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diff --git a/share/doc/gcc/Designated-Inits.html b/share/doc/gcc/Designated-Inits.html new file mode 100644 index 0000000..ca296c0 --- /dev/null +++ b/share/doc/gcc/Designated-Inits.html @@ -0,0 +1,229 @@ +<!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): Designated Inits</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Designated Inits"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Designated Inits"> +<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="Case-Ranges.html#Case-Ranges" rel="next" title="Case Ranges"> +<link href="Compound-Literals.html#Compound-Literals" rel="previous" title="Compound Literals"> +<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="Designated-Inits"></a> +<div class="header"> +<p> +Next: <a href="Case-Ranges.html#Case-Ranges" accesskey="n" rel="next">Case Ranges</a>, Previous: <a href="Compound-Literals.html#Compound-Literals" accesskey="p" rel="previous">Compound Literals</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="Designated-Initializers"></a> +<h3 class="section">6.29 Designated Initializers</h3> +<a name="index-initializers-with-labeled-elements"></a> +<a name="index-labeled-elements-in-initializers"></a> +<a name="index-case-labels-in-initializers"></a> +<a name="index-designated-initializers"></a> + +<p>Standard C90 requires the elements of an initializer to appear in a fixed +order, the same as the order of the elements in the array or structure +being initialized. +</p> +<p>In ISO C99 you can give the elements in any order, specifying the array +indices or structure field names they apply to, and GNU C allows this as +an extension in C90 mode as well. This extension is not +implemented in GNU C++. +</p> +<p>To specify an array index, write +‘<samp>[<var>index</var>] =</samp>’ before the element value. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">int a[6] = { [4] = 29, [2] = 15 }; +</pre></div> + +<p>is equivalent to +</p> +<div class="smallexample"> +<pre class="smallexample">int a[6] = { 0, 0, 15, 0, 29, 0 }; +</pre></div> + +<p>The index values must be constant expressions, even if the array being +initialized is automatic. +</p> +<p>An alternative syntax for this that has been obsolete since GCC 2.5 but +GCC still accepts is to write ‘<samp>[<var>index</var>]</samp>’ before the element +value, with no ‘<samp>=</samp>’. +</p> +<p>To initialize a range of elements to the same value, write +‘<samp>[<var>first</var> ... <var>last</var>] = <var>value</var></samp>’. This is a GNU +extension. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; +</pre></div> + +<p>If the value in it has side effects, the side effects happen only once, +not for each initialized field by the range initializer. +</p> +<p>Note that the length of the array is the highest value specified +plus one. +</p> +<p>In a structure initializer, specify the name of a field to initialize +with ‘<samp>.<var>fieldname</var> =</samp>’ before the element value. For example, +given the following structure, +</p> +<div class="smallexample"> +<pre class="smallexample">struct point { int x, y; }; +</pre></div> + +<p>the following initialization +</p> +<div class="smallexample"> +<pre class="smallexample">struct point p = { .y = yvalue, .x = xvalue }; +</pre></div> + +<p>is equivalent to +</p> +<div class="smallexample"> +<pre class="smallexample">struct point p = { xvalue, yvalue }; +</pre></div> + +<p>Another syntax that has the same meaning, obsolete since GCC 2.5, is +‘<samp><var>fieldname</var>:</samp>’, as shown here: +</p> +<div class="smallexample"> +<pre class="smallexample">struct point p = { y: yvalue, x: xvalue }; +</pre></div> + +<p>Omitted fields are implicitly initialized the same as for objects +that have static storage duration. +</p> +<a name="index-designators"></a> +<p>The ‘<samp>[<var>index</var>]</samp>’ or ‘<samp>.<var>fieldname</var></samp>’ is known as a +<em>designator</em>. You can also use a designator (or the obsolete colon +syntax) when initializing a union, to specify which element of the union +should be used. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">union foo { int i; double d; }; + +union foo f = { .d = 4 }; +</pre></div> + +<p>converts 4 to a <code>double</code> to store it in the union using +the second element. By contrast, casting 4 to type <code>union foo</code> +stores it into the union as the integer <code>i</code>, since it is +an integer. See <a href="Cast-to-Union.html#Cast-to-Union">Cast to Union</a>. +</p> +<p>You can combine this technique of naming elements with ordinary C +initialization of successive elements. Each initializer element that +does not have a designator applies to the next consecutive element of the +array or structure. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">int a[6] = { [1] = v1, v2, [4] = v4 }; +</pre></div> + +<p>is equivalent to +</p> +<div class="smallexample"> +<pre class="smallexample">int a[6] = { 0, v1, v2, 0, v4, 0 }; +</pre></div> + +<p>Labeling the elements of an array initializer is especially useful +when the indices are characters or belong to an <code>enum</code> type. +For example: +</p> +<div class="smallexample"> +<pre class="smallexample">int whitespace[256] + = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, + ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 }; +</pre></div> + +<a name="index-designator-lists"></a> +<p>You can also write a series of ‘<samp>.<var>fieldname</var></samp>’ and +‘<samp>[<var>index</var>]</samp>’ designators before an ‘<samp>=</samp>’ to specify a +nested subobject to initialize; the list is taken relative to the +subobject corresponding to the closest surrounding brace pair. For +example, with the ‘<samp>struct point</samp>’ declaration above: +</p> +<div class="smallexample"> +<pre class="smallexample">struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 }; +</pre></div> + +<p>If the same field is initialized multiple times, or overlapping +fields of a union are initialized, the value from the last +initialization is used. When a field of a union is itself a structure, +the entire structure from the last field initialized is used. If any previous +initializer has side effect, it is unspecified whether the side effect +happens or not. Currently, GCC discards the side-effecting +initializer expressions and issues a warning. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Case-Ranges.html#Case-Ranges" accesskey="n" rel="next">Case Ranges</a>, Previous: <a href="Compound-Literals.html#Compound-Literals" accesskey="p" rel="previous">Compound Literals</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> |