1<html lang="en"> 2<head> 3<title>Compound Literals - 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="Initializers.html#Initializers" title="Initializers"> 10<link rel="next" href="Designated-Inits.html#Designated-Inits" title="Designated Inits"> 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). 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A compound literal looks like 63a cast containing an initializer. Its value is an object of the 64type specified in the cast, containing the elements specified in 65the initializer; it is an lvalue. As an extension, GCC supports 66compound literals in C90 mode and in C++. 67 68 <p>Usually, the specified type is a structure. Assume that 69<code>struct foo</code> and <code>structure</code> are declared as shown: 70 71<pre class="smallexample"> struct foo {int a; char b[2];} structure; 72</pre> 73 <p class="noindent">Here is an example of constructing a <code>struct foo</code> with a compound literal: 74 75<pre class="smallexample"> structure = ((struct foo) {x + y, 'a', 0}); 76</pre> 77 <p class="noindent">This is equivalent to writing the following: 78 79<pre class="smallexample"> { 80 struct foo temp = {x + y, 'a', 0}; 81 structure = temp; 82 } 83</pre> 84 <p>You can also construct an array. If all the elements of the compound literal 85are (made up of) simple constant expressions, suitable for use in 86initializers of objects of static storage duration, then the compound 87literal can be coerced to a pointer to its first element and used in 88such an initializer, as shown here: 89 90<pre class="smallexample"> char **foo = (char *[]) { "x", "y", "z" }; 91</pre> 92 <p>Compound literals for scalar types and union types are is 93also allowed, but then the compound literal is equivalent 94to a cast. 95 96 <p>As a GNU extension, GCC allows initialization of objects with static storage 97duration by compound literals (which is not possible in ISO C99, because 98the initializer is not a constant). 99It is handled as if the object was initialized only with the bracket 100enclosed list if the types of the compound literal and the object match. 101The initializer list of the compound literal must be constant. 102If the object being initialized has array type of unknown size, the size is 103determined by compound literal size. 104 105<pre class="smallexample"> static struct foo x = (struct foo) {1, 'a', 'b'}; 106 static int y[] = (int []) {1, 2, 3}; 107 static int z[] = (int [3]) {1}; 108</pre> 109 <p class="noindent">The above lines are equivalent to the following: 110<pre class="smallexample"> static struct foo x = {1, 'a', 'b'}; 111 static int y[] = {1, 2, 3}; 112 static int z[] = {1, 0, 0}; 113</pre> 114 </body></html> 115 116