docs/UsingLibcxx.rst

.. _using-libcxx:

============
Using libc++
============

.. contents::
  :local:

Usually, libc++ is packaged and shipped by a vendor through some delivery vehicle
(operating system distribution, SDK, toolchain, etc) and users don't need to do
anything special in order to use the library.

This page contains information about configuration knobs that can be used by
users when they know libc++ is used by their toolchain, and how to use libc++
when it is not the default library used by their toolchain.


Using a different version of the C++ Standard
=============================================

Libc++ implements the various versions of the C++ Standard. Changing the version of
the standard can be done by passing ``-std=c++XY`` to the compiler. Libc++ will
automatically detect what Standard is being used and will provide functionality that
matches that Standard in the library.

.. code-block:: bash

  $ clang++ -std=c++17 test.cpp

.. warning::
  Using ``-std=c++XY`` with a version of the Standard that has not been ratified yet
  is considered unstable. Libc++ reserves the right to make breaking changes to the
  library until the standard has been ratified.


Enabling experimental C++ Library features
==========================================

Libc++ provides implementations of some experimental features. Experimental features
are either Technical Specifications (TSes) or official features that were voted to
the Standard but whose implementation is not complete or stable yet in libc++. Those
are disabled by default because they are neither API nor ABI stable. However, the
``-fexperimental-library`` compiler flag can be defined to turn those features on.

.. warning::
  Experimental libraries are experimental.
    * The contents of the ``<experimental/...>`` headers and the associated static
      library will not remain compatible between versions.
    * No guarantees of API or ABI stability are provided.
    * When the standardized version of an experimental feature is implemented,
      the experimental feature is removed two releases after the non-experimental
      version has shipped. The full policy is explained :ref:`here <experimental features>`.

.. note::
  On compilers that do not support the ``-fexperimental-library`` flag, users can
  define the ``_LIBCPP_ENABLE_EXPERIMENTAL`` macro and manually link against the
  appropriate static library (usually shipped as ``libc++experimental.a``) to get
  access to experimental library features.


Using libc++ when it is not the system default
==============================================

On systems where libc++ is provided but is not the default, Clang provides a flag
called ``-stdlib=`` that can be used to decide which standard library is used.
Using ``-stdlib=libc++`` will select libc++:

.. code-block:: bash

  $ clang++ -stdlib=libc++ test.cpp

On systems where libc++ is the library in use by default such as macOS and FreeBSD,
this flag is not required.


.. _alternate libcxx:

Using a custom built libc++
===========================

Most compilers provide a way to disable the default behavior for finding the
standard library and to override it with custom paths. With Clang, this can
be done with:

.. code-block:: bash

  $ clang++ -nostdinc++ -nostdlib++           \
            -isystem <install>/include/c++/v1 \
            -L <install>/lib                  \
            -Wl,-rpath,<install>/lib          \
            -lc++                             \
            test.cpp

The option ``-Wl,-rpath,<install>/lib`` adds a runtime library search path,
which causes the system's dynamic linker to look for libc++ in ``<install>/lib``
whenever the program is loaded.

GCC does not support the ``-nostdlib++`` flag, so one must use ``-nodefaultlibs``
instead. Since that removes all the standard system libraries and not just libc++,
the system libraries must be re-added manually. For example:

.. code-block:: bash

  $ g++ -nostdinc++ -nodefaultlibs           \
        -isystem <install>/include/c++/v1    \
        -L <install>/lib                     \
        -Wl,-rpath,<install>/lib             \
        -lc++ -lc++abi -lm -lc -lgcc_s -lgcc \
        test.cpp


GDB Pretty printers for libc++
==============================

GDB does not support pretty-printing of libc++ symbols by default. However, libc++ does
provide pretty-printers itself. Those can be used as:

.. code-block:: bash

  $ gdb -ex "source <libcxx>/utils/gdb/libcxx/printers.py" \
        -ex "python register_libcxx_printer_loader()" \
        <args>


.. _assertions-mode:

Enabling the "safe libc++" mode
===============================

Libc++ contains a number of assertions whose goal is to catch undefined behavior in the
library, usually caused by precondition violations. Those assertions do not aim to be
exhaustive -- instead they aim to provide a good balance between safety and performance.
In particular, these assertions do not change the complexity of algorithms. However, they
might, in some cases, interfere with compiler optimizations.

By default, these assertions are turned off. Vendors can decide to turn them on while building
the compiled library by defining ``LIBCXX_ENABLE_ASSERTIONS=ON`` at CMake configuration time.
When ``LIBCXX_ENABLE_ASSERTIONS`` is used, the compiled library will be built with assertions
enabled, **and** user code will be built with assertions enabled by default. If
``LIBCXX_ENABLE_ASSERTIONS=OFF`` at CMake configure time, the compiled library will not contain
assertions and the default when building user code will be to have assertions disabled.
As a user, you can consult your vendor to know whether assertions are enabled by default.

Furthermore, independently of any vendor-selected default, users can always control whether
assertions are enabled in their code by defining ``_LIBCPP_ENABLE_ASSERTIONS=0|1`` before
including any libc++ header (we recommend passing ``-D_LIBCPP_ENABLE_ASSERTIONS=X`` to the
compiler). Note that if the compiled library was built by the vendor without assertions,
functions compiled inside the static or shared library won't have assertions enabled even
if the user defines ``_LIBCPP_ENABLE_ASSERTIONS=1`` (the same is true for the inverse case
where the static or shared library was compiled **with** assertions but the user tries to
disable them). However, most of the code in libc++ is in the headers, so the user-selected
value for ``_LIBCPP_ENABLE_ASSERTIONS`` (if any) will usually be respected.

When an assertion fails, an assertion handler function is called. The library provides a default
assertion handler that prints an error message and calls ``std::abort()``. Note that this assertion
handler is provided by the static or shared library, so it is only available when deploying to a
platform where the compiled library is sufficiently recent. However, users can also override that
assertion handler with their own, which can be useful to provide custom behavior, or when deploying
to older platforms where the default assertion handler isn't available.

Replacing the default assertion handler is done by defining the following function:

.. code-block:: cpp

  void __libcpp_assertion_handler(char const* file, int line, char const* expression, char const* message)

This mechanism is similar to how one can replace the default definition of ``operator new``
and ``operator delete``. For example:

.. code-block:: cpp

  // In HelloWorldHandler.cpp
  #include <version> // must include any libc++ header before defining the handler (C compatibility headers excluded)

  void std::__libcpp_assertion_handler(char const* file, int line, char const* expression, char const* message) {
    std::printf("Assertion %s failed at %s:%d, more info: %s", expression, file, line, message);
    std::abort();
  }

  // In HelloWorld.cpp
  #include <vector>

  int main() {
    std::vector<int> v;
    int& x = v[0]; // Your assertion handler will be called here if _LIBCPP_ENABLE_ASSERTIONS=1
  }

Also note that the assertion handler should usually not return. Since the assertions in libc++
catch undefined behavior, your code will proceed with undefined behavior if your assertion
handler is called and does return.

Furthermore, throwing an exception from the assertion handler is not recommended. Indeed, many
functions in the library are ``noexcept``, and any exception thrown from the assertion handler
will result in ``std::terminate`` being called.

Back-deploying with a custom assertion handler
----------------------------------------------
When deploying to an older platform that does not provide a default assertion handler, the
compiler will diagnose the usage of ``std::__libcpp_assertion_handler`` with an error. This
is done to avoid the load-time error that would otherwise happen if the code was being deployed
on the older system.

If you are providing a custom assertion handler, this error is effectively a false positive.
To let the library know that you are providing a custom assertion handler in back-deployment
scenarios, you must define the ``_LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED`` macro,
and the library will assume that you are providing your own definition. If no definition is
provided and the code is back-deployed to the older platform, it will fail to load when the
dynamic linker fails to find a definition for ``std::__libcpp_assertion_handler``, so you
should only remove the guard rails if you really mean it!

Libc++ Configuration Macros
===========================

Libc++ provides a number of configuration macros which can be used to enable
or disable extended libc++ behavior, including enabling "debug mode" or
thread safety annotations.

**_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS**:
  This macro is used to enable -Wthread-safety annotations on libc++'s
  ``std::mutex`` and ``std::lock_guard``. By default, these annotations are
  disabled and must be manually enabled by the user.

**_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS**:
  This macro is used to disable all visibility annotations inside libc++.
  Defining this macro and then building libc++ with hidden visibility gives a
  build of libc++ which does not export any symbols, which can be useful when
  building statically for inclusion into another library.

**_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS**:
  This macro disables the additional diagnostics generated by libc++ using the
  `diagnose_if` attribute. These additional diagnostics include checks for:

    * Giving `set`, `map`, `multiset`, `multimap` and their `unordered_`
      counterparts a comparator which is not const callable.
    * Giving an unordered associative container a hasher that is not const
      callable.

**_LIBCPP_NO_VCRUNTIME**:
  Microsoft's C and C++ headers are fairly entangled, and some of their C++
  headers are fairly hard to avoid. In particular, `vcruntime_new.h` gets pulled
  in from a lot of other headers and provides definitions which clash with
  libc++ headers, such as `nothrow_t` (note that `nothrow_t` is a struct, so
  there's no way for libc++ to provide a compatible definition, since you can't
  have multiple definitions).

  By default, libc++ solves this problem by deferring to Microsoft's vcruntime
  headers where needed. However, it may be undesirable to depend on vcruntime
  headers, since they may not always be available in cross-compilation setups,
  or they may clash with other headers. The `_LIBCPP_NO_VCRUNTIME` macro
  prevents libc++ from depending on vcruntime headers. Consequently, it also
  prevents libc++ headers from being interoperable with vcruntime headers (from
  the aforementioned clashes), so users of this macro are promising to not
  attempt to combine libc++ headers with the problematic vcruntime headers. This
  macro also currently prevents certain `operator new`/`operator delete`
  replacement scenarios from working, e.g. replacing `operator new` and
  expecting a non-replaced `operator new[]` to call the replaced `operator new`.

**_LIBCPP_ENABLE_NODISCARD**:
  Allow the library to add ``[[nodiscard]]`` attributes to entities not specified
  as ``[[nodiscard]]`` by the current language dialect. This includes
  backporting applications of ``[[nodiscard]]`` from newer dialects and
  additional extended applications at the discretion of the library. All
  additional applications of ``[[nodiscard]]`` are disabled by default.
  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>` for
  more information.

**_LIBCPP_DISABLE_NODISCARD_EXT**:
  This macro prevents the library from applying ``[[nodiscard]]`` to entities
  purely as an extension. See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
  for more information.

**_LIBCPP_DISABLE_DEPRECATION_WARNINGS**:
  This macro disables warnings when using deprecated components. For example,
  using `std::auto_ptr` when compiling in C++11 mode will normally trigger a
  warning saying that `std::auto_ptr` is deprecated. If the macro is defined,
  no warning will be emitted. By default, this macro is not defined.

C++17 Specific Configuration Macros
-----------------------------------
**_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
  This macro is used to re-enable all the features removed in C++17. The effect
  is equivalent to manually defining each macro listed below.

**_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
  This macro is used to re-enable `auto_ptr`.

**_LIBCPP_ENABLE_CXX17_REMOVED_BINDERS**:
  This macro is used to re-enable the `binder1st`, `binder2nd`,
  `pointer_to_unary_function`, `pointer_to_binary_function`, `mem_fun_t`,
  `mem_fun1_t`, `mem_fun_ref_t`, `mem_fun1_ref_t`, `const_mem_fun_t`,
  `const_mem_fun1_t`, `const_mem_fun_ref_t`, and `const_mem_fun1_ref_t`
  class templates, and the `bind1st`, `bind2nd`, `mem_fun`, `mem_fun_ref`,
  and `ptr_fun` functions.

**_LIBCPP_ENABLE_CXX17_REMOVED_RANDOM_SHUFFLE**:
  This macro is used to re-enable the `random_shuffle` algorithm.

**_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS**:
  This macro is used to re-enable `set_unexpected`, `get_unexpected`, and
  `unexpected`.

C++20 Specific Configuration Macros
-----------------------------------
**_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
  This macro can be used to disable diagnostics emitted from functions marked
  ``[[nodiscard]]`` in dialects after C++17.  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
  for more information.

**_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES**:
  This macro is used to re-enable all the features removed in C++20. The effect
  is equivalent to manually defining each macro listed below.

**_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS**:
  This macro is used to re-enable redundant members of `allocator<T>`,
  including `pointer`, `reference`, `rebind`, `address`, `max_size`,
  `construct`, `destroy`, and the two-argument overload of `allocate`.

**_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION**:
  This macro is used to re-enable the library-provided specializations of
  `allocator<void>` and `allocator<const void>`.
  Use it in conjunction with `_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS`
  to ensure that removed members of `allocator<void>` can be accessed.

**_LIBCPP_ENABLE_CXX20_REMOVED_BINDER_TYPEDEFS**:
  This macro is used to re-enable the `argument_type`, `result_type`,
  `first_argument_type`, and `second_argument_type` members of class
  templates such as `plus`, `logical_not`, `hash`, and `owner_less`.

**_LIBCPP_ENABLE_CXX20_REMOVED_NEGATORS**:
  This macro is used to re-enable `not1`, `not2`, `unary_negate`,
  and `binary_negate`.

**_LIBCPP_ENABLE_CXX20_REMOVED_RAW_STORAGE_ITERATOR**:
  This macro is used to re-enable `raw_storage_iterator`.

**_LIBCPP_ENABLE_CXX20_REMOVED_TYPE_TRAITS**:
  This macro is used to re-enable `is_literal_type`, `is_literal_type_v`,
  `result_of` and `result_of_t`.


Libc++ Extensions
=================

This section documents various extensions provided by libc++, how they're
provided, and any information regarding how to use them.

.. _nodiscard extension:

Extended applications of ``[[nodiscard]]``
------------------------------------------

The ``[[nodiscard]]`` attribute is intended to help users find bugs where
function return values are ignored when they shouldn't be. After C++17 the
C++ standard has started to declared such library functions as ``[[nodiscard]]``.
However, this application is limited and applies only to dialects after C++17.
Users who want help diagnosing misuses of STL functions may desire a more
liberal application of ``[[nodiscard]]``.

For this reason libc++ provides an extension that does just that! The
extension must be enabled by defining ``_LIBCPP_ENABLE_NODISCARD``. The extended
applications of ``[[nodiscard]]`` takes two forms:

1. Backporting ``[[nodiscard]]`` to entities declared as such by the
   standard in newer dialects, but not in the present one.

2. Extended applications of ``[[nodiscard]]``, at the library's discretion,
   applied to entities never declared as such by the standard.

Users may also opt-out of additional applications ``[[nodiscard]]`` using
additional macros.

Applications of the first form, which backport ``[[nodiscard]]`` from a newer
dialect, may be disabled using macros specific to the dialect in which it was
added. For example, ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17``.

Applications of the second form, which are pure extensions, may be disabled
by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.


Entities declared with ``_LIBCPP_NODISCARD_EXT``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section lists all extended applications of ``[[nodiscard]]`` to entities
which no dialect declares as such (See the second form described above).

* ``adjacent_find``
* ``all_of``
* ``any_of``
* ``binary_search``
* ``clamp``
* ``count_if``
* ``count``
* ``equal_range``
* ``equal``
* ``find_end``
* ``find_first_of``
* ``find_if_not``
* ``find_if``
* ``find``
* ``get_temporary_buffer``
* ``includes``
* ``is_heap_until``
* ``is_heap``
* ``is_partitioned``
* ``is_permutation``
* ``is_sorted_until``
* ``is_sorted``
* ``lexicographical_compare``
* ``lower_bound``
* ``max_element``
* ``max``
* ``min_element``
* ``min``
* ``minmax_element``
* ``minmax``
* ``mismatch``
* ``none_of``
* ``remove_if``
* ``remove``
* ``search_n``
* ``search``
* ``unique``
* ``upper_bound``
* ``lock_guard``'s constructors
* ``as_const``
* ``bit_cast``
* ``forward``
* ``move``
* ``move_if_noexcept``
* ``identity::operator()``
* ``to_integer``
* ``to_underlying``

Additional types supported in random distributions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The `C++ Standard <http://eel.is/c++draft/rand#req.genl-1.5>`_ mentions that instantiating several random number
distributions with types other than ``short``, ``int``, ``long``, ``long long``, and their unsigned versions is
undefined. As an extension, libc++ supports instantiating ``binomial_distribution``, ``discrete_distribution``,
``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``, and ``uniform_int_distribution``
with ``int8_t``, ``__int128_t`` and their unsigned versions.