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- P0174R2 (Deprecating Vestigial Library Parts in C++17)
47- N4190 (Removing auto_ptr, random_shuffle(), And Old <functional> Stuff)
48- P0154R1 (Hardware inference size)
49
50- Marked the following papers as "Complete" (note that some of those might have
51  been implemented in a previous release but not marked as such):
52
53    - P1207R4 (Movability of Single-pass Iterators);
54    - P1474R1 (Helpful pointers for ``ContiguousIterator``);
55    - P1522R1 (Iterator Difference Type and Integer Overflow);
56    - P1523R1 (Views and Size Types);
57    - P1456R1 (Move-only views);
58    - P1870R1 (``forwarding-range`` is too subtle);
59    - P1878R1 (Constraining Readable Types);
60    - P1970R2 (Consistency for ``size()`` functions: Add ``ranges::ssize``);
61    - P1983R0 (Wording for GB301, US296, US292, US291, and US283).
62
63New Features
64------------
65
66- `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
67  "heapsort with bounce" to reduce the number of comparisons, and rearranges
68  elements using move-assignment instead of `swap`.
69
70- Libc++ now supports a variety of assertions that can be turned on to help catch
71  undefined behavior in user code. This new support is now separate from the old
72  (and incomplete) Debug Mode. Vendors can select whether the library they ship
73  should include assertions or not by default. For details, see
74  :ref:`the documentation <assertions-mode>` about this new feature.
75
76- The implementation of the function ``std::to_chars`` for integral types has
77  moved from the dylib to the header. This means the function no longer has a
78  minimum deployment target.
79
80- The format functions (``std::format``, ``std::format_to``, ``std::format_to_n``, and
81  ``std::formatted_size``) now validate the format string at compile time.
82  When the format string is invalid this will make the code ill-formed instead
83  of throwing an exception at run-time.  (This does not affect the ``v``
84  functions.)
85
86
87API Changes
88-----------
89
90- The ``_LIBCPP_ABI_UNSTABLE`` macro has been removed in favour of setting
91  ``_LIBCPP_ABI_VERSION=2``. This should not have any impact on users because
92  they were not supposed to set ``_LIBCPP_ABI_UNSTABLE`` manually, however we
93  still feel that it is worth mentioning in the release notes in case some users
94  had been doing it.
95
96- The header ``<experimental/filesystem>`` has been removed. Instead, use
97  ``<filesystem>`` header. The associated macro
98  ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
99
100- Some libc++ headers no longer transitively include all of:
101    - ``<algorithm>``
102    - ``<chrono>``
103    - ``<exception>``
104    - ``<functional>``
105    - ``<iterator>``
106    - ``<new>``
107    - ``<typeinfo>``
108    - ``<utility>``
109
110  If, after updating libc++, you see compiler errors related to missing declarations
111  in namespace ``std``, it might be because one of your source files now needs to
112  include one or more of the headers listed above.
113
114- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
115  ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
116  and ``uniform_int_distribution`` now conform to the Standard by rejecting
117  template parameter types other than ``short``, ``int``, ``long``, ``long long``,
118  (as an extension) ``__int128_t``, and the unsigned versions thereof.
119  In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
120
121- The C++14 function ``std::quoted(const char*)`` is no longer supported in
122  C++03 or C++11 modes.
123
124- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
125  supported anymore. Please migrate to using the new support for
126  :ref:`assertions <assertions-mode>` instead.
127
128- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
129  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
130
131- The ``_LIBCPP_DEBUG`` macro is not supported anymore. It will be honoured until
132  LLVM 16, and then it will be an error to define that macro. To enable basic
133  assertions (previously ``_LIBCPP_DEBUG=0``), please use ``_LIBCPP_ENABLE_ASSERTIONS=1``.
134  To enable the debug mode (previously ``_LIBCPP_DEBUG=1|2``), please ensure that
135  the library has been built with support for the debug mode, and it will be
136  enabled automatically (no need to define ``_LIBCPP_DEBUG``).
137
138- The ``_LIBCPP_DISABLE_EXTERN_TEMPLATE`` macro is not honored anymore when defined by
139  users of libc++. Instead, users not wishing to take a dependency on libc++ should link
140  against the static version of libc++, which will result in no dependency being
141  taken against the shared library.
142
143- The ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION`` macro has been added to allow
144  re-enabling the ``allocator<void>`` specialization. When used in conjuction with
145  ``_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS``, this ensures that the members of
146  ``allocator<void>`` removed in C++20 can be accessed.
147
148- The experimental versions of ``boyer_moore_searcher`` and ``boyer_moore_horspool_searcher``
149  will be removed in LLVM 17. You can disable the deprecation warnings by defining
150  ``_LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_SEARCHERS``.
151
152- ``std::function`` has been removed in C++03. If you are using it, please remove usages
153  or upgrade to C++11 or later. It is possible to re-enable ``std::function`` in C++03 by defining
154  ``_LIBCPP_ENABLE_CXX03_FUNCTION``. This option will be removed in LLVM 16.
155
156- ``unary_function`` and ``binary_function`` are no longer available in C++17 and C++20.
157  They can be re-enabled by defining ``_LIBCPP_ENABLE_CXX17_REMOVED_UNARY_BINARY_FUNCTION``.
158  They are also marked as ``[[deprecated]]`` in C++11 and later. To disable deprecation warnings
159  you have to define ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS``. Note that this disables
160  all deprecation warnings.
161
162ABI Changes
163-----------
164
165- The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
166  emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
167  ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
168  will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
169
170- On FreeBSD, NetBSD, DragonFlyBSD and Solaris, ``std::random_device`` is now implemented on
171  top of ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
172  token used when constructing a ``std::random_device`` will now be ignored instead of
173  interpreted as a file to read entropy from.
174
175- ``std::valarray``'s unary operators ``!``, ``+``, ``~`` and ``-`` now return an expression
176  object instead of a ``valarray``. This was done to fix an issue where any expression involving
177  other ``valarray`` operators and one of these unary operators would end up with a dangling
178  reference. This is a potential ABI break for code that exposes ``std::valarray`` on an ABI
179  boundary, specifically if the return type of an ABI-boundary function is ``auto``-deduced
180  from an expression involving unary operators on ``valarray``. If you are concerned by this,
181  you can audit whether your executable or library exports any function that returns a
182  ``valarray``, and if so ensure that any such function uses ``std::valarray`` directly
183  as a return type instead of relying on the type of ``valarray``-expressions, which is
184  not guaranteed by the Standard anyway.
185
186Build System Changes
187--------------------
188
189- Support for standalone builds have been entirely removed from libc++, libc++abi and
190  libunwind. Please use :ref:`these instructions <build instructions>` for building
191  libc++, libc++abi and/or libunwind.
192
193- The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and
194  ``{LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN`` CMake variables have been removed. Instead, please
195  use the ``CMAKE_CXX_COMPILER_TARGET``, ``CMAKE_SYSROOT`` and ``CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN``
196  variables provided by CMake.
197
198- When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi
199  library should set ``LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON`` when configuring CMake. The current
200  behavior, which tries to guess the correct dll-export semantics based on whether we're building
201  the libc++ shared library, will be removed in LLVM 16.
202
203- Previously, the C++ ABI library headers would be installed inside ``<prefix>/include/c++/v1``
204  alongside the libc++ headers as part of building libc++. This is not the case anymore -- the
205  ABI library is expected to install its headers where it wants them as part of its own build.
206  Note that no action is required for most users, who build libc++ against libc++abi, since
207  libc++abi already installs its headers in the right location. However, vendors building
208  libc++ against alternate ABI libraries should make sure that their ABI library installs
209  its own headers.
210
211- The legacy testing configuration is now deprecated and will be removed in the next release. For
212  most users, this should not have any impact. However, if you are testing libc++, libc++abi or
213  libunwind in a configuration or on a platform that used to be supported by the legacy testing
214  configuration and isn't supported by one of the configurations in ``libcxx/test/configs``,
215  ``libcxxabi/test/configs`` or ``libunwind/test/configs``, please move to one of those
216  configurations or define your own.
217
218- The ``LIBCXX_ENABLE_DEBUG_MODE_SUPPORT`` CMake configuration is not supported anymore. If you
219  were disabling support for the debug mode with that flag, please use ``LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=OFF``
220  instead.
221
222- MinGW DLL builds of libc++ no longer use dllimport in their headers, which
223  means that the same set of installed headers works for both DLL and static
224  linkage. This means that distributors finally can build both library
225  versions with a single CMake invocation.
226