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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">6.31 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 a union type is a C extension not available in C++.  It looks
just like ordinary casts with the constraint 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.  The result
of a cast to a union is a temporary rvalue of the union type with a member
whose type matches that of the operand initialized to the value of
the operand.  The effect of a cast to a union is similar to a compound
literal except that it yields an rvalue like standard casts do.
See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.
</p>
<p>Expressions that may be cast to the union type are those whose type matches
at least one 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;
union foo z;
</pre></div>

<p>both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code> and
the following assignments
</p><div class="smallexample">
<pre class="smallexample">  z = (union foo) x;
  z = (union foo) y;
</pre></div>
<p>are shorthand equivalents of these
</p><div class="smallexample">
<pre class="smallexample">  z = (union foo) { .i = x };
  z = (union foo) { .d = y };
</pre></div>

<p>However, <code>(union foo) FLT_MAX;</code> is not a valid cast because the union
has no member of type <code>float</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 with
the same type
</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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