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184 lines
8.1 KiB
HTML
184 lines
8.1 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>Compound Literals (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Compound Literals (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Compound Literals (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="Designated-Inits.html#Designated-Inits" rel="next" title="Designated Inits">
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<link href="Initializers.html#Initializers" rel="prev" title="Initializers">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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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="Compound-Literals"></a>
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<div class="header">
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<p>
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Next: <a href="Designated-Inits.html#Designated-Inits" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Initializers.html#Initializers" accesskey="p" rel="prev">Initializers</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="Compound-Literals-1"></a>
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<h3 class="section">6.26 Compound Literals</h3>
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<a name="index-constructor-expressions"></a>
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<a name="index-initializations-in-expressions"></a>
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<a name="index-structures_002c-constructor-expression"></a>
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<a name="index-expressions_002c-constructor"></a>
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<a name="index-compound-literals"></a>
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<p>A compound literal looks like a cast of a brace-enclosed aggregate
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initializer list. Its value is an object of the type specified in
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the cast, containing the elements specified in the initializer.
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Unlike the result of a cast, a compound literal is an lvalue. ISO
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C99 and later support compound literals. As an extension, GCC
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supports compound literals also in C90 mode and in C++, although
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as explained below, the C++ semantics are somewhat different.
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</p>
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<p>Usually, the specified type of a compound literal is a structure. Assume
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that <code>struct foo</code> and <code>structure</code> are declared as shown:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct foo {int a; char b[2];} structure;
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</pre></div>
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<p>Here is an example of constructing a <code>struct foo</code> with a compound literal:
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</p>
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<div class="smallexample">
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<pre class="smallexample">structure = ((struct foo) {x + y, 'a', 0});
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</pre></div>
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<p>This is equivalent to writing the following:
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</p>
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<div class="smallexample">
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<pre class="smallexample">{
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struct foo temp = {x + y, 'a', 0};
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structure = temp;
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}
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</pre></div>
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<p>You can also construct an array, though this is dangerous in C++, as
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explained below. If all the elements of the compound literal are
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(made up of) simple constant expressions suitable for use in
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initializers of objects of static storage duration, then the compound
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literal can be coerced to a pointer to its first element and used in
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such an initializer, as shown here:
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</p>
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<div class="smallexample">
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<pre class="smallexample">char **foo = (char *[]) { "x", "y", "z" };
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</pre></div>
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<p>Compound literals for scalar types and union types are also allowed. In
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the following example the variable <code>i</code> is initialized to the value
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<code>2</code>, the result of incrementing the unnamed object created by
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the compound literal.
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</p>
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<div class="smallexample">
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<pre class="smallexample">int i = ++(int) { 1 };
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</pre></div>
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<p>As a GNU extension, GCC allows initialization of objects with static storage
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duration by compound literals (which is not possible in ISO C99 because
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the initializer is not a constant).
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It is handled as if the object were initialized only with the brace-enclosed
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list if the types of the compound literal and the object match.
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The elements of the compound literal must be constant.
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If the object being initialized has array type of unknown size, the size is
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determined by the size of the compound literal.
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</p>
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<div class="smallexample">
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<pre class="smallexample">static struct foo x = (struct foo) {1, 'a', 'b'};
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static int y[] = (int []) {1, 2, 3};
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static int z[] = (int [3]) {1};
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</pre></div>
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<p>The above lines are equivalent to the following:
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</p><div class="smallexample">
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<pre class="smallexample">static struct foo x = {1, 'a', 'b'};
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static int y[] = {1, 2, 3};
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static int z[] = {1, 0, 0};
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</pre></div>
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<p>In C, a compound literal designates an unnamed object with static or
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automatic storage duration. In C++, a compound literal designates a
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temporary object that only lives until the end of its full-expression.
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As a result, well-defined C code that takes the address of a subobject
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of a compound literal can be undefined in C++, so G++ rejects
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the conversion of a temporary array to a pointer. For instance, if
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the array compound literal example above appeared inside a function,
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any subsequent use of <code>foo</code> in C++ would have undefined behavior
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because the lifetime of the array ends after the declaration of <code>foo</code>.
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</p>
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<p>As an optimization, G++ sometimes gives array compound literals longer
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lifetimes: when the array either appears outside a function or has
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a <code>const</code>-qualified type. If <code>foo</code> and its initializer had
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elements of type <code>char *const</code> rather than <code>char *</code>, or if
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<code>foo</code> were a global variable, the array would have static storage
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duration. But it is probably safest just to avoid the use of array
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compound literals in C++ code.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Designated-Inits.html#Designated-Inits" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Initializers.html#Initializers" accesskey="p" rel="prev">Initializers</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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</body>
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</html>
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