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122 lines
4.8 KiB
HTML
122 lines
4.8 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Cast to Union (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Cast to Union (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Cast to Union (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Mixed-Declarations.html#Mixed-Declarations" rel="next" title="Mixed Declarations">
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<link href="Case-Ranges.html#Case-Ranges" rel="prev" title="Case Ranges">
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<style type="text/css">
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Cast-to-Union"></a>
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<div class="header">
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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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</div>
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<hr>
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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 looks similar to other casts, except that the type
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specified is a union type. You can specify the type either with the
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<code>union</code> keyword or with a <code>typedef</code> name that refers to
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a union. A cast to a union actually creates a compound literal and
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yields an lvalue, not an rvalue like true casts do.
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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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</pre></div>
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</body>
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</html>
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