1<html lang="en"> 2<head> 3<title>Cast to Union - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Designated-Inits.html#Designated-Inits" title="Designated Inits"> 10<link rel="next" href="Case-Ranges.html#Case-Ranges" title="Case Ranges"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Cast-to-Union"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Case-Ranges.html#Case-Ranges">Case Ranges</a>, 53Previous: <a rel="previous" accesskey="p" href="Designated-Inits.html#Designated-Inits">Designated Inits</a>, 54Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">6.28 Cast to a Union Type</h3> 59 60<p><a name="index-cast-to-a-union-2387"></a><a name="index-union_002c-casting-to-a-2388"></a> 61A cast to union type is similar to other casts, except that the type 62specified is a union type. You can specify the type either with 63<code>union </code><var>tag</var> or with a typedef name. A cast to union is actually 64a constructor though, not a cast, and hence does not yield an lvalue like 65normal casts. (See <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>.) 66 67 <p>The types that may be cast to the union type are those of the members 68of the union. Thus, given the following union and variables: 69 70<pre class="smallexample"> union foo { int i; double d; }; 71 int x; 72 double y; 73</pre> 74 <p class="noindent">both <code>x</code> and <code>y</code> can be cast to type <code>union foo</code>. 75 76 <p>Using the cast as the right-hand side of an assignment to a variable of 77union type is equivalent to storing in a member of the union: 78 79<pre class="smallexample"> union foo u; 80 /* <span class="roman">...</span> */ 81 u = (union foo) x == u.i = x 82 u = (union foo) y == u.d = y 83</pre> 84 <p>You can also use the union cast as a function argument: 85 86<pre class="smallexample"> void hack (union foo); 87 /* <span class="roman">...</span> */ 88 hack ((union foo) x); 89</pre> 90 </body></html> 91 92