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    22. 

  22. 

  23. <h1>Clang Language Extensions</h1>
    24. 

  24. 

  25. <ul>
    26. 

  26. <li><a href="#intro">Introduction</a></li>
    27. 

  27. <li><a href="#feature_check">Feature Checking Macros</a></li>
    28. 

  28. <li><a href="#has_include">Include File Checking Macros</a></li>
    29. 

  29. <li><a href="#builtinmacros">Builtin Macros</a></li>
    30. 

  30. <li><a href="#vectors">Vectors and Extended Vectors</a></li>
    31. 

  31. <li><a href="#deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> attributes</a></li>
    32. 

  32. <li><a href="#attributes-on-enumerators">Attributes on enumerators</a></li>
    33. 

  33. <li><a href="#user_specified_system_framework">'User-Specified' System Frameworks</a></li>
    34. 

  34. <li><a href="#availability">Availability attribute</a></li>
    35. 

  35. <li><a href="#checking_language_features">Checks for Standard Language Features</a>
    36. 

  36.   <ul>
    37. 

  37.   <li><a href="#cxx98">C++98</a>
    38. 

  38.     <ul>
    39. 

  39.     <li><a href="#cxx_exceptions">C++ exceptions</a></li>
    40. 

  40.     <li><a href="#cxx_rtti">C++ RTTI</a></li>
    41. 

  41.   </ul></li>
    42. 

  42.   <li><a href="#cxx11">C++11</a>
    43. 

  43.     <ul>
    44. 

  44.     <li><a href="#cxx_access_control_sfinae">C++11 SFINAE includes access control</a></li>
    45. 

  45.     <li><a href="#cxx_alias_templates">C++11 alias templates</a></li>
    46. 

  46.     <li><a href="#cxx_alignas">C++11 alignment specifiers</a></li>
    47. 

  47.     <li><a href="#cxx_attributes">C++11 attributes</a></li>
    48. 

  48.     <li><a href="#cxx_constexpr">C++11 generalized constant expressions</a></li>
    49. 

  49.     <li><a href="#cxx_decltype">C++11 <tt>decltype()</tt></a></li>
    50. 

  50.     <li><a href="#cxx_default_function_template_args">C++11 default template arguments in function templates</a></li>
    51. 

  51.     <li><a href="#cxx_defaulted_functions">C++11 defaulted functions</a></li>
    52. 

  52.     <li><a href="#cxx_delegating_constructor">C++11 delegating constructors</a></li>
    53. 

  53.     <li><a href="#cxx_deleted_functions">C++11 deleted functions</a></li>
    54. 

  54.     <li><a href="#cxx_explicit_conversions">C++11 explicit conversion functions</a></li>
    55. 

  55.     <li><a href="#cxx_generalized_initializers">C++11 generalized initializers</a></li>
    56. 

  56.     <li><a href="#cxx_implicit_moves">C++11 implicit move constructors/assignment operators</a></li>
    57. 

  57.     <li><a href="#cxx_inheriting_constructors">C++11 inheriting constructors</a></li>
    58. 

  58.     <li><a href="#cxx_inline_namespaces">C++11 inline namespaces</a></li>
    59. 

  59.     <li><a href="#cxx_lambdas">C++11 lambdas</a></li>
    60. 

  60.     <li><a href="#cxx_local_type_template_args">C++11 local and unnamed types as template arguments</a></li>
    61. 

  61.     <li><a href="#cxx_noexcept">C++11 noexcept specification</a></li>
    62. 

  62.     <li><a href="#cxx_nonstatic_member_init">C++11 in-class non-static data member initialization</a></li>
    63. 

  63.     <li><a href="#cxx_nullptr">C++11 nullptr</a></li>
    64. 

  64.     <li><a href="#cxx_override_control">C++11 override control</a></li>
    65. 

  65.     <li><a href="#cxx_range_for">C++11 range-based for loop</a></li>
    66. 

  66.     <li><a href="#cxx_raw_string_literals">C++11 raw string literals</a></li>
    67. 

  67.     <li><a href="#cxx_rvalue_references">C++11 rvalue references</a></li>
    68. 

  68.     <li><a href="#cxx_reference_qualified_functions">C++11 reference-qualified functions</a></li>
    69. 

  69.     <li><a href="#cxx_static_assert">C++11 <tt>static_assert()</tt></a></li>
    70. 

  70.     <li><a href="#cxx_auto_type">C++11 type inference</a></li>
    71. 

  71.     <li><a href="#cxx_strong_enums">C++11 strongly-typed enumerations</a></li>
    72. 

  72.     <li><a href="#cxx_trailing_return">C++11 trailing return type</a></li>
    73. 

  73.     <li><a href="#cxx_unicode_literals">C++11 Unicode string literals</a></li>
    74. 

  74.     <li><a href="#cxx_unrestricted_unions">C++11 unrestricted unions</a></li>
    75. 

  75.     <li><a href="#cxx_user_literals">C++11 user-defined literals</a></li>
    76. 

  76.     <li><a href="#cxx_variadic_templates">C++11 variadic templates</a></li>
    77. 

  77.   </ul></li>
    78. 

  78.   <li><a href="#c11">C11</a>
    79. 

  79.     <ul>
    80. 

  80.     <li><a href="#c_alignas">C11 alignment specifiers</a></li>
    81. 

  81.     <li><a href="#c_atomic">C11 atomic operations</a></li>
    82. 

  82.     <li><a href="#c_generic_selections">C11 generic selections</a></li>
    83. 

  83.     <li><a href="#c_static_assert">C11 <tt>_Static_assert()</tt></a></li>
    84. 

  84.   </ul></li>
    85. 

  85. </ul></li>
    86. 

  86. <li><a href="#checking_type_traits">Checks for Type Traits</a></li>
    87. 

  87. <li><a href="#blocks">Blocks</a></li>
    88. 

  88. <li><a href="#objc_features">Objective-C Features</a>
    89. 

  89.   <ul>
    90. 

  90.     <li><a href="#objc_instancetype">Related result types</a></li>
    91. 

  91.     <li><a href="#objc_arc">Automatic reference counting</a></li>
    92. 

  92.     <li><a href="#objc_fixed_enum">Enumerations with a fixed underlying type</a></li>
    93. 

  93.     <li><a href="#objc_lambdas">Interoperability with C++11 lambdas</a></li>
    94. 

  94.     <li><a href="#objc_object_literals_subscripting">Object Literals and Subscripting</a></li>
    95. 

  95.   </ul>
    96. 

  96. </li>
    97. 

  97. <li><a href="#overloading-in-c">Function Overloading in C</a></li>
    98. 

  98. <li><a href="#complex-list-init">Initializer lists for complex numbers in C</a></li>
    99. 

  99. <li><a href="#builtins">Builtin Functions</a>
    100. 

  100.   <ul>
    101. 

  101.   <li><a href="#__builtin_readcyclecounter">__builtin_readcyclecounter</a></li>
    102. 

  102.   <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
    103. 

  103.   <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
    104. 

  104.   <li><a href="#__sync_swap">__sync_swap</a></li>
    105. 

  105.  </ul>
    106. 

  106. </li>
    107. 

  107. <li><a href="#non-standard-attributes">Non-standard C++11 Attributes</a>
    108. 

  108. <ul>
    109. 

  109.   <li><a href="#clang__fallthrough">The <tt>clang::fallthrough</tt> attribute</a></li>
    110. 

  110. </ul>
    111. 

  111. </li>
    112. 

  112. <li><a href="#targetspecific">Target-Specific Extensions</a>
    113. 

  113.   <ul>
    114. 

  114.   <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
    115. 

  115.   </ul>
    116. 

  116. </li>
    117. 

  117. <li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a></li>
    118. 

  118. <li><a href="#dynamicanalyzerspecific">Dynamic Analysis-Specific Extensions</a>
    119. 

  119.   <ul>
    120. 

  120.   <li><a href="#address_sanitizer">AddressSanitizer</a></li>
    121. 

  121.   </ul>
    122. 

  122. </li>
    123. 

  123. <li><a href="#threadsafety">Thread Safety Annotation Checking</a>
    124. 

  124.     <ul>
    125. 

  125.     <li><a href="#ts_noanal"><tt>no_thread_safety_analysis</tt></a></li>   
    126. 

  126.     <li><a href="#ts_lockable"><tt>lockable</tt></a></li>  
    127. 

  127.     <li><a href="#ts_scopedlockable"><tt>scoped_lockable</tt></a></li>  
    128. 

  128.     <li><a href="#ts_guardedvar"><tt>guarded_var</tt></a></li>
    129. 

  129.     <li><a href="#ts_ptguardedvar"><tt>pt_guarded_var</tt></a></li>
    130. 

  130.     <li><a href="#ts_guardedby"><tt>guarded_by(l)</tt></a></li>
    131. 

  131.     <li><a href="#ts_ptguardedby"><tt>pt_guarded_by(l)</tt></a></li>  
    132. 

  132.     <li><a href="#ts_acquiredbefore"><tt>acquired_before(...)</tt></a></li>  
    133. 

  133.     <li><a href="#ts_acquiredafter"><tt>acquired_after(...)</tt></a></li>    
    134. 

  134.     <li><a href="#ts_elf"><tt>exclusive_lock_function(...)</tt></a></li>   
    135. 

  135.     <li><a href="#ts_slf"><tt>shared_lock_function(...)</tt></a></li>   
    136. 

  136.     <li><a href="#ts_etf"><tt>exclusive_trylock_function(...)</tt></a></li>   
    137. 

  137.     <li><a href="#ts_stf"><tt>shared_trylock_function(...)</tt></a></li>   
    138. 

  138.     <li><a href="#ts_uf"><tt>unlock_function(...)</tt></a></li>   
    139. 

  139.     <li><a href="#ts_lr"><tt>lock_returned(l)</tt></a></li>   
    140. 

  140.     <li><a href="#ts_le"><tt>locks_excluded(...)</tt></a></li>   
    141. 

  141.     <li><a href="#ts_elr"><tt>exclusive_locks_required(...)</tt></a></li>   
    142. 

  142.     <li><a href="#ts_slr"><tt>shared_locks_required(...)</tt></a></li>   
    143. 

  143.     </ul>
    144. 

  144. </li>
    145. 

  145. <li><a href="#type_safety">Type Safety Checking</a>
    146. 

  146.   <ul>
    147. 

  147.   <li><a href="#argument_with_type_tag"><tt>argument_with_type_tag(...)</tt></a></li>
    148. 

  148.   <li><a href="#pointer_with_type_tag"><tt>pointer_with_type_tag(...)</tt></a></li>
    149. 

  149.   <li><a href="#type_tag_for_datatype"><tt>type_tag_for_datatype(...)</tt></a></li>
    150. 

  150.   </ul>
    151. 

  151. </li>
    152. 

  152. </ul>
    153. 

  153. 

  154. <!-- ======================================================================= -->
    155. 

  155. <h2 id="intro">Introduction</h2>
    156. 

  156. <!-- ======================================================================= -->
    157. 

  157. 

  158. <p>This document describes the language extensions provided by Clang.  In
    159. 

  159. addition to the language extensions listed here, Clang aims to support a broad
    160. 

  160. range of GCC extensions.  Please see the <a 
    161. 

  161. href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
    162. 

  162. more information on these extensions.</p>
    163. 

  163. 

  164. <!-- ======================================================================= -->
    165. 

  165. <h2 id="feature_check">Feature Checking Macros</h2>
    166. 

  166. <!-- ======================================================================= -->
    167. 

  167. 

  168. <p>Language extensions can be very useful, but only if you know you can depend
    169. 

  169. on them.  In order to allow fine-grain features checks, we support three builtin
    170. 

  170. function-like macros.  This allows you to directly test for a feature in your
    171. 

  171. code without having to resort to something like autoconf or fragile "compiler
    172. 

  172. version checks".</p>
    173. 

    174. 

  174. <!-- ======================================================================= -->
    175. 

  175. <h3><a name="__has_builtin">__has_builtin</a></h3>
    176. 

  176. <!-- ======================================================================= -->
    177. 

  177. 

  178. <p>This function-like macro takes a single identifier argument that is the name
    179. 

  179. of a builtin function.  It evaluates to 1 if the builtin is supported or 0 if
    180. 

  180. not.  It can be used like this:</p>
    181. 

  181. 

  182. <blockquote>
    183. 

  183. <pre>
    184. 

  184. #ifndef __has_builtin         // Optional of course.
    185. 

  185.   #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
    186. 

  186. #endif
    187. 

  187. 

  188. ...
    189. 

  189. #if __has_builtin(__builtin_trap)
    190. 

  190.   __builtin_trap();
    191. 

  191. #else
    192. 

  192.   abort();
    193. 

  193. #endif
    194. 

  194. ...
    195. 

  195. </pre>
    196. 

  196. </blockquote>
    197. 

  197. 

  198. 

  199. <!-- ======================================================================= -->
    200. 

  200. <h3><a name="__has_feature_extension"> __has_feature and __has_extension</a></h3>
    201. 

  201. <!-- ======================================================================= -->
    202. 

  202. 

  203. <p>These function-like macros take a single identifier argument that is the
    204. 

  204. name of a feature.  <code>__has_feature</code> evaluates to 1 if the feature
    205. 

  205. is both supported by Clang and standardized in the current language standard
    206. 

  206. or 0 if not (but see <a href="#has_feature_back_compat">below</a>), while
    207. 

  207. <code>__has_extension</code> evaluates to 1 if the feature is supported by
    208. 

  208. Clang in the current language (either as a language extension or a standard
    209. 

  209. language feature) or 0 if not.  They can be used like this:</p>
    210. 

  210. 

  211. <blockquote>
    212. 

  212. <pre>
    213. 

  213. #ifndef __has_feature         // Optional of course.
    214. 

  214.   #define __has_feature(x) 0  // Compatibility with non-clang compilers.
    215. 

  215. #endif
    216. 

  216. #ifndef __has_extension
    217. 

  217.   #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
    218. 

  218. #endif
    219. 

  219. 

  220. ...
    221. 

  221. #if __has_feature(cxx_rvalue_references)
    222. 

  222. // This code will only be compiled with the -std=c++11 and -std=gnu++11
    223. 

  223. // options, because rvalue references are only standardized in C++11.
    224. 

  224. #endif
    225. 

  225. 

  226. #if __has_extension(cxx_rvalue_references)
    227. 

  227. // This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
    228. 

  228. // and -std=gnu++98 options, because rvalue references are supported as a
    229. 

  229. // language extension in C++98.
    230. 

  230. #endif
    231. 

  231. </pre>
    232. 

  232. </blockquote>
    233. 

  233. 

  234. <p id="has_feature_back_compat">For backwards compatibility reasons,
    235. 

  235. <code>__has_feature</code> can also be used to test for support for
    236. 

  236. non-standardized features, i.e. features not prefixed <code>c_</code>,
    237. 

  237. <code>cxx_</code> or <code>objc_</code>.</p>
    238. 

  238. 

  239. <p id="has_feature_for_non_language_features">
    240. 

  240. Another use of <code>__has_feature</code> is to check for compiler features
    241. 

  241. not related to the language standard, such as e.g.
    242. 

  242. <a href="AddressSanitizer.html">AddressSanitizer</a>.
    243. 

  243. 

  244. <p>If the <code>-pedantic-errors</code> option is given,
    245. 

  245. <code>__has_extension</code> is equivalent to <code>__has_feature</code>.</p>
    246. 

  246. 

  247. <p>The feature tag is described along with the language feature below.</p>
    248. 

  248. 

  249. <p>The feature name or extension name can also be specified with a preceding and
    250. 

  250. following <code>__</code> (double underscore) to avoid interference from a macro
    251. 

  251. with the same name. For instance, <code>__cxx_rvalue_references__</code> can be
    252. 

  252. used instead of <code>cxx_rvalue_references</code>.</p>
    253. 

  253. 

  254. <!-- ======================================================================= -->
    255. 

  255. <h3><a name="__has_attribute">__has_attribute</a></h3>
    256. 

  256. <!-- ======================================================================= -->
    257. 

  257. 

  258. <p>This function-like macro takes a single identifier argument that is the name
    259. 

  259. of an attribute.  It evaluates to 1 if the attribute is supported or 0 if not.  It
    260. 

  260. can be used like this:</p>
    261. 

  261. 

  262. <blockquote>
    263. 

  263. <pre>
    264. 

  264. #ifndef __has_attribute         // Optional of course.
    265. 

  265.   #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
    266. 

  266. #endif
    267. 

  267. 

  268. ...
    269. 

  269. #if __has_attribute(always_inline)
    270. 

  270. #define ALWAYS_INLINE __attribute__((always_inline))
    271. 

  271. #else
    272. 

  272. #define ALWAYS_INLINE
    273. 

  273. #endif
    274. 

  274. ...
    275. 

  275. </pre>
    276. 

  276. </blockquote>
    277. 

  277. 

  278. <p>The attribute name can also be specified with a preceding and
    279. 

  279. following <code>__</code> (double underscore) to avoid interference from a macro
    280. 

  280. with the same name. For instance, <code>__always_inline__</code> can be used
    281. 

  281. instead of <code>always_inline</code>.</p>
    282. 

  282. 

  283. <!-- ======================================================================= -->
    284. 

  284. <h2 id="has_include">Include File Checking Macros</h2>
    285. 

  285. <!-- ======================================================================= -->
    286. 

  286. 

  287. <p>Not all developments systems have the same include files.
    288. 

  288. The <a href="#__has_include">__has_include</a> and
    289. 

  289. <a href="#__has_include_next">__has_include_next</a> macros allow you to
    290. 

  290. check for the existence of an include file before doing
    291. 

  291. a possibly failing #include directive.</p>
    292. 

  292. 

  293. <!-- ======================================================================= -->
    294. 

  294. <h3><a name="__has_include">__has_include</a></h3>
    295. 

  295. <!-- ======================================================================= -->
    296. 

  296. 

  297. <p>This function-like macro takes a single file name string argument that
    298. 

  298. is the name of an include file.  It evaluates to 1 if the file can
    299. 

  299. be found using the include paths, or 0 otherwise:</p>
    300. 

  300. 

  301. <blockquote>
    302. 

  302. <pre>
    303. 

  303. // Note the two possible file name string formats.
    304. 

  304. #if __has_include("myinclude.h") &amp;&amp; __has_include(&lt;stdint.h&gt;)
    305. 

  305. # include "myinclude.h"
    306. 

  306. #endif
    307. 

  307. 

  308. // To avoid problem with non-clang compilers not having this macro.
    309. 

  309. #if defined(__has_include) &amp;&amp; __has_include("myinclude.h")
    310. 

  310. # include "myinclude.h"
    311. 

  311. #endif
    312. 

  312. </pre>
    313. 

  313. </blockquote>
    314. 

  314. 

  315. <p>To test for this feature, use #if defined(__has_include).</p>
    316. 

  316. 

  317. <!-- ======================================================================= -->
    318. 

  318. <h3><a name="__has_include_next">__has_include_next</a></h3>
    319. 

  319. <!-- ======================================================================= -->
    320. 

  320. 

  321. <p>This function-like macro takes a single file name string argument that
    322. 

  322. is the name of an include file.  It is like __has_include except that it
    323. 

  323. looks for the second instance of the given file found in the include
    324. 

  324. paths.  It evaluates to 1 if the second instance of the file can
    325. 

  325. be found using the include paths, or 0 otherwise:</p>
    326. 

  326. 

  327. <blockquote>
    328. 

  328. <pre>
    329. 

  329. // Note the two possible file name string formats.
    330. 

  330. #if __has_include_next("myinclude.h") &amp;&amp; __has_include_next(&lt;stdint.h&gt;)
    331. 

  331. # include_next "myinclude.h"
    332. 

  332. #endif
    333. 

  333. 

  334. // To avoid problem with non-clang compilers not having this macro.
    335. 

  335. #if defined(__has_include_next) &amp;&amp; __has_include_next("myinclude.h")
    336. 

  336. # include_next "myinclude.h"
    337. 

  337. #endif
    338. 

  338. </pre>
    339. 

  339. </blockquote>
    340. 

  340. 

  341. <p>Note that __has_include_next, like the GNU extension
    342. 

  342. #include_next directive, is intended for use in headers only,
    343. 

  343. and will issue a warning if used in the top-level compilation
    344. 

  344. file.  A warning will also be issued if an absolute path
    345. 

  345. is used in the file argument.</p>
    346. 

  346. 

  347. 

  348. <!-- ======================================================================= -->
    349. 

  349. <h3><a name="__has_warning">__has_warning</a></h3>
    350. 

  350. <!-- ======================================================================= -->
    351. 

  351. 

  352. <p>This function-like macro takes a string literal that represents a command
    353. 

  353.   line option for a warning and returns true if that is a valid warning
    354. 

  354.   option.</p>
    355. 

  355.   
    356. 

  356. <blockquote>
    357. 

  357. <pre>
    358. 

  358. #if __has_warning("-Wformat")
    359. 

  359. ...
    360. 

  360. #endif
    361. 

  361. </pre>
    362. 

  362. </blockquote>
    363. 

  363. 

  364. <!-- ======================================================================= -->
    365. 

  365. <h2 id="builtinmacros">Builtin Macros</h2>
    366. 

  366. <!-- ======================================================================= -->
    367. 

  367. 

  368. <dl>
    369. 

  369.   <dt><code>__BASE_FILE__</code></dt>
    370. 

  370.   <dd>Defined to a string that contains the name of the main input
    371. 

  371.   file passed to Clang.</dd> 
    372. 

  372. 

  373.   <dt><code>__COUNTER__</code></dt>
    374. 

  374.   <dd>Defined to an integer value that starts at zero and is
    375. 

  375.   incremented each time the <code>__COUNTER__</code> macro is
    376. 

  376.   expanded.</dd> 
    377. 

  377.     
    378. 

  378.   <dt><code>__INCLUDE_LEVEL__</code></dt>
    379. 

  379.   <dd>Defined to an integral value that is the include depth of the
    380. 

  380.   file currently being translated. For the main file, this value is
    381. 

  381.   zero.</dd> 
    382. 

  382. 

  383.   <dt><code>__TIMESTAMP__</code></dt>
    384. 

  384.   <dd>Defined to the date and time of the last modification of the
    385. 

  385.   current source file.</dd> 
    386. 

  386.     
    387. 

  387.   <dt><code>__clang__</code></dt>
    388. 

  388.   <dd>Defined when compiling with Clang</dd>
    389. 

  389. 

  390.   <dt><code>__clang_major__</code></dt>
    391. 

  391.   <dd>Defined to the major marketing version number of Clang (e.g., the 
    392. 

  392.   2 in 2.0.1).  Note that marketing version numbers should not be used to 
    393. 

  393.   check for language features, as different vendors use different numbering
    394. 

  394.   schemes.  Instead, use the <a href="#feature_check">feature checking
    395. 

  395.   macros</a>.</dd> 
    396. 

  396. 

  397.   <dt><code>__clang_minor__</code></dt>
    398. 

  398.   <dd>Defined to the minor version number of Clang (e.g., the 0 in
    399. 

  399.   2.0.1).  Note that marketing version numbers should not be used to 
    400. 

  400.   check for language features, as different vendors use different numbering
    401. 

  401.   schemes.  Instead, use the <a href="#feature_check">feature checking
    402. 

  402.   macros</a>.</dd> 
    403. 

  403. 

  404.   <dt><code>__clang_patchlevel__</code></dt>
    405. 

  405.   <dd>Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).</dd>
    406. 

  406. 

  407.   <dt><code>__clang_version__</code></dt>
    408. 

  408.   <dd>Defined to a string that captures the Clang marketing version, including
    409. 

  409.   the Subversion tag or revision number, e.g., "1.5 (trunk 102332)".</dd> 
    410. 

  410. </dl>
    411. 

  411. 

  412. <!-- ======================================================================= -->
    413. 

  413. <h2 id="vectors">Vectors and Extended Vectors</h2>
    414. 

  414. <!-- ======================================================================= -->
    415. 

  415. 

  416. <p>Supports the GCC, OpenCL, AltiVec and NEON vector extensions.</p>
    417. 

  417. 

  418. <p>OpenCL vector types are created using <tt>ext_vector_type</tt> attribute. It
    419. 

  419. support for <tt>V.xyzw</tt> syntax and other tidbits as seen in OpenCL. An
    420. 

  420. example is:</p>
    421. 

  421. 

  422. <blockquote>
    423. 

  423. <pre>
    424. 

  424. typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
    425. 

  425. typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
    426. 

  426. 

  427. float4 foo(float2 a, float2 b) {
    428. 

  428.   float4 c;
    429. 

  429.   c.xz = a;
    430. 

  430.   c.yw = b;
    431. 

  431.   return c;
    432. 

  432. }
    433. 

  433. </pre>
    434. 

  434. </blockquote>
    435. 

  435. 

  436. <p>Query for this feature with
    437. 

  437. <tt>__has_extension(attribute_ext_vector_type)</tt>.</p>
    438. 

  438. 

  439. <p>Giving <tt>-faltivec</tt> option to clang enables support for AltiVec vector
    440. 

  440. syntax and functions. For example:</p>
    441. 

  441. 

  442. <blockquote>
    443. 

  443. <pre>
    444. 

  444. vector float foo(vector int a) { 
    445. 

  445.   vector int b;
    446. 

  446.   b = vec_add(a, a) + a; 
    447. 

  447.   return (vector float)b;
    448. 

  448. }
    449. 

  449. </pre>
    450. 

  450. </blockquote>
    451. 

  451. 

  452. <p>NEON vector types are created using <tt>neon_vector_type</tt> and 
    453. 

  453. <tt>neon_polyvector_type</tt> attributes. For example:</p>
    454. 

  454. 

  455. <blockquote>
    456. 

  456. <pre>
    457. 

  457. typedef <b>__attribute__((neon_vector_type(8)))</b> int8_t int8x8_t;
    458. 

  458. typedef <b>__attribute__((neon_polyvector_type(16)))</b> poly8_t poly8x16_t;
    459. 

  459. 

  460. int8x8_t foo(int8x8_t a) {
    461. 

  461.   int8x8_t v;
    462. 

  462.   v = a;
    463. 

  463.   return v;
    464. 

  464. }
    465. 

  465. </pre>
    466. 

  466. </blockquote>
    467. 

  467. 

  468. <!-- ======================================================================= -->
    469. 

  469. <h3><a name="vector_literals">Vector Literals</a></h3>
    470. 

  470. <!-- ======================================================================= -->
    471. 

  471. 

  472. <p>Vector literals can be used to create vectors from a set of scalars, or 
    473. 

  473. vectors. Either parentheses or braces form can be used. In the parentheses form 
    474. 

  474. the number of literal values specified must be one, i.e. referring to a scalar 
    475. 

  475. value, or must match the size of the vector type being created. If a single 
    476. 

  476. scalar literal value is specified, the scalar literal value will be replicated 
    477. 

  477. to all the components of the vector type. In the brackets form any number of 
    478. 

  478. literals can be specified. For example:</p>
    479. 

  479. 

  480. <blockquote>
    481. 

  481. <pre>
    482. 

  482. typedef int v4si __attribute__((__vector_size__(16)));
    483. 

  483. typedef float float4 __attribute__((ext_vector_type(4)));
    484. 

  484. typedef float float2 __attribute__((ext_vector_type(2)));
    485. 

  485. 

  486. v4si vsi = (v4si){1, 2, 3, 4};
    487. 

  487. float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
    488. 

  488. vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
    489. 

  489. vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
    490. 

  490. vector int vi3 = (vector int)(1, 2); // error
    491. 

  491. vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
    492. 

  492. vector int vi5 = (vector int)(1, 2, 3, 4);
    493. 

  493. float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
    494. 

  494. </pre>
    495. 

  495. </blockquote>
    496. 

  496. 

  497. <!-- ======================================================================= -->
    498. 

  498. <h3><a name="vector_operations">Vector Operations</a></h3>
    499. 

  499. <!-- ======================================================================= -->
    500. 

  500. 

  501. <p>The table below shows the support for each operation by vector extension.
    502. 

  502. A dash indicates that an operation is not accepted according to a corresponding 
    503. 

  503. specification.</p>
    504. 

  504. 

  505. <table width="500" border="1" cellspacing="0">
    506. 

  506.  <tr>
    507. 

  507.     <th>Operator</th>
    508. 

  508.     <th>OpenCL</th>
    509. 

  509.     <th>AltiVec</th>
    510. 

  510.     <th>GCC</th>
    511. 

  511.     <th>NEON</th>
    512. 

  512.  </tr>
    513. 

  513.      <tr>
    514. 

  514.       <td>[]</td>
    515. 

  515.       <td align="center">yes</td>
    516. 

  516.       <td align="center">yes</td>
    517. 

  517.       <td align="center">yes</td>
    518. 

  518.       <td align="center">-</td>
    519. 

  519.     </tr>
    520. 

  520.     <tr>
    521. 

  521.       <td>unary operators +, -</td>
    522. 

  522.       <td align="center">yes</td>
    523. 

  523.       <td align="center">yes</td>
    524. 

  524.       <td align="center">yes</td>
    525. 

  525.       <td align="center">-</td>
    526. 

  526.     </tr>
    527. 

  527.     <tr>
    528. 

  528.       <td>++, --</td>
    529. 

  529.       <td align="center">yes</td>
    530. 

  530.       <td align="center">yes</td>
    531. 

  531.       <td align="center">-</td>
    532. 

  532.       <td align="center">-</td>
    533. 

  533.     </tr>
    534. 

  534.     <tr>
    535. 

  535.       <td>+, -, *, /, %</td>
    536. 

  536.       <td align="center">yes</td>
    537. 

  537.       <td align="center">yes</td>
    538. 

  538.       <td align="center">yes</td>
    539. 

  539.       <td align="center">-</td>
    540. 

  540.     </tr>
    541. 

  541.     <tr>
    542. 

  542.       <td>bitwise operators &, |, ^, ~</td>
    543. 

  543.       <td align="center">yes</td>
    544. 

  544.       <td align="center">yes</td>
    545. 

  545.       <td align="center">yes</td>
    546. 

  546.       <td align="center">-</td>
    547. 

  547.     </tr>
    548. 

  548.     <tr>
    549. 

  549.       <td>&gt&gt, &lt&lt</td>
    550. 

  550.       <td align="center">yes</td>
    551. 

  551.       <td align="center">yes</td>
    552. 

  552.       <td align="center">yes</td>
    553. 

  553.       <td align="center">-</td>
    554. 

  554.     </tr>
    555. 

  555.     <tr>
    556. 

  556.       <td>!, &&,||</td>
    557. 

  557.       <td align="center">no</td>
    558. 

  558.       <td align="center">-</td>
    559. 

  559.       <td align="center">-</td>
    560. 

  560.       <td align="center">-</td>
    561. 

  561.     </tr>
    562. 

  562.     <tr>
    563. 

  563.       <td>==,!=, >, <, >=, <=</td>
    564. 

  564.       <td align="center">yes</td>
    565. 

  565.       <td align="center">yes</td>
    566. 

  566.       <td align="center">-</td>
    567. 

  567.       <td align="center">-</td>
    568. 

  568.     </tr>
    569. 

  569.     <tr>
    570. 

  570.       <td>=</td>
    571. 

  571.       <td align="center">yes</td>
    572. 

  572.       <td align="center">yes</td>
    573. 

  573.       <td align="center">yes</td>
    574. 

  574.       <td align="center">yes</td>
    575. 

  575.     </tr>
    576. 

  576.     <tr>
    577. 

  577.       <td>:?</td>
    578. 

  578.       <td align="center">yes</td>
    579. 

  579.       <td align="center">-</td>
    580. 

  580.       <td align="center">-</td>
    581. 

  581.       <td align="center">-</td>
    582. 

  582.     </tr>
    583. 

  583.     <tr>
    584. 

  584.       <td>sizeof</td>
    585. 

  585.       <td align="center">yes</td>
    586. 

  586.       <td align="center">yes</td>
    587. 

  587.       <td align="center">yes</td>
    588. 

  588.       <td align="center">yes</td>
    589. 

  589.     </tr>
    590. 

  590. </table>
    591. 

  591. 

  592. <p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
    593. 

  593. 

  594. <!-- ======================================================================= -->
    595. 

  595. <h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
    596. 

  596. <!-- ======================================================================= -->
    597. 

  597. 

  598. <p>An optional string message can be added to the <tt>deprecated</tt>
    599. 

  599. and <tt>unavailable</tt> attributes.  For example:</p>
    600. 

  600. 

  601. <blockquote>
    602. 

  602. <pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
    603. 

  603. </blockquote>
    604. 

  604. 

  605. <p>If the deprecated or unavailable declaration is used, the message
    606. 

  606. will be incorporated into the appropriate diagnostic:</p>
    607. 

  607. 

  608. <blockquote>
    609. 

  609. <pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
    610. 

  610.       [-Wdeprecated-declarations]
    611. 

  611.   explode();
    612. 

  612.   ^</pre>
    613. 

  613. </blockquote>
    614. 

  614. 

  615. <p>Query for this feature
    616. 

  616. with <tt>__has_extension(attribute_deprecated_with_message)</tt>
    617. 

  617. and <tt>__has_extension(attribute_unavailable_with_message)</tt>.</p>
    618. 

  618. 

  619. <!-- ======================================================================= -->
    620. 

  620. <h2 id="attributes-on-enumerators">Attributes on Enumerators</h2>
    621. 

  621. <!-- ======================================================================= -->
    622. 

  622. 

  623. <p>Clang allows attributes to be written on individual enumerators.
    624. 

  624. This allows enumerators to be deprecated, made unavailable, etc.  The
    625. 

  625. attribute must appear after the enumerator name and before any
    626. 

  626. initializer, like so:</p>
    627. 

  627. 

  628. <blockquote>
    629. 

  629. <pre>enum OperationMode {
    630. 

  630.   OM_Invalid,
    631. 

  631.   OM_Normal,
    632. 

  632.   OM_Terrified __attribute__((deprecated)),
    633. 

  633.   OM_AbortOnError __attribute__((deprecated)) = 4
    634. 

  634. };</pre>
    635. 

  635. </blockquote>
    636. 

  636. 

  637. <p>Attributes on the <tt>enum</tt> declaration do not apply to
    638. 

  638. individual enumerators.</p>
    639. 

  639. 

  640. <p>Query for this feature with <tt>__has_extension(enumerator_attributes)</tt>.</p>
    641. 

  641. 

  642. <!-- ======================================================================= -->
    643. 

  643. <h2 id="user_specified_system_framework">'User-Specified' System Frameworks</h2>
    644. 

  644. <!-- ======================================================================= -->
    645. 

  645. 

  646. <p>Clang provides a mechanism by which frameworks can be built in such a way
    647. 

  647. that they will always be treated as being 'system frameworks', even if they are
    648. 

  648. not present in a system framework directory. This can be useful to system
    649. 

  649. framework developers who want to be able to test building other applications
    650. 

  650. with development builds of their framework, including the manner in which the
    651. 

  651. compiler changes warning behavior for system headers.</p>
    652. 

  652. 

  653. <p>Framework developers can opt-in to this mechanism by creating a
    654. 

  654. '.system_framework' file at the top-level of their framework. That is, the
    655. 

  655. framework should have contents like:</p>
    656. 

  656. 

  657. <pre>
    658. 

  658.  .../TestFramework.framework
    659. 

  659.  .../TestFramework.framework/.system_framework
    660. 

  660.  .../TestFramework.framework/Headers
    661. 

  661.  .../TestFramework.framework/Headers/TestFramework.h
    662. 

  662.  ...
    663. 

  663. </pre>
    664. 

  664. 

  665. <p>Clang will treat the presence of this file as an indicator that the framework
    666. 

  666. should be treated as a system framework, regardless of how it was found in the
    667. 

  667. framework search path. For consistency, we recommend that such files never be
    668. 

  668. included in installed versions of the framework.</p>
    669. 

  669. 

  670. <!-- ======================================================================= -->
    671. 

  671. <h2 id="availability">Availability attribute</h2>
    672. 

  672. <!-- ======================================================================= -->
    673. 

  673. 

  674. <p>Clang introduces the <code>availability</code> attribute, which can
    675. 

  675. be placed on declarations to describe the lifecycle of that
    676. 

  676. declaration relative to operating system versions. Consider the function declaration for a hypothetical function <code>f</code>:</p>
    677. 

  677. 

  678. <pre>
    679. 

  679. void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));
    680. 

  680. </pre>
    681. 

  681. 

  682. <p>The availability attribute states that <code>f</code> was introduced in Mac OS X 10.4, deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information is used by Clang to determine when it is safe to use <code>f</code>: for example, if Clang is instructed to compile code for Mac OS X 10.5, a call to <code>f()</code> succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call succeeds but Clang emits a warning specifying that the function is deprecated. Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call fails because <code>f()</code> is no longer available.</p>
    683. 

  683. 

  684. <p>The availablility attribute is a comma-separated list starting with the platform name and then including clauses specifying important milestones in the declaration's lifetime (in any order) along with additional information. Those clauses can be:</p>
    685. 

    686. 

  686. <dl>
    687. 

  687.   <dt>introduced=<i>version</i></dt>
    688. 

  688.   <dd>The first version in which this declaration was introduced.</dd>
    689. 

  689. 

  690.   <dt>deprecated=<i>version</i></dt>
    691. 

  691.   <dd>The first version in which this declaration was deprecated, meaning that users should migrate away from this API.</dd>
    692. 

  692. 

  693.   <dt>obsoleted=<i>version</i></dt>
    694. 

  694.   <dd>The first version in which this declaration was obsoleted, meaning that it was removed completely and can no longer be used.</dd>
    695. 

  695. 

  696.   <dt>unavailable</dt>
    697. 

  697.   <dd>This declaration is never available on this platform.</dd>
    698. 

  698. 

  699.   <dt>message=<i>string-literal</i></dt>
    700. 

  700.   <dd>Additional message text that Clang will provide when emitting a warning or error about use of a deprecated or obsoleted declaration. Useful to direct users to replacement APIs.</dd>
    701. 

  701. </dl>
    702. 

  702. 

  703. <p>Multiple availability attributes can be placed on a declaration, which may correspond to different platforms. Only the availability attribute with the platform corresponding to the target platform will be used; any others will be ignored. If no availability attribute specifies availability for the current target platform, the availability attributes are ignored. Supported platforms are:</p>
    704. 

  704. 

  705. <dl>
    706. 

  706.   <dt>ios</dt>
    707. 

  707.   <dd>Apple's iOS operating system. The minimum deployment target is specified by the <code>-mios-version-min=<i>version</i></code> or <code>-miphoneos-version-min=<i>version</i></code> command-line arguments.</dd>
    708. 

    709. 

  709.   <dt>macosx</dt>
    710. 

  710.   <dd>Apple's Mac OS X operating system. The minimum deployment target is specified by the <code>-mmacosx-version-min=<i>version</i></code> command-line argument.</dd>
    711. 

  711. </dl>
    712. 

  712. 

  713. <p>A declaration can be used even when deploying back to a platform
    714. 

  714. version prior to when the declaration was introduced. When this
    715. 

  715. happens, the declaration is <a
    716. 

  716.  href="https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html">weakly
    717. 

  717. linked</a>, as if the <code>weak_import</code> attribute were added to the declaration. A weakly-linked declaration may or may not be present a run-time, and a program can determine whether the declaration is present by checking whether the address of that declaration is non-NULL.</p>
    718. 

  718. 

  719. <!-- ======================================================================= -->
    720. 

  720. <h2 id="checking_language_features">Checks for Standard Language Features</h2>
    721. 

  721. <!-- ======================================================================= -->
    722. 

  722. 

  723. <p>The <tt>__has_feature</tt> macro can be used to query if certain standard
    724. 

  724. language features are enabled.  The <tt>__has_extension</tt> macro can be used
    725. 

  725. to query if language features are available as an extension when compiling for
    726. 

  726. a standard which does not provide them. The features which can be tested are
    727. 

  727. listed here.</p>
    728. 

  728. 

  729. <h3 id="cxx98">C++98</h3>
    730. 

  730. 

  731. <p>The features listed below are part of the C++98 standard. These features are
    732. 

  732. enabled by default when compiling C++ code.</p>
    733. 

  733. 

  734. <h4 id="cxx_exceptions">C++ exceptions</h4>
    735. 

  735. 

  736. <p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
    737. 

  737. example, compiling code with <tt>-fno-exceptions</tt> disables C++ exceptions.</p>
    738. 

  738. 

  739. <h4 id="cxx_rtti">C++ RTTI</h4>
    740. 

  740. 

  741. <p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
    742. 

  742. compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
    743. 

  743. 

  744. <h3 id="cxx11">C++11</h3>
    745. 

  745. 

  746. <p>The features listed below are part of the C++11 standard. As a result, all
    747. 

  747. these features are enabled with the <tt>-std=c++11</tt> or <tt>-std=gnu++11</tt>
    748. 

  748. option when compiling C++ code.</p>
    749. 

  749. 

  750. <h4 id="cxx_access_control_sfinae">C++11 SFINAE includes access control</h4>
    751. 

  751. 

  752. <p>Use <tt>__has_feature(cxx_access_control_sfinae)</tt> or <tt>__has_extension(cxx_access_control_sfinae)</tt> to determine whether access-control errors (e.g., calling a private constructor) are considered to be template argument deduction errors (aka SFINAE errors), per <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170">C++ DR1170</a>.</p>
    753. 

  753. 

  754. <h4 id="cxx_alias_templates">C++11 alias templates</h4>
    755. 

  755. 

  756. <p>Use <tt>__has_feature(cxx_alias_templates)</tt> or
    757. 

  757. <tt>__has_extension(cxx_alias_templates)</tt> to determine if support for
    758. 

  758. C++11's alias declarations and alias templates is enabled.</p>
    759. 

    760. 

  760. <h4 id="cxx_alignas">C++11 alignment specifiers</h4>
    761. 

  761. 

  762. <p>Use <tt>__has_feature(cxx_alignas)</tt> or
    763. 

  763. <tt>__has_extension(cxx_alignas)</tt> to determine if support for alignment
    764. 

  764. specifiers using <tt>alignas</tt> is enabled.</p>
    765. 

  765. 

  766. <h4 id="cxx_attributes">C++11 attributes</h4>
    767. 

  767. 

  768. <p>Use <tt>__has_feature(cxx_attributes)</tt> or
    769. 

  769. <tt>__has_extension(cxx_attributes)</tt> to determine if support for attribute
    770. 

  770. parsing with C++11's square bracket notation is enabled.</p>
    771. 

    772. 

  772. <h4 id="cxx_constexpr">C++11 generalized constant expressions</h4>
    773. 

  773. 

  774. <p>Use <tt>__has_feature(cxx_constexpr)</tt> to determine if support
    775. 

  775. for generalized constant expressions (e.g., <tt>constexpr</tt>) is
    776. 

  776. enabled.</p>
    777. 

  777. 

  778. <h4 id="cxx_decltype">C++11 <tt>decltype()</tt></h4>
    779. 

  779. 

  780. <p>Use <tt>__has_feature(cxx_decltype)</tt> or
    781. 

  781. <tt>__has_extension(cxx_decltype)</tt> to determine if support for the
    782. 

  782. <tt>decltype()</tt> specifier is enabled. C++11's <tt>decltype</tt>
    783. 

  783. does not require type-completeness of a function call expression.
    784. 

  784. Use <tt>__has_feature(cxx_decltype_incomplete_return_types)</tt>
    785. 

  785. or <tt>__has_extension(cxx_decltype_incomplete_return_types)</tt>
    786. 

  786. to determine if support for this feature is enabled.</p>
    787. 

  787. 

  788. <h4 id="cxx_default_function_template_args">C++11 default template arguments in function templates</h4>
    789. 

  789. 

  790. <p>Use <tt>__has_feature(cxx_default_function_template_args)</tt> or
    791. 

  791. <tt>__has_extension(cxx_default_function_template_args)</tt> to determine
    792. 

  792. if support for default template arguments in function templates is enabled.</p>
    793. 

  793. 

  794. <h4 id="cxx_defaulted_functions">C++11 <tt>default</tt>ed functions</h4>
    795. 

  795. 

  796. <p>Use <tt>__has_feature(cxx_defaulted_functions)</tt> or
    797. 

  797. <tt>__has_extension(cxx_defaulted_functions)</tt> to determine if support for
    798. 

  798. defaulted function definitions (with <tt>= default</tt>) is enabled.</p>
    799. 

  799. 

  800. <h4 id="cxx_delegating_constructors">C++11 delegating constructors</h4>
    801. 

  801. 

  802. <p>Use <tt>__has_feature(cxx_delegating_constructors)</tt> to determine if
    803. 

  803. support for delegating constructors is enabled.</p>
    804. 

  804. 

  805. <h4 id="cxx_deleted_functions">C++11 <tt>delete</tt>d functions</h4>
    806. 

  806. 

  807. <p>Use <tt>__has_feature(cxx_deleted_functions)</tt> or
    808. 

  808. <tt>__has_extension(cxx_deleted_functions)</tt> to determine if support for
    809. 

  809. deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
    810. 

  810. 

  811. <h4 id="cxx_explicit_conversions">C++11 explicit conversion functions</h4>
    812. 

  812. <p>Use <tt>__has_feature(cxx_explicit_conversions)</tt> to determine if support for <tt>explicit</tt> conversion functions is enabled.</p>
    813. 

  813. 

  814. <h4 id="cxx_generalized_initializers">C++11 generalized initializers</h4>
    815. 

  815. 

  816. <p>Use <tt>__has_feature(cxx_generalized_initializers)</tt> to determine if
    817. 

  817. support for generalized initializers (using braced lists and
    818. 

  818. <tt>std::initializer_list</tt>) is enabled.</p>
    819. 

  819. 

  820. <h4 id="cxx_implicit_moves">C++11 implicit move constructors/assignment operators</h4>
    821. 

  821. 

  822. <p>Use <tt>__has_feature(cxx_implicit_moves)</tt> to determine if Clang will
    823. 

  823. implicitly generate move constructors and move assignment operators where needed.</p>
    824. 

  824. 

  825. <h4 id="cxx_inheriting_constructors">C++11 inheriting constructors</h4>
    826. 

  826. 

  827. <p>Use <tt>__has_feature(cxx_inheriting_constructors)</tt> to determine if support for inheriting constructors is enabled. Clang does not currently implement this feature.</p>
    828. 

  828. 

  829. <h4 id="cxx_inline_namespaces">C++11 inline namespaces</h4>
    830. 

  830. 

  831. <p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> or
    832. 

  832. <tt>__has_extension(cxx_inline_namespaces)</tt> to determine if support for
    833. 

  833. inline namespaces is enabled.</p>
    834. 

  834. 

  835. <h4 id="cxx_lambdas">C++11 lambdas</h4>
    836. 

  836. 

  837. <p>Use <tt>__has_feature(cxx_lambdas)</tt> or
    838. 

  838. <tt>__has_extension(cxx_lambdas)</tt> to determine if support for lambdas
    839. 

  839. is enabled. </p>
    840. 

  840. 

  841. <h4 id="cxx_local_type_template_args">C++11 local and unnamed types as template arguments</h4>
    842. 

  842. 

  843. <p>Use <tt>__has_feature(cxx_local_type_template_args)</tt> or
    844. 

  844. <tt>__has_extension(cxx_local_type_template_args)</tt> to determine if
    845. 

  845. support for local and unnamed types as template arguments is enabled.</p>
    846. 

  846. 

  847. <h4 id="cxx_noexcept">C++11 noexcept</h4>
    848. 

  848. 

  849. <p>Use <tt>__has_feature(cxx_noexcept)</tt> or
    850. 

  850. <tt>__has_extension(cxx_noexcept)</tt> to determine if support for noexcept
    851. 

  851. exception specifications is enabled.</p>
    852. 

  852. 

  853. <h4 id="cxx_nonstatic_member_init">C++11 in-class non-static data member initialization</h4>
    854. 

  854. 

  855. <p>Use <tt>__has_feature(cxx_nonstatic_member_init)</tt> to determine whether in-class initialization of non-static data members is enabled.</p>
    856. 

  856. 

  857. <h4 id="cxx_nullptr">C++11 <tt>nullptr</tt></h4>
    858. 

  858. 

  859. <p>Use <tt>__has_feature(cxx_nullptr)</tt> or
    860. 

  860. <tt>__has_extension(cxx_nullptr)</tt> to determine if support for
    861. 

  861. <tt>nullptr</tt> is enabled.</p>
    862. 

  862. 

  863. <h4 id="cxx_override_control">C++11 <tt>override control</tt></h4>
    864. 

  864. 

  865. <p>Use <tt>__has_feature(cxx_override_control)</tt> or
    866. 

  866. <tt>__has_extension(cxx_override_control)</tt> to determine if support for
    867. 

  867. the override control keywords is enabled.</p>
    868. 

  868. 

  869. <h4 id="cxx_reference_qualified_functions">C++11 reference-qualified functions</h4>
    870. 

  870. <p>Use <tt>__has_feature(cxx_reference_qualified_functions)</tt> or
    871. 

  871. <tt>__has_extension(cxx_reference_qualified_functions)</tt> to determine
    872. 

  872. if support for reference-qualified functions (e.g., member functions with
    873. 

  873. <code>&amp;</code> or <code>&amp;&amp;</code> applied to <code>*this</code>)
    874. 

  874. is enabled.</p>
    875. 

  875. 

  876. <h4 id="cxx_range_for">C++11 range-based <tt>for</tt> loop</h4>
    877. 

  877. 

  878. <p>Use <tt>__has_feature(cxx_range_for)</tt> or
    879. 

  879. <tt>__has_extension(cxx_range_for)</tt> to determine if support for the
    880. 

  880. range-based for loop is enabled. </p>
    881. 

  881. 

  882. <h4 id="cxx_raw_string_literals">C++11 raw string literals</h4>
    883. 

  883. <p>Use <tt>__has_feature(cxx_raw_string_literals)</tt> to determine if support
    884. 

  884. for raw string literals (e.g., <tt>R"x(foo\bar)x"</tt>) is enabled.</p>
    885. 

  885. 

  886. <h4 id="cxx_rvalue_references">C++11 rvalue references</h4>
    887. 

  887. 

  888. <p>Use <tt>__has_feature(cxx_rvalue_references)</tt> or
    889. 

  889. <tt>__has_extension(cxx_rvalue_references)</tt> to determine if support for
    890. 

  890. rvalue references is enabled. </p>
    891. 

  891. 

  892. <h4 id="cxx_static_assert">C++11 <tt>static_assert()</tt></h4>
    893. 

  893. 

  894. <p>Use <tt>__has_feature(cxx_static_assert)</tt> or
    895. 

  895. <tt>__has_extension(cxx_static_assert)</tt> to determine if support for
    896. 

  896. compile-time assertions using <tt>static_assert</tt> is enabled.</p>
    897. 

  897. 

  898. <h4 id="cxx_auto_type">C++11 type inference</h4>
    899. 

  899. 

  900. <p>Use <tt>__has_feature(cxx_auto_type)</tt> or
    901. 

  901. <tt>__has_extension(cxx_auto_type)</tt> to determine C++11 type inference is
    902. 

  902. supported using the <tt>auto</tt> specifier. If this is disabled, <tt>auto</tt>
    903. 

  903. will instead be a storage class specifier, as in C or C++98.</p>
    904. 

  904. 

  905. <h4 id="cxx_strong_enums">C++11 strongly typed enumerations</h4>
    906. 

  906. 

  907. <p>Use <tt>__has_feature(cxx_strong_enums)</tt> or
    908. 

  908. <tt>__has_extension(cxx_strong_enums)</tt> to determine if support for
    909. 

  909. strongly typed, scoped enumerations is enabled.</p>
    910. 

  910. 

  911. <h4 id="cxx_trailing_return">C++11 trailing return type</h4>
    912. 

  912. 

  913. <p>Use <tt>__has_feature(cxx_trailing_return)</tt> or
    914. 

  914. <tt>__has_extension(cxx_trailing_return)</tt> to determine if support for the
    915. 

  915. alternate function declaration syntax with trailing return type is enabled.</p>
    916. 

  916. 

  917. <h4 id="cxx_unicode_literals">C++11 Unicode string literals</h4>
    918. 

  918. <p>Use <tt>__has_feature(cxx_unicode_literals)</tt> to determine if
    919. 

  919. support for Unicode string literals is enabled.</p>
    920. 

  920. 

  921. <h4 id="cxx_unrestricted_unions">C++11 unrestricted unions</h4>
    922. 

  922. 

  923. <p>Use <tt>__has_feature(cxx_unrestricted_unions)</tt> to determine if support for unrestricted unions is enabled.</p>
    924. 

  924. 

  925. <h4 id="cxx_user_literals">C++11 user-defined literals</h4>
    926. 

  926. 

  927. <p>Use <tt>__has_feature(cxx_user_literals)</tt> to determine if support for user-defined literals is enabled.</p>
    928. 

  928. 

  929. <h4 id="cxx_variadic_templates">C++11 variadic templates</h4>
    930. 

  930. 

  931. <p>Use <tt>__has_feature(cxx_variadic_templates)</tt> or
    932. 

  932. <tt>__has_extension(cxx_variadic_templates)</tt> to determine if support
    933. 

  933. for variadic templates is enabled.</p>
    934. 

  934. 

  935. <h3 id="c11">C11</h3>
    936. 

  936. 

  937. <p>The features listed below are part of the C11 standard. As a result, all
    938. 

  938. these features are enabled with the <tt>-std=c11</tt> or <tt>-std=gnu11</tt>
    939. 

  939. option when compiling C code. Additionally, because these features are all
    940. 

  940. backward-compatible, they are available as extensions in all language modes.</p>
    941. 

  941. 

  942. <h4 id="c_alignas">C11 alignment specifiers</h4>
    943. 

  943. 

  944. <p>Use <tt>__has_feature(c_alignas)</tt> or <tt>__has_extension(c_alignas)</tt>
    945. 

  945. to determine if support for alignment specifiers using <tt>_Alignas</tt>
    946. 

  946. is enabled.</p>
    947. 

  947. 

  948. <h4 id="c_atomic">C11 atomic operations</h4>
    949. 

  949. 

  950. <p>Use <tt>__has_feature(c_atomic)</tt> or <tt>__has_extension(c_atomic)</tt>
    951. 

  951. to determine if support for atomic types using <tt>_Atomic</tt> is enabled.
    952. 

  952. Clang also provides <a href="#__c11_atomic">a set of builtins</a> which can be
    953. 

  953. used to implement the <tt>&lt;stdatomic.h&gt;</tt> operations on
    954. 

  954. <tt>_Atomic</tt> types.</p>
    955. 

  955. 

  956. <h4 id="c_generic_selections">C11 generic selections</h4>
    957. 

  957. 

  958. <p>Use <tt>__has_feature(c_generic_selections)</tt> or
    959. 

  959. <tt>__has_extension(c_generic_selections)</tt> to determine if support for
    960. 

  960. generic selections is enabled.</p>
    961. 

  961. 

  962. <p>As an extension, the C11 generic selection expression is available in all
    963. 

  963. languages supported by Clang.  The syntax is the same as that given in the
    964. 

  964. C11 standard.</p>
    965. 

  965. 

  966. <p>In C, type compatibility is decided according to the rules given in the
    967. 

  967. appropriate standard, but in C++, which lacks the type compatibility rules
    968. 

  968. used in C, types are considered compatible only if they are equivalent.</p>
    969. 

  969. 

  970. <h4 id="c_static_assert">C11 <tt>_Static_assert()</tt></h4>
    971. 

  971. 

  972. <p>Use <tt>__has_feature(c_static_assert)</tt> or
    973. 

  973. <tt>__has_extension(c_static_assert)</tt> to determine if support for
    974. 

  974. compile-time assertions using <tt>_Static_assert</tt> is enabled.</p>
    975. 

  975. 

  976. <!-- ======================================================================= -->
    977. 

  977. <h2 id="checking_type_traits">Checks for Type Traits</h2>
    978. 

  978. <!-- ======================================================================= -->
    979. 

  979. 

  980. <p>Clang supports the <a href="http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html">GNU C++ type traits</a> and a subset of the <a href="http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx">Microsoft Visual C++ Type traits</a>. For each supported type trait <code>__X</code>, <code>__has_extension(X)</code> indicates the presence of the type trait. For example:
    981. 

  981. <blockquote>
    982. 

  982. <pre>
    983. 

  983. #if __has_extension(is_convertible_to)
    984. 

  984. template&lt;typename From, typename To&gt;
    985. 

  985. struct is_convertible_to {
    986. 

  986.   static const bool value = __is_convertible_to(From, To);
    987. 

  987. };
    988. 

  988. #else
    989. 

  989. // Emulate type trait
    990. 

  990. #endif
    991. 

  991. </pre>
    992. 

  992. </blockquote>
    993. 

  993. 

  994. <p>The following type traits are supported by Clang:</p>
    995. 

  995. <ul>
    996. 

  996.   <li><code>__has_nothrow_assign</code> (GNU, Microsoft)</li>
    997. 

  997.   <li><code>__has_nothrow_copy</code> (GNU, Microsoft)</li>
    998. 

  998.   <li><code>__has_nothrow_constructor</code> (GNU, Microsoft)</li>
    999. 

  999.   <li><code>__has_trivial_assign</code> (GNU, Microsoft)</li>
    1000. 

  1000.   <li><code>__has_trivial_copy</code> (GNU, Microsoft)</li>
    1001. 

  1001.   <li><code>__has_trivial_constructor</code> (GNU, Microsoft)</li>
    1002. 

  1002.   <li><code>__has_trivial_destructor</code> (GNU, Microsoft)</li>
    1003. 

  1003.   <li><code>__has_virtual_destructor</code> (GNU, Microsoft)</li>
    1004. 

  1004.   <li><code>__is_abstract</code> (GNU, Microsoft)</li>
    1005. 

  1005.   <li><code>__is_base_of</code> (GNU, Microsoft)</li>
    1006. 

  1006.   <li><code>__is_class</code> (GNU, Microsoft)</li>
    1007. 

  1007.   <li><code>__is_convertible_to</code> (Microsoft)</li>
    1008. 

  1008.   <li><code>__is_empty</code> (GNU, Microsoft)</li>
    1009. 

  1009.   <li><code>__is_enum</code> (GNU, Microsoft)</li>
    1010. 

  1010.   <li><code>__is_pod</code> (GNU, Microsoft)</li>
    1011. 

  1011.   <li><code>__is_polymorphic</code> (GNU, Microsoft)</li>
    1012. 

  1012.   <li><code>__is_union</code> (GNU, Microsoft)</li>
    1013. 

  1013.   <li><code>__is_literal(type)</code>: Determines whether the given type is a literal type</li>
    1014. 

  1014.   <li><code>__is_final</code>: Determines whether the given type is declared with a <code>final</code> class-virt-specifier.</li>
    1015. 

  1015.   <li><code>__underlying_type(type)</code>: Retrieves the underlying type for a given <code>enum</code> type. This trait is required to implement the C++11 standard library.</li>
    1016. 

  1016.   <li><code>__is_trivially_assignable(totype, fromtype)</code>: Determines whether a value of type <tt>totype</tt> can be assigned to from a value of type <tt>fromtype</tt> such that no non-trivial functions are called as part of that assignment. This trait is required to implement the C++11 standard library.</li>
    1017. 

  1017.   <li><code>__is_trivially_constructible(type, argtypes...)</code>: Determines whether a value of type <tt>type</tt> can be direct-initialized with arguments of types <tt>argtypes...</tt> such that no non-trivial functions are called as part of that initialization. This trait is required to implement the C++11 standard library.</li>
    1018. 

  1018. </ul>
    1019. 

  1019. 

  1020. <!-- ======================================================================= -->
    1021. 

  1021. <h2 id="blocks">Blocks</h2>
    1022. 

  1022. <!-- ======================================================================= -->
    1023. 

  1023. 

  1024. <p>The syntax and high level language feature description is in <a
    1025. 

  1025. href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>.  Implementation and ABI
    1026. 

  1026. details for the clang implementation are in <a 
    1027. 

  1027. href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
    1028. 

  1028. 

  1029. 

  1030. <p>Query for this feature with __has_extension(blocks).</p>
    1031. 

  1031. 

  1032. <!-- ======================================================================= -->
    1033. 

  1033. <h2 id="objc_features">Objective-C Features</h2>
    1034. 

  1034. <!-- ======================================================================= -->
    1035. 

  1035. 

  1036. <h3 id="objc_instancetype">Related result types</h3>
    1037. 

  1037. 

  1038. <p>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:</p>
    1039. 

    1040. 

  1040. <blockquote>
    1041. 

  1041. <pre>
    1042. 

  1042. @interface NSObject
    1043. 

  1043. + (id)alloc;
    1044. 

  1044. - (id)init;
    1045. 

  1045. @end
    1046. 

  1046. 

  1047. @interface NSArray : NSObject
    1048. 

  1048. @end
    1049. 

  1049. </pre>
    1050. 

  1050. </blockquote>
    1051. 

  1051. 

  1052. <p>and this common initialization pattern</p>
    1053. 

  1053. 

  1054. <blockquote>
    1055. 

  1055. <pre>
    1056. 

  1056. NSArray *array = [[NSArray alloc] init];
    1057. 

  1057. </pre>
    1058. 

  1058. </blockquote>
    1059. 

  1059. 

  1060. <p>the type of the expression <code>[NSArray alloc]</code> is
    1061. 

  1061. <code>NSArray*</code> because <code>alloc</code> implicitly has a
    1062. 

  1062. related result type. Similarly, the type of the expression
    1063. 

  1063. <code>[[NSArray alloc] init]</code> is <code>NSArray*</code>, since
    1064. 

  1064. <code>init</code> has a related result type and its receiver is known
    1065. 

  1065. to have the type <code>NSArray *</code>. If neither <code>alloc</code> nor <code>init</code> had a related result type, the expressions would have had type <code>id</code>, as declared in the method signature.</p>
    1066. 

  1066. 

  1067. <p>A method with a related result type can be declared by using the
    1068. 

  1068. type <tt>instancetype</tt> as its result type. <tt>instancetype</tt>
    1069. 

  1069. is a contextual keyword that is only permitted in the result type of
    1070. 

  1070. an Objective-C method, e.g.</p>
    1071. 

  1071. 

  1072. <pre>
    1073. 

  1073. @interface A
    1074. 

  1074. + (<b>instancetype</b>)constructAnA;
    1075. 

  1075. @end
    1076. 

  1076. </pre>
    1077. 

  1077. 

  1078. <p>The related result type can also be inferred for some methods.
    1079. 

  1079. To determine whether a method has an inferred related result type, the first
    1080. 

  1080. word in the camel-case selector (e.g., "init" in "initWithObjects") is
    1081. 

  1081. considered, and the method will have a related result type if its return
    1082. 

  1082. type is compatible with the type of its class and if</p>
    1083. 

  1083. 

  1084. <ul>
    1085. 

  1085.   
    1086. 

  1086.   <li>the first word is "alloc" or "new", and the method is a class
    1087. 

  1087.   method, or</li>
    1088. 

  1088.   
    1089. 

  1089.   <li>the first word is "autorelease", "init", "retain", or "self",
    1090. 

  1090.   and the method is an instance method.</li>
    1091. 

  1091.   
    1092. 

  1092. </ul>
    1093. 

  1093. 

  1094. <p>If a method with a related result type is overridden by a subclass
    1095. 

  1095. method, the subclass method must also return a type that is compatible
    1096. 

  1096. with the subclass type. For example:</p>
    1097. 

  1097. 

  1098. <blockquote>
    1099. 

  1099. <pre>
    1100. 

  1100. @interface NSString : NSObject
    1101. 

  1101. - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
    1102. 

  1102. @end
    1103. 

  1103. </pre>
    1104. 

  1104. </blockquote>
    1105. 

  1105. 

  1106. <p>Related result types only affect the type of a message send or
    1107. 

  1107. property access via the given method. In all other respects, a method
    1108. 

  1108. with a related result type is treated the same way as method that
    1109. 

  1109. returns <tt>id</tt>.</p>
    1110. 

  1110. 

  1111. <p>Use <tt>__has_feature(objc_instancetype)</tt> to determine whether
    1112. 

  1112. the <tt>instancetype</tt> contextual keyword is available.</p>
    1113. 

  1113. 

  1114. <!-- ======================================================================= -->
    1115. 

  1115. <h2 id="objc_arc">Automatic reference counting </h2>
    1116. 

  1116. <!-- ======================================================================= -->
    1117. 

  1117. 

  1118. <p>Clang provides support for <a href="AutomaticReferenceCounting.html">automated reference counting</a> in Objective-C, which eliminates the need for manual retain/release/autorelease message sends. There are two feature macros associated with automatic reference counting: <code>__has_feature(objc_arc)</code> indicates the availability of automated reference counting in general, while <code>__has_feature(objc_arc_weak)</code> indicates that automated reference counting also includes support for <code>__weak</code> pointers to Objective-C objects.</p>
    1119. 

  1119. 

  1120. <!-- ======================================================================= -->
    1121. 

  1121. <h2 id="objc_fixed_enum">Enumerations with a fixed underlying type</h2>
    1122. 

  1122. <!-- ======================================================================= -->
    1123. 

  1123. 

  1124. <p>Clang provides support for C++11 enumerations with a fixed
    1125. 

  1125. underlying type within Objective-C. For example, one can write an
    1126. 

  1126. enumeration type as:</p>
    1127. 

  1127. 

  1128. <pre>
    1129. 

  1129. typedef enum : unsigned char { Red, Green, Blue } Color;
    1130. 

  1130. </pre>
    1131. 

  1131. 

  1132. <p>This specifies that the underlying type, which is used to store the
    1133. 

  1133. enumeration value, is <tt>unsigned char</tt>.</p>
    1134. 

  1134. 

  1135. <p>Use <tt>__has_feature(objc_fixed_enum)</tt> to determine whether
    1136. 

  1136. support for fixed underlying types is available in Objective-C
    1137.