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- `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
50  "heapsort with bounce" to reduce the number of comparisons, and rearranges
51  elements using move-assignment instead of `swap`.
52
53 - Libc++ now supports a variety of assertions that can be turned on to help catch
54   undefined behavior in user code. This new support is now separate from the old
55   (and incomplete) Debug Mode. Vendors can select whether the library they ship
56   should include assertions or not by default. For details, see
57   :ref:`the documentation <assertions-mode>` about this new feature.
58
59API Changes
60-----------
61
62- The ``_LIBCPP_ABI_UNSTABLE`` macro has been removed in favour of setting
63  ``_LIBCPP_ABI_VERSION=2``. This should not have any impact on users because
64  they were not supposed to set ``_LIBCPP_ABI_UNSTABLE`` manually, however we
65  still feel that it is worth mentioning in the release notes in case some users
66  had been doing it.
67
68- The header ``<experimental/filesystem>`` has been removed. Instead, use
69  ``<filesystem>`` header. The associated macro
70  ``_LIBCPP_DEPRECATED_EXPERIMENTAL_FILESYSTEM`` has also been removed.
71
72- Some libc++ headers no longer transitively include all of:
73    - ``<algorithm>``
74    - ``<chrono>``
75    - ``<functional>``
76    - ``<utility>``
77
78  If, after updating libc++, you see compiler errors related to missing declarations
79  in namespace ``std``, it might be because one of your source files now needs to
80  include one or more of the headers listed above.
81
82- The integer distributions ``binomial_distribution``, ``discrete_distribution``,
83  ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``,
84  and ``uniform_int_distribution`` now conform to the Standard by rejecting
85  template parameter types other than ``short``, ``int``, ``long``, ``long long``,
86  (as an extension) ``__int128_t``, and the unsigned versions thereof.
87  In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
88
89- The C++14 function ``std::quoted(const char*)`` is no longer supported in
90  C++03 or C++11 modes.
91
92- Setting a custom debug handler with ``std::__libcpp_debug_function`` is not
93  supported anymore. Please migrate to using the new support for
94  :ref:`assertions <assertions-mode>` instead.
95
96- ``vector<bool>::const_reference``, ``vector<bool>::const_iterator::reference``
97  and ``bitset::const_reference`` are now aliases for `bool` in the unstable ABI.
98
99ABI Changes
100-----------
101
102- The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
103  emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
104  ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
105  will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
106
107- On FreeBSD, NetBSD and DragonFlyBSD, ``std::random_device`` is now implemented on top of
108  ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
109  token used when constructing a ``std::random_device`` will now be ignored instead of
110  interpreted as a file to read entropy from.
111
112Build System Changes
113--------------------
114
115- Support for standalone builds have been entirely removed from libc++, libc++abi and
116  libunwind. Please use :ref:`these instructions <build instructions>` for building
117  libc++, libc++abi and/or libunwind.
118
119- The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and
120  ``{LIBCXX,LIBCXXABI,LIBUNWIND}_GCC_TOOLCHAIN`` CMake variables have been removed. Instead, please
121  use the ``CMAKE_CXX_COMPILER_TARGET``, ``CMAKE_SYSROOT`` and ``CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN``
122  variables provided by CMake.
123
124- When building for Windows, vendors who want to avoid dll-exporting symbols from the static libc++abi
125  library should set ``LIBCXXABI_HERMETIC_STATIC_LIBRARY=ON`` when configuring CMake. The current
126  behavior, which tries to guess the correct dll-export semantics based on whether we're building
127  the libc++ shared library, will be removed in LLVM 16.
128
129- Previously, the C++ ABI library headers would be installed inside ``<prefix>/include/c++/v1``
130  alongside the libc++ headers as part of building libc++. This is not the case anymore -- the
131  ABI library is expected to install its headers where it wants them as part of its own build.
132  Note that no action is required for most users, who build libc++ against libc++abi, since
133  libc++abi already installs its headers in the right location. However, vendors building
134  libc++ against alternate ABI libraries should make sure that their ABI library installs
135  its own headers.
136
137- The legacy testing configuration is now deprecated and will be removed in the next release. For
138  most users, this should not have any impact. However, if you are testing libc++ in a configuration
139  or on a platform that used to be supported by the legacy testing configuration and isn't supported
140  by one of the configurations in ``libcxx/test/configs``, please reach out to the libc++ developers
141  to get your configuration supported officially.
142