LanguageExtensions.rst

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

.. contents::
   :local:
   :depth: 1

.. toctree::
   :hidden:

   ObjectiveCLiterals
   BlockLanguageSpec
   Block-ABI-Apple
   AutomaticReferenceCounting

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 backward compatibility, ``__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. :doc:`AddressSanitizer
<AddressSanitizer>`.

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_cpp_attribute``
-----------------------

This function-like macro takes a single argument that is the name of a
C++11-style attribute. The argument can either be a single identifier, or a
scoped identifier. If the attribute is supported, a nonzero value is returned.
If the attribute is a standards-based attribute, this macro returns a nonzero
value based on the year and month in which the attribute was voted into the
working draft. If the attribute is not supported by the current compliation
target, this macro evaluates to 0.  It can be used like this:

.. code-block:: c++

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

  ...
  #if __has_cpp_attribute(clang::fallthrough)
  #define FALLTHROUGH [[clang::fallthrough]]
  #else
  #define FALLTHROUGH
  #endif
  ...

The attribute identifier (but not scope) can also be specified with a preceding
and following ``__`` (double underscore) to avoid interference from a macro with
the same name.  For instance, ``gnu::__const__`` can be used instead of
``gnu::const``.

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

This function-like macro takes a single identifier argument that is the name of
a GNU-style attribute.  It evaluates to 1 if the attribute is supported by the
current compilation target, 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``.


``__has_declspec_attribute``
----------------------------

This function-like macro takes a single identifier argument that is the name of
an attribute implemented as a Microsoft-style ``__declspec`` attribute.  It
evaluates to 1 if the attribute is supported by the current compilation target,
or 0 if not.  It can be used like this:

.. code-block:: c++

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

  ...
  #if __has_declspec_attribute(dllexport)
  #define DLLEXPORT __declspec(dllexport)
  #else
  #define DLLEXPORT
  #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, ``__dllexport__`` can be used instead of ``dllexport``.

``__is_identifier``
-------------------

This function-like macro takes a single identifier argument that might be either
a reserved word or a regular identifier. It evaluates to 1 if the argument is just
a regular identifier and not a reserved word, in the sense that it can then be
used as the name of a user-defined function or variable. Otherwise it evaluates