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<p>Specifies the <a href="objects.html#Alignment" title="cpp/language/object">alignment requirement</a> of a type or an object.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="alignas.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="alignas.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="alignas.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="alignas.html#Keywords"><span class="tocnumber">4</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="alignas.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="alignas.html#See_also"><span class="tocnumber">6</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/alignas&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">expression</span> <code><b>)</b></code>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">type-id</span> <code><b>)</b></code>
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<td class="t-sdsc-nopad"> <code><b>alignas(</b></code> <span class="t-spar">pack</span> <code><b>...</b></code> <code><b>)</b></code>
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<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span>expression</span></span> must be an <a href="constant_expression.html#Constant_expression" title="cpp/language/constant expression">integral constant expression</a> that evaluates to zero, or to a valid value for an <a href="objects.html#Alignment" title="cpp/language/object">alignment</a> or extended alignment.</div>
<div class="t-li1"><span class="t-li">2)</span> Equivalent to <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span>alignof<span class="br0">(</span>type<span class="br0">)</span><span class="br0">)</span></span></span></div>
<div class="t-li1"><span class="t-li">3)</span> Equivalent to multiple alignas specifiers applied to the same declaration, one for each member of the <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a>, which can be either type or non-type parameter pack.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/alignas&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> specifier may be applied to the declaration of a variable or a non-bitfield class data member, or it can be applied to the declaration or definition of a <a href="classes.html" title="cpp/language/classes">class/struct/union</a> or <a href="enum.html" title="cpp/language/enum">enumeration</a>. It cannot be applied to a function parameter or to the exception parameter of a catch clause.
</p><p>The object or the type declared by such a declaration will have its <a href="objects.html#Alignment" title="cpp/language/object">alignment requirement</a> equal to the stricted (largest) non-zero <span class="t-spar">expression</span> of all <code>alignas</code> specifiers used in the declaration, unless it would weaken the natural alignment of the type.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/alignas&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Invalid non-zero alignments, such as <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span></span></span> are ill-formed, but alignments that are not used because they are weaker than natural alignment or weaker than another <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> on the same declaration are simply ignored. <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span></span></span> is always ignored.
</p><p>As of the ISO C11 standard, the C language has the <span class="t-c"><span class="mw-geshi cpp source-cpp">_Alignas</span></span> keyword and defines <span class="t-c"><span class="mw-geshi cpp source-cpp">alignas</span></span> as a preprocessor macro expanding to the keyword in the header <a href="../../c/types.html" title="c/types"><tt><stdalign.h></tt></a>, but in C++ this is a keyword, and the headers <a href="../header.html#Deprecated_headers" title="cpp/header"><tt><stdalign.h></tt></a> and <a href="../header/cstdalign.html" title="cpp/header/cstdalign"><tt><cstdalign></tt></a> do not define such macro. They do, however, define the macro constant <span class="t-c"><span class="mw-geshi cpp source-cpp">__alignas_is_defined</span></span>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/alignas&action=edit&section=4" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/alignas.html" title="cpp/keyword/alignas"><tt>alignas</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/alignas&action=edit&section=5" 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="co1">// every object of type sse_t will be aligned to 16-byte boundary</span>
<span class="kw1">struct</span> alignas<span class="br0">(</span><span class="nu0">16</span><span class="br0">)</span> sse_t
<span class="br0">{</span>
<span class="kw4">float</span> sse_data<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// the array "cacheline" will be aligned to 128-byte boundary</span>
alignas<span class="br0">(</span><span class="nu0">128</span><span class="br0">)</span> <span class="kw4">char</span> cacheline<span class="br0">[</span><span class="nu0">128</span><span class="br0">]</span><span class="sy4">;</span></pre></div></div>
<p><br />
</p>
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<p><br />
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/alignas&action=edit&section=6" 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> <a href="alignof.html" title="cpp/language/alignof"> alignof operator </a>
</td>
<td> queries alignment requirements of a type <span class="t-mark-rev t-since-cxx11">(since C++11)</span> <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/dsc_alignof&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="../types/alignment_of.html" title="cpp/types/alignment of"> <span class="t-lines"><span>alignment_of</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td> obtains the type's alignment requirements <br /> <span class="t-mark">(class template)</span> <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/types/dsc_alignment_of&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../c/keyword/_Alignas.html" title="c/keyword/ Alignas">C documentation</a></span> for <span class="t-dsc-see-tt"><span>_Alignas</span></span></div>
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