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