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
38New Features
39------------
40
41- Implemented P0627R6 (Function to mark unreachable code)
42
43- Implemented P1165R1 (Make stateful allocator propagation more consistent for ``operator+(basic_string)``)
44
45- Implemented P0674R1 (Support arrays in ``make_shared`` and ``allocate_shared``)
46
47- Implemented P0980R1 (Making ``std::string`` constexpr)
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
62- `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
63  "heapsort with bounce" to reduce the number of comparisons, and rearranges
64  elements using move-assignment instead of `swap`.
65
66- Libc++ now supports a variety of assertions that can be turned on to help catch
67  undefined behavior in user code. This new support is now separate from the old
68  (and incomplete) Debug Mode. Vendors can select whether the library they ship
69  should include assertions or not by default. For details, see
70  :ref:`the documentation <assertions-mode>` about this new feature.
71
72API Changes
73-----------
74
75- The ``_LIBCPP_ABI_UNSTABLE`` macro has been removed in favour of setting
76  ``_LIBCPP_ABI_VERSION=2``. This should not have any impact on users because
77  they were not supposed to set ``_LIBCPP_ABI_UNSTABLE`` manually, however we
78  still feel that it is worth mentioning in the release notes in case some users
79  had been doing it.
80
81- The header ``<experimental/filesystem>`` has been removed. Instead, use
82  ``<filesystem>`` header. The associated macro
83  ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
84
85- Some libc++ headers no longer transitively include all of:
86    - ``<algorithm>``
87    - ``<chrono>``
88    - ``<functional>``
89    - ``<utility>``
90
91  If, after updating libc++, you see compiler errors related to missing declarations
92  in namespace ``std``, it might be because one of your source files now needs to
93  include one or more of the headers listed above.
94
95- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
96  ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
97  and ``uniform_int_distribution`` now conform to the Standard by rejecting
98  template parameter types other than ``short``, ``int``, ``long``, ``long long``,
99  (as an extension) ``__int128_t``, and the unsigned versions thereof.
100  In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
101
102- The C++14 function ``std::quoted(const char*)`` is no longer supported in
103  C++03 or C++11 modes.
104
105- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
106  supported anymore. Please migrate to using the new support for
107  :ref:`assertions <assertions-mode>` instead.
108
109- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
110  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
111
112ABI Changes
113-----------
114
115- The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
116  emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
117  ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
118  will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
119
120- On FreeBSD, NetBSD, DragonFlyBSD and Solaris, ``std::random_device`` is now implemented on
121  top of ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
122  token used when constructing a ``std::random_device`` will now be ignored instead of
123  interpreted as a file to read entropy from.
124
125Build System Changes
126--------------------
127
128- Support for standalone builds have been entirely removed from libc++, libc++abi and
129  libunwind. Please use :ref:`these instructions <build instructions>` for building
130  libc++, libc++abi and/or libunwind.
131
132- The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and
133  ``{LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN`` CMake variables have been removed. Instead, please
134  use the ``CMAKE_CXX_COMPILER_TARGET``, ``CMAKE_SYSROOT`` and ``CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN``
135  variables provided by CMake.
136
137- When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi
138  library should set ``LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON`` when configuring CMake. The current
139  behavior, which tries to guess the correct dll-export semantics based on whether we're building
140  the libc++ shared library, will be removed in LLVM 16.
141
142- Previously, the C++ ABI library headers would be installed inside ``<prefix>/include/c++/v1``
143  alongside the libc++ headers as part of building libc++. This is not the case anymore -- the
144  ABI library is expected to install its headers where it wants them as part of its own build.
145  Note that no action is required for most users, who build libc++ against libc++abi, since
146  libc++abi already installs its headers in the right location. However, vendors building
147  libc++ against alternate ABI libraries should make sure that their ABI library installs
148  its own headers.
149
150- The legacy testing configuration is now deprecated and will be removed in the next release. For
151  most users, this should not have any impact. However, if you are testing libc++, libc++abi or
152  libunwind in a configuration or on a platform that used to be supported by the legacy testing
153  configuration and isn't supported by one of the configurations in ``libcxx/test/configs``,
154  ``libcxxabi/test/configs`` or ``libunwind/test/configs``, please move to one of those
155  configurations or define your own.
156