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Using libc++
.. contents::
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 macOS 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 \
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 macOS) 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++ \
-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 <>`__
.. 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.
* When we implement the standardized version of an experimental feature,
the experimental feature is removed two releases after the non-experimental
version has shipped. The full policy is explained :ref:`here <experimental features>`.
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 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 <>`_.
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.
See :ref:`using-debug-mode` for more information.
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.
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.
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.
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 <>`_ 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.
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
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`.
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.
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.
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
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.
This macro is used to re-enable the `set_unexpected`, `get_unexpected`, and
`unexpected` functions, which were removed in C++17.
This macro is used to re-enable `std::auto_ptr` in C++17.
C++2a Specific Configuration Macros:
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
Applications of the second form, which are pure extensions, may be disabled
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