
193  alongside the libc++ headers as part of building libc++. This is not the case anymore -- the
194  ABI library is expected to install its headers where it wants them as part of its own build.
195  Note that no action is required for most users, who build libc++ against libc++abi, since
196  libc++abi already installs its headers in the right location. However, vendors building
197  libc++ against alternate ABI libraries should make sure that their ABI library installs
198  its own headers.
199
200- The legacy testing configuration is now deprecated and will be removed in the next release. For
201  most users, this should not have any impact. However, if you are testing libc++, libc++abi or
202  libunwind in a configuration or on a platform that used to be supported by the legacy testing
203  configuration and isn't supported by one of the configurations in ``libcxx/test/configs``,
204  ``libcxxabi/test/configs`` or ``libunwind/test/configs``, please move to one of those
205  configurations or define your own.
206
207- The ``LIBCXX_ENABLE_DEBUG_MODE_SUPPORT`` CMake configuration is not supported anymore. If you
208  were disabling support for the debug mode with that flag, please use ``LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=OFF``
209  instead.
210
211- MinGW DLL builds of libc++ no longer use dllimport in their headers, which
212  means that the same set of installed headers works for both DLL and static
213  linkage. This means that distributors finally can build both library
214  versions with a single CMake invocation.
215