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<tr class="t-nv"><td colspan="5"> <a href="reference_initialization.html" title="cpp/language/reference initialization"> Reference initialization</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> Expressions</td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="operator_alternative.html" title="cpp/language/operator alternative"> Alternative representations</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Literals</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bool_literal.html" title="cpp/language/bool literal"> Boolean</a> - <a href="integer_literal.html" title="cpp/language/integer literal"> Integer</a> - <a href="floating_literal.html" title="cpp/language/floating literal"> Floating-point</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="character_literal.html" title="cpp/language/character literal"> Character</a> - <a href="string_literal.html" title="cpp/language/string literal"> String</a> - <a href="nullptr.html" title="cpp/language/nullptr"> <code>nullptr</code></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr> 
<tr class="t-nv"><td colspan="5"> <a href="user_literal.html" title="cpp/language/user literal"> User-defined</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"> <code>static_cast</code></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"> <code>dynamic_cast</code></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"> <code>const_cast</code></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"> <code>reinterpret_cast</code></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv-h1"><td colspan="5"> Classes</td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> override specifier</a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> final specifier</a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"> <code>explicit</code></a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv-h1"><td colspan="5">Templates </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/navbar_content&amp;action=edit">&#91;edit&#93;</a></span></div></div></div></div><div class="t-navbar-sep">&#160;</div></div>
<p>A union is a special class type that can hold only one of its non-static <a href="data_members.html" title="cpp/language/data members">data members</a> at a time.
</p><p>The class specifier for a union declaration is similar to <a href="class.html" title="cpp/language/class">class or struct</a> declaration:
</p>
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<td class="t-sdsc-nopad"> <code><b>union</b></code> <span class="t-spar">attr</span> <span class="t-spar">class-head-name</span> <code><b>{ </b></code> <span class="t-spar">member-specification</span> <code><b>} </b></code>
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<td class="t-sdsc-nopad">
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<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td>  optional sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">class-head-name</span>
</td>
<td> -
</td>
<td>  the name of the union that's being defined. Optionally prepended by <span class="t-spar">nested-name-specifier</span> (sequence of names and scope-resolution operators, ending with scope-resolution operator). The name may be omitted, in which case the union is <i>unnamed</i>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">member-specification</span>
</td>
<td> -
</td>
<td>  list of access specifiers, member object and member function declarations and definitions.
</td></tr></table>
<p>A union can have member functions (including constructors and destructors), but not virtual functions. 
</p><p>A union cannot have base classes and cannot be used as a base class.
</p><p>A union cannot have data members of reference types.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td> Unions cannot contain a non-static data member with a non-trivial special member function (<a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>, <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">copy-assignment</a> operator, or <a href="destructor.html" title="cpp/language/destructor">destructor</a>).
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
<tr class="t-rev t-since-cxx11"><td> If a union contains a non-static data member with a non-trivial special member function (<a href="default_constructor.html" title="cpp/language/default constructor">default</a> constructor, <a href="copy_constructor.html" title="cpp/language/copy constructor">copy</a>/<a href="move_constructor.html" title="cpp/language/move constructor">move</a> constructor, <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">copy</a>/<a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">move</a> assignment, or <a href="destructor.html" title="cpp/language/destructor">destructor</a>) that function is  deleted by default in the union and needs to be defined explicitly by the programmer.
<p>At most one data member can have a <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializer</a>
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>Just like in <a href="classes.html" title="cpp/language/classes">struct</a> declaration, the default member access in a union is <a href="access.html" title="cpp/language/access">public</a>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="union.html#Explanation"><span class="tocnumber">1</span> <span class="toctext">Explanation</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="union.html#Anonymous_unions"><span class="tocnumber">1.1</span> <span class="toctext">Anonymous unions</span></a></li>
<li class="toclevel-2 tocsection-3"><a href="union.html#Union-like_classes"><span class="tocnumber">1.2</span> <span class="toctext">Union-like classes</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-4"><a href="union.html#Defect_reports"><span class="tocnumber">2</span> <span class="toctext">Defect reports</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="union.html#See_also"><span class="tocnumber">3</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/union&amp;action=edit&amp;section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The union is only as big as necessary to hold its largest data member. The other data members are allocated in the same bytes as part of that largest member. The details of that allocation are implementation-defined, and it's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">union</span> S <span class="br0">&#123;</span>
    <a href="../types/integer.html"><span class="kw108">std::<span class="me2">int32_t</span></span></a> n<span class="sy4">;</span> <span class="co1">// occupies 4 bytes</span>
    <a href="../types/integer.html"><span class="kw121">std::<span class="me2">uint16_t</span></span></a> s<span class="br0">&#91;</span><span class="nu0">2</span><span class="br0">&#93;</span><span class="sy4">;</span> <span class="co1">// occupies 4 bytes</span>
    <a href="../types/integer.html"><span class="kw120">std::<span class="me2">uint8_t</span></span></a> c<span class="sy4">;</span> <span class="co1">// occupies 1 byte</span>
<span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// the whole union occupies 4 bytes</span>
&#160;
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    S s <span class="sy1">=</span> <span class="br0">&#123;</span><span class="nu12">0x12345678</span><span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// initializes the first member, s.n is now the active member</span>
    <span class="co1">// at this point, reading from s.s or s.c is UB</span>
    <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/hex.html"><span class="kw1505">std::<span class="me2">hex</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;s.n = &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">n</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    s.<span class="me1">s</span><span class="br0">&#91;</span><span class="nu0">0</span><span class="br0">&#93;</span> <span class="sy1">=</span> <span class="nu12">0x0011</span><span class="sy4">;</span> <span class="co1">// s.s is now the active member</span>
    <span class="co1">// at this point, reading from n or c is UB but most compilers define this</span>
    <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;s.c is now &quot;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">+</span>s.<span class="me1">c</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span> <span class="co1">// 11 or 00, depending on platform</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;s.n is now &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">n</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 12340011 or 00115678 </span>
<span class="br0">&#125;</span></pre></div></div>
<p>Each member is allocated as if it is the only member of the class.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>If members of a union are classes with user-defined constructors and destructors, to switch the active member, explicit destructor and placement new are generally needed:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="kw1">union</span> S <span class="br0">&#123;</span>
    <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> str<span class="sy4">;</span>
    <a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> vec<span class="sy4">;</span>
    ~S<span class="br0">&#40;</span><span class="br0">&#41;</span> <span class="br0">&#123;</span><span class="br0">&#125;</span> <span class="co1">// needs to know which member is active, only possible in union-like class </span>
<span class="br0">&#125;</span><span class="sy4">;</span> <span class="co1">// the whole union occupies max(sizeof(string), sizeof(vector&lt;int&gt;))</span>
&#160;
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    S s <span class="sy1">=</span> <span class="br0">&#123;</span><span class="st0">&quot;Hello, world&quot;</span><span class="br0">&#125;</span><span class="sy4">;</span>
    <span class="co1">// at this point, reading from s.vec is UB</span>
    <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;s.str = &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">str</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    s.<span class="me1">str</span>.~basic_string<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span>
    new <span class="br0">&#40;</span><span class="sy3">&amp;</span>s.<span class="me1">vec</span><span class="br0">&#41;</span> <a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span>
    <span class="co1">// now, s.vec is the active member of the union</span>
    s.<span class="me1">vec</span>.<span class="me1">push_back</span><span class="br0">&#40;</span><span class="nu0">10</span><span class="br0">&#41;</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">vec</span>.<span class="me1">size</span><span class="br0">&#40;</span><span class="br0">&#41;</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    s.<span class="me1">vec</span>.~vector<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>If two union members are standard-layout types, it's well-defined to examine their common subsequence on any compiler.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/union&amp;action=edit&amp;section=2" title="Edit section: Anonymous unions">edit</a>]</span> <span class="mw-headline" id="Anonymous_unions">Anonymous unions</span></h4>
<p>An unnamed union definition that does not define any objects, pointers, or references is an <i>anonymous union</i> definition.
</p>
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<td class="t-sdsc-nopad"> <code><b>union</b></code> <code><b>{ </b></code> <span class="t-spar">member-specification</span> <code><b>} </b></code> <code><b>;</b></code>
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<p>Anonymous unions have further restrictions: they cannot have member functions, cannot have static data members, and all their data members must be public. The only declarations allowed are non-static data members <span class="t-rev-inl t-since-cxx14"><span> and <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declarations</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>.
</p><p>Members of an anonymous union are injected in the enclosing scope (and must not conflict with other names declared there).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    <span class="kw1">union</span> <span class="br0">&#123;</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="br0">&#125;</span><span class="sy4">;</span>
    a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    p <span class="sy1">=</span> <span class="st0">&quot;Jennifer&quot;</span><span class="sy4">;</span>
<span class="br0">&#125;</span></pre></div></div>
<p>Namespace-scope anonymous unions must be static.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/union&amp;action=edit&amp;section=3" title="Edit section: Union-like classes">edit</a>]</span> <span class="mw-headline" id="Union-like_classes">Union-like classes</span></h4>
<p>A <i>union-like</i> class is any class with at least one anonymous union as a member. The members of all member anonymous unions of a class are called its <i>variant members</i>.
</p><p>A union is also a union-like class. The variant members of a union consists of its non-static data members that is not an anonymous union, and the members of all of its member anonymous unions.
</p><p>Union-like classes can be used to implement <a href="http://en.wikipedia.com/wiki/Tagged_union" class="extiw" title="enwiki:Tagged union">tagged unions</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co1">// S has one non-static data member (tag), three enumerator members, </span>
<span class="co1">// and three variant members (c, n, d)</span>
<span class="kw1">struct</span> S
<span class="br0">&#123;</span>
    <span class="kw2">enum</span> <span class="br0">&#123;</span>CHAR, INT, DOUBLE<span class="br0">&#125;</span> tag<span class="sy4">;</span>
    <span class="kw1">union</span> <span class="br0">&#123;</span>
        <span class="kw4">char</span> c<span class="sy4">;</span>
        <span class="kw4">int</span> n<span class="sy4">;</span>
        <span class="kw4">double</span> d<span class="sy4">;</span>
    <span class="br0">&#125;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
&#160;
<span class="kw4">void</span> print_s<span class="br0">&#40;</span><span class="kw4">const</span> S<span class="sy3">&amp;</span> s<span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    <span class="kw1">switch</span><span class="br0">&#40;</span>s.<span class="me1">tag</span><span class="br0">&#41;</span>
    <span class="br0">&#123;</span>
        <span class="kw1">case</span> S<span class="sy4">::</span><span class="me2">CHAR</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">c</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
        <span class="kw1">case</span> S<span class="sy4">::</span><span class="me2">INT</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">n</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
        <span class="kw1">case</span> S<span class="sy4">::</span><span class="me2">DOUBLE</span><span class="sy4">:</span> <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">d</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
    <span class="br0">&#125;</span>
<span class="br0">&#125;</span>
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    S s <span class="sy1">=</span> <span class="br0">&#123;</span>S<span class="sy4">::</span><span class="me2">CHAR</span>, <span class="st0">'a'</span><span class="br0">&#125;</span><span class="sy4">;</span>
    print_s<span class="br0">&#40;</span>s<span class="br0">&#41;</span><span class="sy4">;</span>
    s.<span class="me1">tag</span> <span class="sy1">=</span> S<span class="sy4">::</span><span class="me2">INT</span><span class="sy4">;</span> s.<span class="me1">n</span> <span class="sy1">=</span> <span class="nu0">123</span><span class="sy4">;</span>
    print_s<span class="br0">&#40;</span>s<span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/union&amp;action=edit&amp;section=4" title="Edit section: Defect reports">edit</a>]</span> <span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1940">CWG 1940</a>
</td>
<td> c++14
</td>
<td> anonymous unions only allowed non-static data members
</td>
<td> static_assert also allowed
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/union&amp;action=edit&amp;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/union.html" title="c/language/union">C documentation</a></span> for <span class="t-dsc-see-tt"><span>Union declaration</span></span></div>
</td></tr>

</table>

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