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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gcc/Compound-Literals.html | |
https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads
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diff --git a/share/doc/gcc/Compound-Literals.html b/share/doc/gcc/Compound-Literals.html new file mode 100644 index 0000000..db24cbd --- /dev/null +++ b/share/doc/gcc/Compound-Literals.html @@ -0,0 +1,186 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Compound Literals</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Compound Literals"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Compound Literals"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> +<link href="Designated-Inits.html#Designated-Inits" rel="next" title="Designated Inits"> +<link href="Initializers.html#Initializers" rel="previous" title="Initializers"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Compound-Literals"></a> +<div class="header"> +<p> +Next: <a href="Designated-Inits.html#Designated-Inits" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Initializers.html#Initializers" accesskey="p" rel="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Compound-Literals-1"></a> +<h3 class="section">6.28 Compound Literals</h3> +<a name="index-constructor-expressions"></a> +<a name="index-initializations-in-expressions"></a> +<a name="index-structures_002c-constructor-expression"></a> +<a name="index-expressions_002c-constructor"></a> +<a name="index-compound-literals"></a> + +<p>A compound literal looks like a cast of a brace-enclosed aggregate +initializer list. Its value is an object of the type specified in +the cast, containing the elements specified in the initializer. +Unlike the result of a cast, a compound literal is an lvalue. ISO +C99 and later support compound literals. As an extension, GCC +supports compound literals also in C90 mode and in C++, although +as explained below, the C++ semantics are somewhat different. +</p> +<p>Usually, the specified type of a compound literal is a structure. Assume +that <code>struct foo</code> and <code>structure</code> are declared as shown: +</p> +<div class="smallexample"> +<pre class="smallexample">struct foo {int a; char b[2];} structure; +</pre></div> + +<p>Here is an example of constructing a <code>struct foo</code> with a compound literal: +</p> +<div class="smallexample"> +<pre class="smallexample">structure = ((struct foo) {x + y, 'a', 0}); +</pre></div> + +<p>This is equivalent to writing the following: +</p> +<div class="smallexample"> +<pre class="smallexample">{ + struct foo temp = {x + y, 'a', 0}; + structure = temp; +} +</pre></div> + +<p>You can also construct an array, though this is dangerous in C++, as +explained below. If all the elements of the compound literal are +(made up of) simple constant expressions suitable for use in +initializers of objects of static storage duration, then the compound +literal can be coerced to a pointer to its first element and used in +such an initializer, as shown here: +</p> +<div class="smallexample"> +<pre class="smallexample">char **foo = (char *[]) { "x", "y", "z" }; +</pre></div> + +<p>Compound literals for scalar types and union types are also allowed. In +the following example the variable <code>i</code> is initialized to the value +<code>2</code>, the result of incrementing the unnamed object created by +the compound literal. +</p> +<div class="smallexample"> +<pre class="smallexample">int i = ++(int) { 1 }; +</pre></div> + +<p>As a GNU extension, GCC allows initialization of objects with static storage +duration by compound literals (which is not possible in ISO C99 because +the initializer is not a constant). +It is handled as if the object were initialized only with the brace-enclosed +list if the types of the compound literal and the object match. +The elements of the compound literal must be constant. +If the object being initialized has array type of unknown size, the size is +determined by the size of the compound literal. +</p> +<div class="smallexample"> +<pre class="smallexample">static struct foo x = (struct foo) {1, 'a', 'b'}; +static int y[] = (int []) {1, 2, 3}; +static int z[] = (int [3]) {1}; +</pre></div> + +<p>The above lines are equivalent to the following: +</p><div class="smallexample"> +<pre class="smallexample">static struct foo x = {1, 'a', 'b'}; +static int y[] = {1, 2, 3}; +static int z[] = {1, 0, 0}; +</pre></div> + +<p>In C, a compound literal designates an unnamed object with static or +automatic storage duration. In C++, a compound literal designates a +temporary object that only lives until the end of its full-expression. +As a result, well-defined C code that takes the address of a subobject +of a compound literal can be undefined in C++, so G++ rejects +the conversion of a temporary array to a pointer. For instance, if +the array compound literal example above appeared inside a function, +any subsequent use of <code>foo</code> in C++ would have undefined behavior +because the lifetime of the array ends after the declaration of <code>foo</code>. +</p> +<p>As an optimization, G++ sometimes gives array compound literals longer +lifetimes: when the array either appears outside a function or has +a <code>const</code>-qualified type. If <code>foo</code> and its initializer had +elements of type <code>char *const</code> rather than <code>char *</code>, or if +<code>foo</code> were a global variable, the array would have static storage +duration. But it is probably safest just to avoid the use of array +compound literals in C++ code. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Designated-Inits.html#Designated-Inits" accesskey="n" rel="next">Designated Inits</a>, Previous: <a href="Initializers.html#Initializers" accesskey="p" rel="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |