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 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. :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_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 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``.


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.  Include file checking macros must be used
as expressions in ``#if`` or ``#elif`` preprocessing directives.

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

.. code-block:: c++

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

.. _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)
  #if __has_include_next("myinclude.h")
  # include_next "myinclude.h"
  #endif
  #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
==============

``__BASE_FILE__``
  Defined to a string that contains the name of the main input file passed to
  Clang.

``__COUNTER__``
  Defined to an integer value that starts at zero and is incremented each time
  the ``__COUNTER__`` macro is expanded.

``__INCLUDE_LEVEL__``
  Defined to an integral value that is the include depth of the file currently
  being translated.  For the main file, this value is zero.

``__TIMESTAMP__``
  Defined to the date and time of the last modification of the current source
  file.

``__clang__``
  Defined when compiling with Clang

``__clang_major__``
  Defined to the major marketing version number of Clang (e.g., the 2 in
  2.0.1).  Note that marketing version numbers should not be used to check for
  language features, as different vendors use different numbering schemes.
  Instead, use the :ref:`langext-feature_check`.

``__clang_minor__``
  Defined to the minor version number of Clang (e.g., the 0 in 2.0.1).  Note
  that marketing version numbers should not be used to check for language
  features, as different vendors use different numbering schemes.  Instead, use
  the :ref:`langext-feature_check`.

``__clang_patchlevel__``
  Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).

``__clang_version__``
  Defined to a string that captures the Clang marketing version, including the
  Subversion tag or revision number, e.g., "``1.5 (trunk 102332)``".

.. _langext-vectors:

Vectors and Extended Vectors
============================

Supports the GCC, OpenCL, AltiVec and NEON vector extensions.

OpenCL vector types are created using ``ext_vector_type`` attribute.  It
support for ``V.xyzw`` syntax and other tidbits as seen in OpenCL.  An example
is:

.. code-block:: c++

  typedef float float4 __attribute__((ext_vector_type(4)));
  typedef float float2 __attribute__((ext_vector_type(2)));

  float4 foo(float2 a, float2 b) {
    float4 c;
    c.xz = a;
    c.yw = b;
    return c;
  }

Query for this feature with ``__has_extension(attribute_ext_vector_type)``.

Giving ``-faltivec`` option to clang enables support for AltiVec vector syntax
and functions.  For example:

.. code-block:: c++

  vector float foo(vector int a) {
    vector int b;
    b = vec_add(a, a) + a;
    return (vector float)b;
  }

NEON vector types are created using ``neon_vector_type`` and
``neon_polyvector_type`` attributes.  For example:

.. code-block:: c++

  typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
  typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;

  int8x8_t foo(int8x8_t a) {
    int8x8_t v;
    v = a;
    return v;
  }

Vector Literals
---------------

Vector literals can be used to create vectors from a set of scalars, or
vectors.  Either parentheses or braces form can be used.  In the parentheses
form the number of literal values specified must be one, i.e. referring to a
scalar value, or must match the size of the vector type being created.  If a
single scalar literal value is specified, the scalar literal value will be
replicated to all the components of the vector type.  In the brackets form any
number of literals can be specified.  For example:

.. code-block:: c++

  typedef int v4si __attribute__((__vector_size__(16)));
  typedef float float4 __attribute__((ext_vector_type(4)));
  typedef float float2 __attribute__((ext_vector_type(2)));

  v4si vsi = (v4si){1, 2, 3, 4};
  float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
  vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
  vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
  vector int vi3 = (vector int)(1, 2); // error
  vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
  vector int vi5 = (vector int)(1, 2, 3, 4);
  float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));

Vector Operations
-----------------

The table below shows the support for each operation by vector extension.  A
dash indicates that an operation is not accepted according to a corresponding
specification.

============================== ====== ======= === ====
         Opeator               OpenCL AltiVec GCC NEON
============================== ====== ======= === ====
[]                              yes     yes   yes  --
unary operators +, --           yes     yes   yes  --
++, -- --                       yes     yes   yes  --
+,--,*,/,%                      yes     yes   yes  --
bitwise operators &,|,^,~       yes     yes   yes  --
>>,<<                           yes     yes   yes  --
!, &&, ||                       no      --    --   --
==, !=, >, <, >=, <=            yes     yes   --   --
=                               yes     yes   yes yes
:?                              yes     --    --   --
sizeof                          yes     yes   yes yes
============================== ====== ======= === ====

See also :ref:`langext-__builtin_shufflevector`.

Messages on ``deprecated`` and ``unavailable`` Attributes
=========================================================

An optional string message can be added to the ``deprecated`` and
``unavailable`` attributes.  For example:

.. code-block:: c++

  void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));

If the deprecated or unavailable declaration is used, the message will be
incorporated into the appropriate diagnostic:

.. code-block:: c++

  harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
        [-Wdeprecated-declarations]
    explode();
    ^

Query for this feature with
``__has_extension(attribute_deprecated_with_message)`` and
``__has_extension(attribute_unavailable_with_message)``.

Attributes on Enumerators
=========================

Clang allows attributes to be written on individual enumerators.  This allows
enumerators to be deprecated, made unavailable, etc.  The attribute must appear
after the enumerator name and before any initializer, like so:

.. code-block:: c++

  enum OperationMode {
    OM_Invalid,
    OM_Normal,
    OM_Terrified __attribute__((deprecated)),
    OM_AbortOnError __attribute__((deprecated)) = 4
  };

Attributes on the ``enum`` declaration do not apply to individual enumerators.

Query for this feature with ``__has_extension(enumerator_attributes)``.

'User-Specified' System Frameworks
==================================

Clang provides a mechanism by which frameworks can be built in such a way that
they will always be treated as being "system frameworks", even if they are not
present in a system framework directory.  This can be useful to system
framework developers who want to be able to test building other applications
with development builds of their framework, including the manner in which the
compiler changes warning behavior for system headers.

Framework developers can opt-in to this mechanism by creating a
"``.system_framework``" file at the top-level of their framework.  That is, the
framework should have contents like:

.. code-block:: none

  .../TestFramework.framework
  .../TestFramework.framework/.system_framework
  .../TestFramework.framework/Headers
  .../TestFramework.framework/Headers/TestFramework.h
  ...

Clang will treat the presence of this file as an indicator that the framework
should be treated as a system framework, regardless of how it was found in the
framework search path.  For consistency, we recommend that such files never be
included in installed versions of the framework.

Checks for Standard Language Features
=====================================

The ``__has_feature`` macro can be used to query if certain standard language
features are enabled.  The ``__has_extension`` macro can be used to query if
language features are available as an extension when compiling for a standard
which does not provide them.  The features which can be tested are listed here.

C++98
-----

The features listed below are part of the C++98 standard.  These features are
enabled by default when compiling C++ code.

C++ exceptions
^^^^^^^^^^^^^^

Use ``__has_feature(cxx_exceptions)`` to determine if C++ exceptions have been
enabled.  For example, compiling code with ``-fno-exceptions`` disables C++
exceptions.

C++ RTTI
^^^^^^^^

Use ``__has_feature(cxx_rtti)`` to determine if C++ RTTI has been enabled.  For
example, compiling code with ``-fno-rtti`` disables the use of RTTI.

C++11
-----

The features listed below are part of the C++11 standard.  As a result, all
these features are enabled with the ``-std=c++11`` or ``-std=gnu++11`` option
when compiling C++ code.

C++11 SFINAE includes access control
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_access_control_sfinae)`` or
``__has_extension(cxx_access_control_sfinae)`` to determine whether
access-control errors (e.g., calling a private constructor) are considered to
be template argument deduction errors (aka SFINAE errors), per `C++ DR1170
<http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170>`_.

C++11 alias templates
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_alias_templates)`` or
``__has_extension(cxx_alias_templates)`` to determine if support for C++11's
alias declarations and alias templates is enabled.

C++11 alignment specifiers
^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_alignas)`` or ``__has_extension(cxx_alignas)`` to
determine if support for alignment specifiers using ``alignas`` is enabled.

C++11 attributes
^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_attributes)`` or ``__has_extension(cxx_attributes)`` to
determine if support for attribute parsing with C++11's square bracket notation
is enabled.

C++11 generalized constant expressions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_constexpr)`` to determine if support for generalized
constant expressions (e.g., ``constexpr``) is enabled.

C++11 ``decltype()``
^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_decltype)`` or ``__has_extension(cxx_decltype)`` to
determine if support for the ``decltype()`` specifier is enabled.  C++11's
``decltype`` does not require type-completeness of a function call expression.
Use ``__has_feature(cxx_decltype_incomplete_return_types)`` or
``__has_extension(cxx_decltype_incomplete_return_types)`` to determine if
support for this feature is enabled.

C++11 default template arguments in function templates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_default_function_template_args)`` or
``__has_extension(cxx_default_function_template_args)`` to determine if support
for default template arguments in function templates is enabled.

C++11 ``default``\ ed functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_defaulted_functions)`` or
``__has_extension(cxx_defaulted_functions)`` to determine if support for
defaulted function definitions (with ``= default``) is enabled.

C++11 delegating constructors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_delegating_constructors)`` to determine if support for
delegating constructors is enabled.

C++11 ``deleted`` functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_deleted_functions)`` or
``__has_extension(cxx_deleted_functions)`` to determine if support for deleted
function definitions (with ``= delete``) is enabled.

C++11 explicit conversion functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_explicit_conversions)`` to determine if support for
``explicit`` conversion functions is enabled.

C++11 generalized initializers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_generalized_initializers)`` to determine if support for
generalized initializers (using braced lists and ``std::initializer_list``) is
enabled.

C++11 implicit move constructors/assignment operators
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_implicit_moves)`` to determine if Clang will implicitly
generate move constructors and move assignment operators where needed.

C++11 inheriting constructors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_inheriting_constructors)`` to determine if support for
inheriting constructors is enabled.

C++11 inline namespaces
^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_inline_namespaces)`` or
``__has_extension(cxx_inline_namespaces)`` to determine if support for inline
namespaces is enabled.

C++11 lambdas
^^^^^^^^^^^^^

Use ``__has_feature(cxx_lambdas)`` or ``__has_extension(cxx_lambdas)`` to
determine if support for lambdas is enabled.

C++11 local and unnamed types as template arguments
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_local_type_template_args)`` or
``__has_extension(cxx_local_type_template_args)`` to determine if support for
local and unnamed types as template arguments is enabled.

C++11 noexcept
^^^^^^^^^^^^^^

Use ``__has_feature(cxx_noexcept)`` or ``__has_extension(cxx_noexcept)`` to
determine if support for noexcept exception specifications is enabled.

C++11 in-class non-static data member initialization
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_nonstatic_member_init)`` to determine whether in-class
initialization of non-static data members is enabled.

C++11 ``nullptr``
^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_nullptr)`` or ``__has_extension(cxx_nullptr)`` to
determine if support for ``nullptr`` is enabled.

C++11 ``override control``
^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_override_control)`` or
``__has_extension(cxx_override_control)`` to determine if support for the
override control keywords is enabled.

C++11 reference-qualified functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_reference_qualified_functions)`` or
``__has_extension(cxx_reference_qualified_functions)`` to determine if support
for reference-qualified functions (e.g., member functions with ``&`` or ``&&``
applied to ``*this``) is enabled.

C++11 range-based ``for`` loop
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_range_for)`` or ``__has_extension(cxx_range_for)`` to
determine if support for the range-based for loop is enabled.

C++11 raw string literals
^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_raw_string_literals)`` to determine if support for raw
string literals (e.g., ``R"x(foo\bar)x"``) is enabled.

C++11 rvalue references
^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_rvalue_references)`` or
``__has_extension(cxx_rvalue_references)`` to determine if support for rvalue
references is enabled.

C++11 ``static_assert()``
^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_static_assert)`` or
``__has_extension(cxx_static_assert)`` to determine if support for compile-time
assertions using ``static_assert`` is enabled.

C++11 ``thread_local``
^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_thread_local)`` to determine if support for
``thread_local`` variables is enabled.

C++11 type inference
^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_auto_type)`` or ``__has_extension(cxx_auto_type)`` to
determine C++11 type inference is supported using the ``auto`` specifier.  If
this is disabled, ``auto`` will instead be a storage class specifier, as in C
or C++98.

C++11 strongly typed enumerations
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_strong_enums)`` or
``__has_extension(cxx_strong_enums)`` to determine if support for strongly
typed, scoped enumerations is enabled.

C++11 trailing return type
^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_trailing_return)`` or
``__has_extension(cxx_trailing_return)`` to determine if support for the
alternate function declaration syntax with trailing return type is enabled.

C++11 Unicode string literals
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_unicode_literals)`` to determine if support for Unicode
string literals is enabled.

C++11 unrestricted unions
^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_unrestricted_unions)`` to determine if support for
unrestricted unions is enabled.

C++11 user-defined literals
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_user_literals)`` to determine if support for
user-defined literals is enabled.

C++11 variadic templates
^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_variadic_templates)`` or
``__has_extension(cxx_variadic_templates)`` to determine if support for
variadic templates is enabled.

C++1y
-----

The features listed below are part of the committee draft for the C++1y
standard.  As a result, all these features are enabled with the ``-std=c++1y``
or ``-std=gnu++1y`` option when compiling C++ code.

C++1y binary literals
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_binary_literals)`` or
``__has_extension(cxx_binary_literals)`` to determine whether
binary literals (for instance, ``0b10010``) are recognized. Clang supports this
feature as an extension in all language modes.

C++1y contextual conversions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_contextual_conversions)`` or
``__has_extension(cxx_contextual_conversions)`` to determine if the C++1y rules
are used when performing an implicit conversion for an array bound in a
*new-expression*, the operand of a *delete-expression*, an integral constant
expression, or a condition in a ``switch`` statement.

C++1y decltype(auto)
^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_decltype_auto)`` or
``__has_extension(cxx_decltype_auto)`` to determine if support
for the ``decltype(auto)`` placeholder type is enabled.

C++1y default initializers for aggregates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_aggregate_nsdmi)`` or
``__has_extension(cxx_aggregate_nsdmi)`` to determine if support
for default initializers in aggregate members is enabled.

C++1y generalized lambda capture
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_init_capture)`` or
``__has_extension(cxx_init_capture)`` to determine if support for
lambda captures with explicit initializers is enabled
(for instance, ``[n(0)] { return ++n; }``).

C++1y generic lambdas
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_generic_lambda)`` or
``__has_extension(cxx_generic_lambda)`` to determine if support for generic
(polymorphic) lambdas is enabled
(for instance, ``[] (auto x) { return x + 1; }``).

C++1y relaxed constexpr
^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_relaxed_constexpr)`` or
``__has_extension(cxx_relaxed_constexpr)`` to determine if variable
declarations, local variable modification, and control flow constructs
are permitted in ``constexpr`` functions.

C++1y return type deduction
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_return_type_deduction)`` or
``__has_extension(cxx_return_type_deduction)`` to determine if support
for return type deduction for functions (using ``auto`` as a return type)
is enabled.

C++1y runtime-sized arrays
^^^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_runtime_array)`` or
``__has_extension(cxx_runtime_array)`` to determine if support
for arrays of runtime bound (a restricted form of variable-length arrays)
is enabled.
Clang's implementation of this feature is incomplete.

C++1y variable templates
^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(cxx_variable_templates)`` or
``__has_extension(cxx_variable_templates)`` to determine if support for
templated variable declarations is enabled.

C11
---

The features listed below are part of the C11 standard.  As a result, all these
features are enabled with the ``-std=c11`` or ``-std=gnu11`` option when
compiling C code.  Additionally, because these features are all
backward-compatible, they are available as extensions in all language modes.

C11 alignment specifiers
^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_alignas)`` or ``__has_extension(c_alignas)`` to determine
if support for alignment specifiers using ``_Alignas`` is enabled.

C11 atomic operations
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_atomic)`` or ``__has_extension(c_atomic)`` to determine
if support for atomic types using ``_Atomic`` is enabled.  Clang also provides
:ref:`a set of builtins <langext-__c11_atomic>` which can be used to implement
the ``<stdatomic.h>`` operations on ``_Atomic`` types.

C11 generic selections
^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_generic_selections)`` or
``__has_extension(c_generic_selections)`` to determine if support for generic
selections is enabled.

As an extension, the C11 generic selection expression is available in all
languages supported by Clang.  The syntax is the same as that given in the C11
standard.

In C, type compatibility is decided according to the rules given in the
appropriate standard, but in C++, which lacks the type compatibility rules used
in C, types are considered compatible only if they are equivalent.

C11 ``_Static_assert()``
^^^^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_static_assert)`` or ``__has_extension(c_static_assert)``
to determine if support for compile-time assertions using ``_Static_assert`` is
enabled.

C11 ``_Thread_local``
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_thread_local)`` or ``__has_extension(c_thread_local)``
to determine if support for ``_Thread_local`` variables is enabled.

Checks for Type Trait Primitives
================================

Type trait primitives are special builtin constant expressions that can be used
by the standard C++ library to facilitate or simplify the implementation of
user-facing type traits in the <type_traits> header.

They are not intended to be used directly by user code because they are
implementation-defined and subject to change -- as such they're tied closely to
the supported set of system headers, currently:

* LLVM's own libc++
* GNU libstdc++
* The Microsoft standard C++ library

Clang supports the `GNU C++ type traits
<http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html>`_ and a subset of the
`Microsoft Visual C++ Type traits
<http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx>`_.

Feature detection is supported only for some of the primitives at present. User
code should not use these checks because they bear no direct relation to the
actual set of type traits supported by the C++ standard library.

For type trait ``__X``, ``__has_extension(X)`` indicates the presence of the
type trait primitive in the compiler. A simplistic usage example as might be
seen in standard C++ headers follows:

.. code-block:: c++

  #if __has_extension(is_convertible_to)
  template<typename From, typename To>
  struct is_convertible_to {
    static const bool value = __is_convertible_to(From, To);
  };
  #else
  // Emulate type trait for compatibility with other compilers.
  #endif

The following type trait primitives are supported by Clang:

* ``__has_nothrow_assign`` (GNU, Microsoft)
* ``__has_nothrow_copy`` (GNU, Microsoft)
* ``__has_nothrow_constructor`` (GNU, Microsoft)
* ``__has_trivial_assign`` (GNU, Microsoft)
* ``__has_trivial_copy`` (GNU, Microsoft)
* ``__has_trivial_constructor`` (GNU, Microsoft)
* ``__has_trivial_destructor`` (GNU, Microsoft)
* ``__has_virtual_destructor`` (GNU, Microsoft)
* ``__is_abstract`` (GNU, Microsoft)
* ``__is_base_of`` (GNU, Microsoft)
* ``__is_class`` (GNU, Microsoft)
* ``__is_convertible_to`` (Microsoft)
* ``__is_empty`` (GNU, Microsoft)
* ``__is_enum`` (GNU, Microsoft)
* ``__is_interface_class`` (Microsoft)
* ``__is_pod`` (GNU, Microsoft)
* ``__is_polymorphic`` (GNU, Microsoft)
* ``__is_union`` (GNU, Microsoft)
* ``__is_literal(type)``: Determines whether the given type is a literal type
* ``__is_final``: Determines whether the given type is declared with a
  ``final`` class-virt-specifier.
* ``__underlying_type(type)``: Retrieves the underlying type for a given
  ``enum`` type.  This trait is required to implement the C++11 standard
  library.
* ``__is_trivially_assignable(totype, fromtype)``: Determines whether a value
  of type ``totype`` can be assigned to from a value of type ``fromtype`` such
  that no non-trivial functions are called as part of that assignment.  This
  trait is required to implement the C++11 standard library.
* ``__is_trivially_constructible(type, argtypes...)``: Determines whether a
  value of type ``type`` can be direct-initialized with arguments of types
  ``argtypes...`` such that no non-trivial functions are called as part of
  that initialization.  This trait is required to implement the C++11 standard
  library.
* ``__is_destructible`` (MSVC 2013): partially implemented
* ``__is_nothrow_destructible`` (MSVC 2013): partially implemented
* ``__is_nothrow_assignable`` (MSVC 2013, clang)
* ``__is_constructible`` (MSVC 2013, clang)
* ``__is_nothrow_constructible`` (MSVC 2013, clang)

Blocks
======

The syntax and high level language feature description is in
:doc:`BlockLanguageSpec<BlockLanguageSpec>`. Implementation and ABI details for
the clang implementation are in :doc:`Block-ABI-Apple<Block-ABI-Apple>`.

Query for this feature with ``__has_extension(blocks)``.

Objective-C Features
====================

Related result types
--------------------

According to Cocoa conventions, Objective-C methods with certain names
("``init``", "``alloc``", etc.) always return objects that are an instance of
the receiving class's type.  Such methods are said to have a "related result
type", meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class.  For example, given the
following classes:

.. code-block:: objc

  @interface NSObject
  + (id)alloc;
  - (id)init;
  @end

  @interface NSArray : NSObject
  @end

and this common initialization pattern

.. code-block:: objc

  NSArray *array = [[NSArray alloc] init];

the type of the expression ``[NSArray alloc]`` is ``NSArray*`` because
``alloc`` implicitly has a related result type.  Similarly, the type of the
expression ``[[NSArray alloc] init]`` is ``NSArray*``, since ``init`` has a
related result type and its receiver is known to have the type ``NSArray *``.
If neither ``alloc`` nor ``init`` had a related result type, the expressions
would have had type ``id``, as declared in the method signature.

A method with a related result type can be declared by using the type
``instancetype`` as its result type.  ``instancetype`` is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.

.. code-block:: objc

  @interface A
  + (instancetype)constructAnA;
  @end

The related result type can also be inferred for some methods.  To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., "``init``" in "``initWithObjects``") is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:

* the first word is "``alloc``" or "``new``", and the method is a class method,
  or

* the first word is "``autorelease``", "``init``", "``retain``", or "``self``",
  and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the
subclass method must also return a type that is compatible with the subclass
type.  For example:

.. code-block:: objc

  @interface NSString : NSObject
  - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
  @end

Related result types only affect the type of a message send or property access
via the given method.  In all other respects, a method with a related result
type is treated the same way as method that returns ``id``.

Use ``__has_feature(objc_instancetype)`` to determine whether the
``instancetype`` contextual keyword is available.

Automatic reference counting
----------------------------

Clang provides support for :doc:`automated reference counting
<AutomaticReferenceCounting>` in Objective-C, which eliminates the need
for manual ``retain``/``release``/``autorelease`` message sends.  There are two
feature macros associated with automatic reference counting:
``__has_feature(objc_arc)`` indicates the availability of automated reference
counting in general, while ``__has_feature(objc_arc_weak)`` indicates that