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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>
</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
&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 fields are implicitly initialized the same as for 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, 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="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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