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<title>Using the GNU Compiler Collection (GCC): Cast to Union</title>
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<a name="Cast-to-Union"></a>
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<p>
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Next: <a href="Mixed-Declarations.html#Mixed-Declarations" accesskey="n" rel="next">Mixed Declarations</a>, Previous: <a href="Case-Ranges.html#Case-Ranges" accesskey="p" rel="prev">Case Ranges</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Cast-to-a-Union-Type"></a>
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<h3 class="section">6.29 Cast to a Union Type</h3>
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<a name="index-cast-to-a-union"></a>
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<a name="index-union_002c-casting-to-a"></a>
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<p>A cast to union type is similar to other casts, except that the type
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specified is a union type. You can specify the type either with
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<code>union <var>tag</var></code> or with a typedef name. A cast to union is actually
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a constructor, not a cast, and hence does not yield an lvalue like
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normal casts. (See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.)
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</p>
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<p>The types that may be cast to the union type are those of the members
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of the union. Thus, given the following union and variables:
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</p>
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<div class="smallexample">
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<pre class="smallexample">union foo { int i; double d; };
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int x;
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double y;
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</pre></div>
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<p>both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code>.
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</p>
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<p>Using the cast as the right-hand side of an assignment to a variable of
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union type is equivalent to storing in a member of the union:
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</p>
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<div class="smallexample">
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<pre class="smallexample">union foo u;
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/* <span class="roman">…</span> */
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u = (union foo) x ≡ u.i = x
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u = (union foo) y ≡ u.d = y
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</pre></div>
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<p>You can also use the union cast as a function argument:
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</p>
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<div class="smallexample">
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<pre class="smallexample">void hack (union foo);
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/* <span class="roman">…</span> */
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hack ((union foo) x);
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