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<h1 id="firstHeading" class="firstHeading">attribute specifier sequence<span class="t-mark-rev t-since-cxx11">(since C++11)</span></h1>
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<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
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</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/declarations/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Introduces implementation-defined attributes for types, objects, code, etc.
</p>
<dl><dd><code><b>[[</b></code><span class="t-spar">attr</span><code><b>]]</b></code> <code><b>[[</b></code><span class="t-spar">attr1</span>, <span class="t-spar">attr2</span>, <span class="t-spar">attr3</span><code><b>(</b></code><span class="t-spar">args</span><code><b>)</b></code><code><b>]]</b></code> <code><b>[[</b></code><span class="t-spar">namespace</span><code><b>::</b></code><span class="t-spar">attr</span><code><b>(</b></code><span class="t-spar">args</span><code><b>)</b></code><code><b>]]</b></code> <span class="t-spar">alignas_specifier</span>
</dd></dl>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="attributes.html#Explanation"><span class="tocnumber">1</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="attributes.html#Standard_attributes"><span class="tocnumber">2</span> <span class="toctext">Standard attributes</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="attributes.html#Example"><span class="tocnumber">3</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="attributes.html#External_links"><span class="tocnumber">4</span> <span class="toctext">External links</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="attributes.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/attributes&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Attributes provide the unified standard syntax for implementation-defined language extensions, such as the GNU and IBM language extensions <code>__attribute__((...))</code>, Microsoft extension <code>__declspec()</code>, etc.
</p><p>An attribute can be used almost everywhere in the C++ program, and can be applied to almost everything: to types, to variables, to functions, to names, to code blocks, to entire translation units, although each particular attribute is only valid where it is permitted by the implementation: <code>[[probably(true)]]</code> can only be used with an <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span></span></span>, and not with an class declaration. <code>[[omp::parallel()]]</code> can apply to a code block or to a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">for</span></span></span> loop, but not to the type <code>int</code>, etc.
</p><p>In declarations, attributes may appear both before and directly after the name of the entity that is declared, in which case they are combined. In most other situations, attributes apply to the directly preceding entity.
</p><p><a href="alignas.html" title="cpp/language/alignas">alignas_specifier</a> is a part of the attribute specifier sequence, although it has different syntax. It may appear where the <code>[[...]]</code> attributes appear and may mix with them (provided it is used where <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> is permitted)
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/attributes&action=edit&section=2" title="Edit section: Standard attributes">edit</a>]</span> <span class="mw-headline" id="Standard_attributes">Standard attributes</span></h3>
<p>Only the following attributes are defined by the C++ standard. All other attributes are implementation-specific.
</p>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td> <code>[[noreturn]]</code>
</td>
<td> Indicates that the function does not return. <br /> This attribute applies to function declarations only. The behavior is undefined if the function with this attribute actually returns. <br />The following standard functions have this attribute: <span class="t-lc"><a href="../utility/program/_Exit.html" title="cpp/utility/program/ Exit">std::_Exit</a></span>, <span class="t-lc"><a href="../utility/program/abort.html" title="cpp/utility/program/abort">std::abort</a></span>, <span class="t-lc"><a href="../utility/program/exit.html" title="cpp/utility/program/exit">std::exit</a></span>, <span class="t-lc"><a href="../utility/program/quick_exit.html" title="cpp/utility/program/quick exit">std::quick_exit</a></span>, <span class="t-lc"><a href="../error/unexpected.html" title="cpp/error/unexpected">std::unexpected</a></span>, <span class="t-lc"><a href="../error/terminate.html" title="cpp/error/terminate">std::terminate</a></span>, <span class="t-lc"><a href="../error/rethrow_exception.html" title="cpp/error/rethrow exception">std::rethrow_exception</a></span>, <span class="t-lc"><a href="../error/throw_with_nested.html" title="cpp/error/throw with nested">std::throw_with_nested</a></span>, <span class="t-lc"><a href="../error/nested_exception/rethrow_nested.html" title="cpp/error/nested exception/rethrow nested">std::nested_exception::rethrow_nested</a></span>
</td></tr>
<tr class="t-dsc">
<td> <code>[[carries_dependency]]</code>
</td>
<td> Indicates that dependency chain in release-consume <span class="t-lc"><a href="../atomic/memory_order.html" title="cpp/atomic/memory order">std::memory_order</a></span> propagates in and out of the function, which allows the compiler to skip unnecessary memory fence instructions.<br />This attribute may appear in two situations:<br />
<p>1) it may apply to the parameter declarations of a function or lambda-expressions, in which case it indicates that initialization of the parameter carries dependency into lvalue-to-rvalue conversion of that object.<br />
2) It may apply to the function declaration as a whole, in which case it indicates that the return value carries dependency to the evaluation of the function call expression.
</p><p>This attribute must appear on the first declaration of a function or one of its parameters in any translation unit. If it is not used on the first declaration of a function or one of its parameters in another translation unit, the program is ill-formed; no diagnostic required.
</p><p>See <span class="t-lc"><a href="../atomic/kill_dependency.html" title="cpp/atomic/kill dependency">std::kill_dependency</a></span> for example usage
</p>
</td></tr>
<tr class="t-dsc">
<td> <code>[[deprecated]]</code><span class="t-mark-rev t-since-cxx14">(C++14)</span><br /><code>[[deprecated("reason")]]</code><span class="t-mark-rev t-since-cxx14">(C++14)</span>
</td>
<td> Indicates that the use of the name or entity declared with this attribute is allowed, but discouraged for some <span class="t-spar">reason</span>. This attribute is allowed in declarations of classes, typedef-names, variables, non-static data members, functions, enumerations, and template specializations. A name declared non-deprecated may be redeclared deprecated. A name declared deprecated cannot be un-deprecated by redeclaring it without this attribute.
</td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/attributes&action=edit&section=3" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="br0">[</span><span class="br0">[</span> noreturn <span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">throw</span> <span class="st0">"error"</span><span class="sy4">;</span>
<span class="co1">// OK</span>
<span class="br0">}</span>
<span class="br0">[</span><span class="br0">[</span> noreturn <span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> q<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
<span class="co1">// behavior is undefined if called with an argument <= 0</span>
<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">></span> <span class="nu0">0</span><span class="br0">)</span>
<span class="kw1">throw</span> <span class="st0">"positive"</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/attributes&action=edit&section=4" title="Edit section: External links">edit</a>]</span> <span class="mw-headline" id="External_links">External links</span></h3>
<p><a rel="nofollow" class="external text" href="http://clang.llvm.org/docs/LanguageExtensions.html#non-standard-c-11-attributes">C++ attributes supported by Clang</a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/attributes&action=edit&section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../c/language/noreturn.html" title="c/language/ Noreturn">C documentation</a></span> for <span class="t-dsc-see-tt"><span>_Noreturn</span></span></div>
</td></tr>
</table>
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