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<title>Using the GNU Compiler Collection (GCC): Designated Inits</title>

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<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="prev">Compound Literals</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<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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<hr>
<a name="Designated-Initializers"></a>
<h3 class="section">6.27 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
&lsquo;<samp>[<var>index</var>] =</samp>&rsquo; 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 &lsquo;<samp>[<var>index</var>]</samp>&rsquo; before the element
value, with no &lsquo;<samp>=</samp>&rsquo;.
</p>
<p>To initialize a range of elements to the same value, write
&lsquo;<samp>[<var>first</var> ... <var>last</var>] = <var>value</var></samp>&rsquo;.  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 &lsquo;<samp>.<var>fieldname</var> =</samp>&rsquo; 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
&lsquo;<samp><var>fieldname</var>:</samp>&rsquo;, as shown here:
</p>
<div class="smallexample">
<pre class="smallexample">struct point p = { y: yvalue, x: xvalue };
</pre></div>

<p>Omitted field members are implicitly initialized the same as objects
that have static storage duration.
</p>
<a name="index-designators"></a>
<p>The &lsquo;<samp>[<var>index</var>]</samp>&rsquo; or &lsquo;<samp>.<var>fieldname</var></samp>&rsquo; 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 &lsquo;<samp>.<var>fieldname</var></samp>&rsquo; and
&lsquo;<samp>[<var>index</var>]</samp>&rsquo; designators before an &lsquo;<samp>=</samp>&rsquo; 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 &lsquo;<samp>struct point</samp>&rsquo; 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, 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.
</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="prev">Compound Literals</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<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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