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