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<title>Designated Inits - Using the GNU Compiler Collection (GCC)</title>
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<a name="Designated-Inits"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Case-Ranges.html#Case-Ranges">Case Ranges</a>,
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Previous: <a rel="previous" accesskey="p" href="Compound-Literals.html#Compound-Literals">Compound Literals</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<h3 class="section">6.27 Designated Initializers</h3>
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<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>
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Standard C90 requires the elements of an initializer to appear in a fixed
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order, the same as the order of the elements in the array or structure
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being initialized.
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<p>In ISO C99 you can give the elements in any order, specifying the array
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indices or structure field names they apply to, and GNU C allows this as
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an extension in C90 mode as well. This extension is not
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implemented in GNU C++.
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<p>To specify an array index, write
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‘<samp><span class="samp">[</span><var>index</var><span class="samp">] =</span></samp>’ before the element value. For example,
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<pre class="smallexample"> int a[6] = { [4] = 29, [2] = 15 };
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</pre>
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<p class="noindent">is equivalent to
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<pre class="smallexample"> int a[6] = { 0, 0, 15, 0, 29, 0 };
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</pre>
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<p class="noindent">The index values must be constant expressions, even if the array being
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initialized is automatic.
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<p>An alternative syntax for this that has been obsolete since GCC 2.5 but
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GCC still accepts is to write ‘<samp><span class="samp">[</span><var>index</var><span class="samp">]</span></samp>’ before the element
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value, with no ‘<samp><span class="samp">=</span></samp>’.
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<p>To initialize a range of elements to the same value, write
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‘<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
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extension. For example,
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<pre class="smallexample"> int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
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</pre>
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<p class="noindent">If the value in it has side-effects, the side-effects happen only once,
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not for each initialized field by the range initializer.
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<p class="noindent">Note that the length of the array is the highest value specified
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plus one.
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<p>In a structure initializer, specify the name of a field to initialize
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with ‘<samp><span class="samp">.</span><var>fieldname</var><span class="samp"> =</span></samp>’ before the element value. For example,
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given the following structure,
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<pre class="smallexample"> struct point { int x, y; };
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</pre>
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<p class="noindent">the following initialization
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<pre class="smallexample"> struct point p = { .y = yvalue, .x = xvalue };
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</pre>
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<p class="noindent">is equivalent to
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<pre class="smallexample"> struct point p = { xvalue, yvalue };
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</pre>
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<p>Another syntax that has the same meaning, obsolete since GCC 2.5, is
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‘<samp><var>fieldname</var><span class="samp">:</span></samp>’, as shown here:
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<pre class="smallexample"> struct point p = { y: yvalue, x: xvalue };
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</pre>
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<p>Omitted field members are implicitly initialized the same as objects
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that have static storage duration.
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<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
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<dfn>designator</dfn>. You can also use a designator (or the obsolete colon
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syntax) when initializing a union, to specify which element of the union
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should be used. For example,
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<pre class="smallexample"> union foo { int i; double d; };
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union foo f = { .d = 4 };
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</pre>
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<p class="noindent">converts 4 to a <code>double</code> to store it in the union using
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the second element. By contrast, casting 4 to type <code>union foo</code>
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stores it into the union as the integer <code>i</code>, since it is
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an integer. (See <a href="Cast-to-Union.html#Cast-to-Union">Cast to Union</a>.)
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<p>You can combine this technique of naming elements with ordinary C
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initialization of successive elements. Each initializer element that
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does not have a designator applies to the next consecutive element of the
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array or structure. For example,
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<pre class="smallexample"> int a[6] = { [1] = v1, v2, [4] = v4 };
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</pre>
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<p class="noindent">is equivalent to
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<pre class="smallexample"> int a[6] = { 0, v1, v2, 0, v4, 0 };
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</pre>
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<p>Labeling the elements of an array initializer is especially useful
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when the indices are characters or belong to an <code>enum</code> type.
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For example:
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<pre class="smallexample"> int whitespace[256]
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= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
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['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
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</pre>
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<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
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‘<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
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nested subobject to initialize; the list is taken relative to the
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subobject corresponding to the closest surrounding brace pair. For
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example, with the ‘<samp><span class="samp">struct point</span></samp>’ declaration above:
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<pre class="smallexample"> struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
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</pre>
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<p class="noindent">If the same field is initialized multiple times, it has the value from
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the last initialization. If any such overridden initialization has
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side-effect, it is unspecified whether the side-effect happens or not.
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Currently, GCC discards them and issues a warning.
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</body></html>
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