225 lines
9.1 KiB
HTML
225 lines
9.1 KiB
HTML
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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) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Designated Inits</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Designated Inits">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Designated Inits">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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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="Case-Ranges.html#Case-Ranges" rel="next" title="Case Ranges">
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<link href="Compound-Literals.html#Compound-Literals" rel="prev" title="Compound Literals">
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<style type="text/css">
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<!--
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Designated-Inits"></a>
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<div class="header">
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<p>
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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="prev">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="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="Designated-Initializers"></a>
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<h3 class="section">6.27 Designated Initializers</h3>
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<a name="index-initializers-with-labeled-elements"></a>
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<a name="index-labeled-elements-in-initializers"></a>
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<a name="index-case-labels-in-initializers"></a>
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<a name="index-designated-initializers"></a>
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<p>Standard C90 requires the elements of an initializer to appear in a fixed
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order, the same as the order of the elements in the array or structure
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being initialized.
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</p>
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<p>In ISO C99 you can give the elements in any order, specifying the array
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indices or structure field names they apply to, and GNU C allows this as
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an extension in C90 mode as well. This extension is not
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implemented in GNU C++.
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</p>
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<p>To specify an array index, write
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‘<samp>[<var>index</var>] =</samp>’ before the element value. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">int a[6] = { [4] = 29, [2] = 15 };
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</pre></div>
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<p>is equivalent to
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</p>
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<div class="smallexample">
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<pre class="smallexample">int a[6] = { 0, 0, 15, 0, 29, 0 };
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</pre></div>
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<p>The index values must be constant expressions, even if the array being
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initialized is automatic.
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</p>
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<p>An alternative syntax for this that has been obsolete since GCC 2.5 but
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GCC still accepts is to write ‘<samp>[<var>index</var>]</samp>’ before the element
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value, with no ‘<samp>=</samp>’.
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</p>
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<p>To initialize a range of elements to the same value, write
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‘<samp>[<var>first</var> ... <var>last</var>] = <var>value</var></samp>’. This is a GNU
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extension. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
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</pre></div>
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<p>If the value in it has side-effects, the side-effects happen only once,
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not for each initialized field by the range initializer.
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</p>
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<p>Note that the length of the array is the highest value specified
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plus one.
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</p>
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<p>In a structure initializer, specify the name of a field to initialize
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with ‘<samp>.<var>fieldname</var> =</samp>’ before the element value. For example,
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given the following structure,
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct point { int x, y; };
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</pre></div>
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<p>the following initialization
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct point p = { .y = yvalue, .x = xvalue };
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</pre></div>
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<p>is equivalent to
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct point p = { xvalue, yvalue };
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</pre></div>
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<p>Another syntax that has the same meaning, obsolete since GCC 2.5, is
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‘<samp><var>fieldname</var>:</samp>’, as shown here:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct point p = { y: yvalue, x: xvalue };
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</pre></div>
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<p>Omitted field members are implicitly initialized the same as objects
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that have static storage duration.
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</p>
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<a name="index-designators"></a>
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<p>The ‘<samp>[<var>index</var>]</samp>’ or ‘<samp>.<var>fieldname</var></samp>’ is known as a
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<em>designator</em>. You can also use a designator (or the obsolete colon
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syntax) when initializing a union, to specify which element of the union
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should be used. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">union foo { int i; double d; };
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union foo f = { .d = 4 };
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</pre></div>
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<p>converts 4 to a <code>double</code> to store it in the union using
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the second element. By contrast, casting 4 to type <code>union foo</code>
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stores it into the union as the integer <code>i</code>, since it is
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an integer. (See <a href="Cast-to-Union.html#Cast-to-Union">Cast to Union</a>.)
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</p>
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<p>You can combine this technique of naming elements with ordinary C
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initialization of successive elements. Each initializer element that
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does not have a designator applies to the next consecutive element of the
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array or structure. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">int a[6] = { [1] = v1, v2, [4] = v4 };
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</pre></div>
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<p>is equivalent to
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</p>
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<div class="smallexample">
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<pre class="smallexample">int a[6] = { 0, v1, v2, 0, v4, 0 };
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</pre></div>
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<p>Labeling the elements of an array initializer is especially useful
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when the indices are characters or belong to an <code>enum</code> type.
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For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">int whitespace[256]
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= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
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['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
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</pre></div>
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<a name="index-designator-lists"></a>
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<p>You can also write a series of ‘<samp>.<var>fieldname</var></samp>’ and
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‘<samp>[<var>index</var>]</samp>’ designators before an ‘<samp>=</samp>’ to specify a
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nested subobject to initialize; the list is taken relative to the
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subobject corresponding to the closest surrounding brace pair. For
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example, with the ‘<samp>struct point</samp>’ declaration above:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
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</pre></div>
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<p>If the same field is initialized multiple times, it has the value from
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the last initialization. If any such overridden initialization has
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side-effect, it is unspecified whether the side-effect happens or not.
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Currently, GCC discards them and issues a warning.
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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="Case-Ranges.html#Case-Ranges" accesskey="n" rel="next">Case Ranges</a>, Previous: <a href="Compound-Literals.html#Compound-Literals" accesskey="p" rel="prev">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="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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