LanguageExtensions.rst

=========================
Clang Language Extensions
=========================

.. contents::
   :local:

Introduction
============

This document describes the language extensions provided by Clang.  In addition
to the language extensions listed here, Clang aims to support a broad range of
GCC extensions.  Please see the `GCC manual
<http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html>`_ for more information on
these extensions.

.. _langext-feature_check:

Feature Checking Macros
=======================

Language extensions can be very useful, but only if you know you can depend on
them.  In order to allow fine-grain features checks, we support three builtin
function-like macros.  This allows you to directly test for a feature in your
code without having to resort to something like autoconf or fragile "compiler
version checks".

``__has_builtin``
-----------------

This function-like macro takes a single identifier argument that is the name of
a builtin function.  It evaluates to 1 if the builtin is supported or 0 if not.
It can be used like this:

.. code-block:: c++

  #ifndef __has_builtin         // Optional of course.
    #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
  #endif

  ...
  #if __has_builtin(__builtin_trap)
    __builtin_trap();
  #else
    abort();
  #endif
  ...

.. _langext-__has_feature-__has_extension:

``__has_feature`` and ``__has_extension``
-----------------------------------------

These function-like macros take a single identifier argument that is the name
of a feature.  ``__has_feature`` evaluates to 1 if the feature is both
supported by Clang and standardized in the current language standard or 0 if
not (but see :ref:`below <langext-has-feature-back-compat>`), while
``__has_extension`` evaluates to 1 if the feature is supported by Clang in the
current language (either as a language extension or a standard language
feature) or 0 if not.  They can be used like this:

.. code-block:: c++

  #ifndef __has_feature         // Optional of course.
    #define __has_feature(x) 0  // Compatibility with non-clang compilers.
  #endif
  #ifndef __has_extension
    #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
  #endif

  ...
  #if __has_feature(cxx_rvalue_references)
  // This code will only be compiled with the -std=c++11 and -std=gnu++11
  // options, because rvalue references are only standardized in C++11.
  #endif

  #if __has_extension(cxx_rvalue_references)
  // This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
  // and -std=gnu++98 options, because rvalue references are supported as a
  // language extension in C++98.
  #endif

.. _langext-has-feature-back-compat:

For backwards compatibility reasons, ``__has_feature`` can also be used to test
for support for non-standardized features, i.e. features not prefixed ``c_``,
``cxx_`` or ``objc_``.

Another use of ``__has_feature`` is to check for compiler features not related
to the language standard, such as e.g. `AddressSanitizer
<AddressSanitizer.html>`_.

If the ``-pedantic-errors`` option is given, ``__has_extension`` is equivalent
to ``__has_feature``.

The feature tag is described along with the language feature below.

The feature name or extension name can also be specified with a preceding and
following ``__`` (double underscore) to avoid interference from a macro with
the same name.  For instance, ``__cxx_rvalue_references__`` can be used instead
of ``cxx_rvalue_references``.

``__has_attribute``
-------------------

This function-like macro takes a single identifier argument that is the name of
an attribute.  It evaluates to 1 if the attribute is supported or 0 if not.  It
can be used like this:

.. code-block:: c++

  #ifndef __has_attribute         // Optional of course.
    #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
  #endif

  ...
  #if __has_attribute(always_inline)
  #define ALWAYS_INLINE __attribute__((always_inline))
  #else
  #define ALWAYS_INLINE
  #endif
  ...

The attribute name can also be specified with a preceding and following ``__``
(double underscore) to avoid interference from a macro with the same name.  For
instance, ``__always_inline__`` can be used instead of ``always_inline``.

Include File Checking Macros
============================

Not all developments systems have the same include files.  The
:ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
you to check for the existence of an include file before doing a possibly
failing ``#include`` directive.

.. _langext-__has_include:

``__has_include``
-----------------

This function-like macro takes a single file name string argument that is the
name of an include file.  It evaluates to 1 if the file can be found using the
include paths, or 0 otherwise:

.. code-block:: c++

  // Note the two possible file name string formats.
  #if __has_include("myinclude.h") && __has_include(<stdint.h>)
  # include "myinclude.h"
  #endif

  // To avoid problem with non-clang compilers not having this macro.
  #if defined(__has_include) && __has_include("myinclude.h")
  # include "myinclude.h"
  #endif

To test for this feature, use ``#if defined(__has_include)``.

.. _langext-__has_include_next:

``__has_include_next``
----------------------

This function-like macro takes a single file name string argument that is the
name of an include file.  It is like ``__has_include`` except that it looks for
the second instance of the given file found in the include paths.  It evaluates
to 1 if the second instance of the file can be found using the include paths,
or 0 otherwise:

.. code-block:: c++

  // Note the two possible file name string formats.
  #if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
  # include_next "myinclude.h"
  #endif

  // To avoid problem with non-clang compilers not having this macro.
  #if defined(__has_include_next) && __has_include_next("myinclude.h")
  # include_next "myinclude.h"
  #endif

Note that ``__has_include_next``, like the GNU extension ``#include_next``
directive, is intended for use in headers only, and will issue a warning if
used in the top-level compilation file.  A warning will also be issued if an
absolute path is used in the file argument.

``__has_warning``
-----------------

This function-like macro takes a string literal that represents a command line
option for a warning and returns true if that is a valid warning option.

.. code-block:: c++

  #if __has_warning("-Wformat")
  ...
  #endif

Builtin Macros
==============