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89 lines
3.7 KiB
HTML
89 lines
3.7 KiB
HTML
4 years ago
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<title>Cast to Union - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Case-Ranges.html#Case-Ranges" title="Case Ranges">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<div class="node">
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<a name="Cast-to-Union"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>,
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Previous: <a rel="previous" accesskey="p" href="Case-Ranges.html#Case-Ranges">Case Ranges</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.29 Cast to a Union Type</h3>
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<p><a name="index-cast-to-a-union-3029"></a><a name="index-union_002c-casting-to-a-3030"></a>
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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 </code><var>tag</var> 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>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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<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>
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<p class="noindent">both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code>.
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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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<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>
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<p>You can also use the union cast as a function argument:
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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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</pre>
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</body></html>
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