You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
165 lines
7.8 KiB
HTML
165 lines
7.8 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Designated Inits - Using the GNU Compiler Collection (GCC)</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Using the GNU Compiler Collection (GCC)">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
|
||
|
|
<link rel="prev" href="Compound-Literals.html#Compound-Literals" title="Compound Literals">
|
||
|
|
<link rel="next" href="Case-Ranges.html#Case-Ranges" title="Case Ranges">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
Copyright (C) 1988-2015 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.-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Designated-Inits"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Case-Ranges.html#Case-Ranges">Case Ranges</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Compound-Literals.html#Compound-Literals">Compound Literals</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h3 class="section">6.27 Designated Initializers</h3>
|
||
|
|
|
||
|
|
<p><a name="index-initializers-with-labeled-elements-3021"></a><a name="index-labeled-elements-in-initializers-3022"></a><a name="index-case-labels-in-initializers-3023"></a><a name="index-designated-initializers-3024"></a>
|
||
|
|
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>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>To specify an array index, write
|
||
|
|
‘<samp><span class="samp">[</span><var>index</var><span class="samp">] =</span></samp>’ before the element value. For example,
|
||
|
|
|
||
|
|
<pre class="smallexample"> int a[6] = { [4] = 29, [2] = 15 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">is equivalent to
|
||
|
|
|
||
|
|
<pre class="smallexample"> int a[6] = { 0, 0, 15, 0, 29, 0 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">The index values must be constant expressions, even if the array being
|
||
|
|
initialized is automatic.
|
||
|
|
|
||
|
|
<p>An alternative syntax for this that has been obsolete since GCC 2.5 but
|
||
|
|
GCC still accepts is to write ‘<samp><span class="samp">[</span><var>index</var><span class="samp">]</span></samp>’ before the element
|
||
|
|
value, with no ‘<samp><span class="samp">=</span></samp>’.
|
||
|
|
|
||
|
|
<p>To initialize a range of elements to the same value, write
|
||
|
|
‘<samp><span class="samp">[</span><var>first</var><span class="samp"> ... </span><var>last</var><span class="samp">] = </span><var>value</var></samp>’. This is a GNU
|
||
|
|
extension. For example,
|
||
|
|
|
||
|
|
<pre class="smallexample"> int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">If the value in it has side-effects, the side-effects happen only once,
|
||
|
|
not for each initialized field by the range initializer.
|
||
|
|
|
||
|
|
<p class="noindent">Note that the length of the array is the highest value specified
|
||
|
|
plus one.
|
||
|
|
|
||
|
|
<p>In a structure initializer, specify the name of a field to initialize
|
||
|
|
with ‘<samp><span class="samp">.</span><var>fieldname</var><span class="samp"> =</span></samp>’ before the element value. For example,
|
||
|
|
given the following structure,
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct point { int x, y; };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">the following initialization
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct point p = { .y = yvalue, .x = xvalue };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">is equivalent to
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct point p = { xvalue, yvalue };
|
||
|
|
</pre>
|
||
|
|
<p>Another syntax that has the same meaning, obsolete since GCC 2.5, is
|
||
|
|
‘<samp><var>fieldname</var><span class="samp">:</span></samp>’, as shown here:
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct point p = { y: yvalue, x: xvalue };
|
||
|
|
</pre>
|
||
|
|
<p>Omitted field members are implicitly initialized the same as objects
|
||
|
|
that have static storage duration.
|
||
|
|
|
||
|
|
<p><a name="index-designators-3025"></a>The ‘<samp><span class="samp">[</span><var>index</var><span class="samp">]</span></samp>’ or ‘<samp><span class="samp">.</span><var>fieldname</var></samp>’ is known as a
|
||
|
|
<dfn>designator</dfn>. 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,
|
||
|
|
|
||
|
|
<pre class="smallexample"> union foo { int i; double d; };
|
||
|
|
|
||
|
|
union foo f = { .d = 4 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">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>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,
|
||
|
|
|
||
|
|
<pre class="smallexample"> int a[6] = { [1] = v1, v2, [4] = v4 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">is equivalent to
|
||
|
|
|
||
|
|
<pre class="smallexample"> int a[6] = { 0, v1, v2, 0, v4, 0 };
|
||
|
|
</pre>
|
||
|
|
<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:
|
||
|
|
|
||
|
|
<pre class="smallexample"> int whitespace[256]
|
||
|
|
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
|
||
|
|
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
|
||
|
|
</pre>
|
||
|
|
<p><a name="index-designator-lists-3026"></a>You can also write a series of ‘<samp><span class="samp">.</span><var>fieldname</var></samp>’ and
|
||
|
|
‘<samp><span class="samp">[</span><var>index</var><span class="samp">]</span></samp>’ designators before an ‘<samp><span class="samp">=</span></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><span class="samp">struct point</span></samp>’ declaration above:
|
||
|
|
|
||
|
|
<pre class="smallexample"> struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">If the same field is initialized multiple times, it has the value from
|
||
|
|
the last initialization. If any such overridden initialization has
|
||
|
|
side-effect, it is unspecified whether the side-effect happens or not.
|
||
|
|
Currently, GCC discards them and issues a warning.
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|