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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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<a name="Cast-to-a-Union-Type"></a>
<h3 class="section">6.29 Cast to a Union Type</h3>
<a name="index-cast-to-a-union"></a>
<a name="index-union_002c-casting-to-a"></a>

<p>A cast to union type looks similar to other casts, except that the type
specified is a union type.  You can specify the type either with the
<code>union</code> keyword or with a <code>typedef</code> name that refers to
a union.  A cast to a union actually creates a compound literal and
yields an lvalue, not an rvalue like true casts do.
See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.
</p>
<p>The types that may be cast to the union type are those of the members
of the union.  Thus, given the following union and variables:
</p>
<div class="smallexample">
<pre class="smallexample">union foo { int i; double d; };
int x;
double y;
</pre></div>

<p>both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code>.
</p>
<p>Using the cast as the right-hand side of an assignment to a variable of
union type is equivalent to storing in a member of the union:
</p>
<div class="smallexample">
<pre class="smallexample">union foo u;
/* <span class="roman">&hellip;</span> */
u = (union foo) x  &equiv;  u.i = x
u = (union foo) y  &equiv;  u.d = y
</pre></div>

<p>You can also use the union cast as a function argument:
</p>
<div class="smallexample">
<pre class="smallexample">void hack (union foo);
/* <span class="roman">&hellip;</span> */
hack ((union foo) x);
</pre></div>


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