llvm-project/libcxx/docs/UsingLibcxx.rst

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

.. contents::
  :local:

Getting Started
===============

If you already have libc++ installed you can use it with clang.

.. code-block:: bash

    $ clang++ -stdlib=libc++ test.cpp
    $ clang++ -std=c++11 -stdlib=libc++ test.cpp

On OS X and FreeBSD libc++ is the default standard library
and the ``-stdlib=libc++`` is not required.

.. _alternate libcxx:

If you want to select an alternate installation of libc++ you
can use the following options.

.. code-block:: bash

  $ clang++ -std=c++11 -stdlib=libc++ -nostdinc++ \
            -I<libcxx-install-prefix>/include/c++/v1 \
            -L<libcxx-install-prefix>/lib \
            -Wl,-rpath,<libcxx-install-prefix>/lib \
            test.cpp

The option ``-Wl,-rpath,<libcxx-install-prefix>/lib`` adds a runtime library
search path. Meaning that the systems dynamic linker will look for libc++ in
``<libcxx-install-prefix>/lib`` whenever the program is run. Alternatively the
environment variable ``LD_LIBRARY_PATH`` (``DYLD_LIBRARY_PATH`` on OS X) can
be used to change the dynamic linkers search paths after a program is compiled.

An example of using ``LD_LIBRARY_PATH``:

.. code-block:: bash

  $ clang++ -stdlib=libc++ -nostdinc++ \
            -I<libcxx-install-prefix>/include/c++/v1
            -L<libcxx-install-prefix>/lib \
            test.cpp -o
  $ ./a.out # Searches for libc++ in the systems library paths.
  $ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
  $ ./a.out # Searches for libc++ along LD_LIBRARY_PATH

Using ``<filesystem>``
======================

Prior to LLVM 9.0, libc++ provides the implementation of the filesystem library
in a separate static library. Users of ``<filesystem>`` and ``<experimental/filesystem>``
are required to link ``-lc++fs``. Prior to libc++ 7.0, users of
``<experimental/filesystem>`` were required to link libc++experimental.

Starting with LLVM 9.0, support for ``<filesystem>`` is provided in the main
library and nothing special is required to use ``<filesystem>``.

Using libc++experimental and ``<experimental/...>``
=====================================================

Libc++ provides implementations of experimental technical specifications
in a separate library, ``libc++experimental.a``. Users of ``<experimental/...>``
headers may be required to link ``-lc++experimental``.

.. code-block:: bash

  $ clang++ -std=c++14 -stdlib=libc++ test.cpp -lc++experimental

Libc++experimental.a may not always be available, even when libc++ is already
installed. For information on building libc++experimental from source see
:ref:`Building Libc++ <build instructions>` and
:ref:`libc++experimental CMake Options <libc++experimental options>`.

Also see the `Experimental Library Implementation Status <http://libcxx.llvm.org/ts1z_status.html>`__
page.

.. warning::
  Experimental libraries are Experimental.
    * The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
      library will not remain compatible between versions.
    * No guarantees of API or ABI stability are provided.

Using libc++ on Linux
=====================

On Linux libc++ can typically be used with only '-stdlib=libc++'. However
some libc++ installations require the user manually link libc++abi themselves.
If you are running into linker errors when using libc++ try adding '-lc++abi'
to the link line.  For example:

.. code-block:: bash

  $ clang++ -stdlib=libc++ test.cpp -lc++ -lc++abi -lm -lc -lgcc_s -lgcc

Alternately, you could just add libc++abi to your libraries list, which in
most situations will give the same result:

.. code-block:: bash

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


Using libc++ with GCC
---------------------

GCC does not provide a way to switch from libstdc++ to libc++. You must manually
configure the compile and link commands.

In particular you must tell GCC to remove the libstdc++ include directories
using ``-nostdinc++`` and to not link libstdc++.so using ``-nodefaultlibs``.

Note that ``-nodefaultlibs`` removes all of the standard system libraries and
not just libstdc++ so they must be manually linked. For example:

.. code-block:: bash

  $ g++ -nostdinc++ -I<libcxx-install-prefix>/include/c++/v1 \
         test.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc


GDB Pretty printers for libc++
------------------------------

GDB does not support pretty-printing of libc++ symbols by default. Unfortunately
libc++ does not provide pretty-printers itself. However there are 3rd
party implementations available and although they are not officially
supported by libc++ they may be useful to users.

Known 3rd Party Implementations Include:

* `Koutheir's libc++ pretty-printers <https://github.com/koutheir/libcxx-pretty-printers>`_.


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_DEBUG**:
  See :ref:`using-debug-mode` for more information.

**_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_EXTERN_TEMPLATE**:
  This macro is used to disable extern template declarations in the libc++
  headers. The intended use case is for clients who wish to use the libc++
  headers without taking a dependency on the libc++ library itself.

**_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
  This macro is used to re-enable an extension in `std::tuple` which allowed
  it to be implicitly constructed from fewer initializers than contained
  elements. Elements without an initializer are default constructed. For example:

  .. code-block:: cpp

    std::tuple<std::string, int, std::error_code> foo() {
      return {"hello world", 42}; // default constructs error_code
    }


  Since libc++ 4.0 this extension has been disabled by default. This macro
  may be defined to re-enable it in order to support existing code that depends
  on the extension. New use of this extension should be discouraged.
  See `PR 27374 <http://llvm.org/PR27374>`_ for more information.

  Note: The "reduced-arity-initialization" extension is still offered but only
  for explicit conversions. Example:

  .. code-block:: cpp

    auto foo() {
      using Tup = std::tuple<std::string, int, std::error_code>;
      return Tup{"hello world", 42}; // explicit constructor called. OK.
    }

**_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_UNEXPECTED_FUNCTIONS**:
  This macro is used to re-enable the `set_unexpected`, `get_unexpected`, and
  `unexpected` functions, which were removed in C++17.

**_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
  This macro is used to re-enable `std::auto_ptr` in C++17.

C++2a 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.


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 libraries 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 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).

* ``get_temporary_buffer``