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<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.
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