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| <h3 class="section">6.25 Designated Initializers</h3> |
| |
| <p><a name="index-initializers-with-labeled-elements-2319"></a><a name="index-labeled-elements-in-initializers-2320"></a><a name="index-case-labels-in-initializers-2321"></a><a name="index-designated-initializers-2322"></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 which 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 will 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 which 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><a name="index-designators-2323"></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">will convert 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> |
| would store 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-2324"></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 will have 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 will discard them and issue a warning. |
| |
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