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<title>Cast to Union - Using the GNU Compiler Collection (GCC)</title>
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<h3 class="section">5.23 Cast to a Union Type</h3>

<p><a name="index-cast-to-a-union-1820"></a><a name="index-union_002c-casting-to-a-1821"></a>
A cast to union type is similar to other casts, except that the type
specified is a union type.  You can specify the type either with
<code>union </code><var>tag</var> or with a typedef name.  A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts.  (See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.)

 <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:

<pre class="smallexample">     union foo { int i; double d; };
     int x;
     double y;
</pre>
 <p class="noindent">both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code>.

 <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:

<pre class="smallexample">     union foo u;
     /* <span class="roman">...</span> */
     u = (union foo) x  ==  u.i = x
     u = (union foo) y  ==  u.d = y
</pre>
 <p>You can also use the union cast as a function argument:

<pre class="smallexample">     void hack (union foo);
     /* <span class="roman">...</span> */
     hack ((union foo) x);
</pre>
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