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<html lang="en"><head><title>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 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>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 }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Designated%20Inits">Designated Inits</a>,Next:&nbsp;<a rel="next" accesskey="n" href="Cast-to-Union.html#Cast%20to%20Union">Cast to Union</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Compound-Literals.html#Compound%20Literals">Compound Literals</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Designated Initializers</h3><p>Standard C89 requires the elements of an initializer to appear in a fixedorder, the same as the order of the elements in the array or structurebeing initialized.   <p>In ISO C99 you can give the elements in any order, specifying the arrayindices or structure field names they apply to, and GNU C allows this asan extension in C89 mode as well.  This extension is notimplemented in GNU C++.   <p>To specify an array index, write<code>[</code><var>index</var><code>] =</code> before the element value.  For example,<pre class="smallexample">     int a[6] = { [4] = 29, [2] = 15 };     </pre><p>is equivalent to<pre class="smallexample">     int a[6] = { 0, 0, 15, 0, 29, 0 };     </pre><p>The index values must be constant expressions, even if the array beinginitialized is automatic.   <p>An alternative syntax for this which has been obsolete since GCC 2.5 butGCC still accepts is to write <code>[</code><var>index</var><code>]</code> before the elementvalue, with no <code>=</code>.   <p>To initialize a range of elements to the same value, write<code>[</code><var>first</var><code> ... </code><var>last</var><code>] = </code><var>value</var><code></code>.  This is a GNUextension.  For example,<pre class="smallexample">     int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };     </pre><p>If the value in it has side-effects, the side-effects will happen only once,not for each initialized field by the range initializer.<p>Note that the length of the array is the highest value specifiedplus one.   <p>In a structure initializer, specify the name of a field to initializewith <code>.</code><var>fieldname</var><code> =</code> before the element value.  For example,given the following structure,<pre class="smallexample">     struct point { int x, y; };     </pre><p>the following initialization<pre class="smallexample">     struct point p = { .y = yvalue, .x = xvalue };     </pre><p>is equivalent to<pre class="smallexample">     struct point p = { xvalue, yvalue };     </pre>   <p>Another syntax which has the same meaning, obsolete since GCC 2.5, is<code></code><var>fieldname</var><code>:</code>, as shown here:<pre class="smallexample">     struct point p = { y: yvalue, x: xvalue };     </pre>   <p>The <code>[</code><var>index</var><code>]</code> or <code>.</code><var>fieldname</var><code></code> is known as a<dfn>designator</dfn>.  You can also use a designator (or the obsolete colonsyntax) when initializing a union, to specify which element of the unionshould be used.  For example,<pre class="smallexample">     union foo { int i; double d; };          union foo f = { .d = 4 };     </pre><p>will convert 4 to a <code>double</code> to store it in the union usingthe second element.  By contrast, casting 4 to type <code>union foo</code>would store it into the union as the integer <code>i</code>, since it isan integer.  (See <a href="Cast-to-Union.html#Cast%20to%20Union">Cast to Union</a>.)   <p>You can combine this technique of naming elements with ordinary Cinitialization of successive elements.  Each initializer element thatdoes not have a designator applies to the next consecutive element of thearray or structure.  For example,<pre class="smallexample">     int a[6] = { [1] = v1, v2, [4] = v4 };     </pre><p>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 usefulwhen 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>You can also write a series of <code>.</code><var>fieldname</var><code></code> and<code>[</code><var>index</var><code>]</code> designators before an <code>=</code> to specify anested subobject to initialize; the list is taken relative to thesubobject corresponding to the closest surrounding brace pair.  Forexample, with the <code>struct point</code> declaration above:<pre class="smallexample">     struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };     </pre><p>If the same field is initialized multiple times, it will have value fromthe last initialization.  If any such overridden initialization hasside-effect, it is unspecified whether the side-effect happens or not. Currently, GCC will discard them and issue a warning.   </body></html>
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