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| Next: <a href="Fast-enumeration-protocol.html#Fast-enumeration-protocol" accesskey="n" rel="next">Fast enumeration protocol</a>, Previous: <a href="c99_002dlike-fast-enumeration-syntax.html#c99_002dlike-fast-enumeration-syntax" accesskey="p" rel="prev">c99-like fast enumeration syntax</a>, Up: <a href="Fast-enumeration.html#Fast-enumeration" accesskey="u" rel="up">Fast enumeration</a> [<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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| <a name="Fast-enumeration-details-1"></a> |
| <h4 class="subsection">8.9.3 Fast enumeration details</h4> |
| |
| <p>Here is a more technical description with the gory details. Consider the code |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> for (<var>object expression</var> in <var>collection expression</var>) |
| { |
| <var>statements</var> |
| } |
| </pre></div> |
| |
| <p>here is what happens when you run it: |
| </p> |
| <ul> |
| <li> <code><var>collection expression</var></code> is evaluated exactly once and the |
| result is used as the collection object to iterate over. This means |
| it is safe to write code such as <code>for (object in [NSDictionary |
| keyEnumerator]) …</code>. |
| |
| </li><li> the iteration is implemented by the compiler by repeatedly getting |
| batches of objects from the collection object using the fast |
| enumeration protocol (see below), then iterating over all objects in |
| the batch. This is faster than a normal enumeration where objects are |
| retrieved one by one (hence the name “fast enumeration”). |
| |
| </li><li> if there are no objects in the collection, then |
| <code><var>object expression</var></code> is set to <code>nil</code> and the loop |
| immediately terminates. |
| |
| </li><li> if there are objects in the collection, then for each object in the |
| collection (in the order they are returned) <code><var>object expression</var></code> |
| is set to the object, then <code><var>statements</var></code> are executed. |
| |
| </li><li> <code><var>statements</var></code> can contain <code>break</code> and <code>continue</code> |
| commands, which will abort the iteration or skip to the next loop |
| iteration as expected. |
| |
| </li><li> when the iteration ends because there are no more objects to iterate |
| over, <code><var>object expression</var></code> is set to <code>nil</code>. This allows |
| you to determine whether the iteration finished because a <code>break</code> |
| command was used (in which case <code><var>object expression</var></code> will remain |
| set to the last object that was iterated over) or because it iterated |
| over all the objects (in which case <code><var>object expression</var></code> will be |
| set to <code>nil</code>). |
| |
| </li><li> <code><var>statements</var></code> must not make any changes to the collection |
| object; if they do, it is a hard error and the fast enumeration |
| terminates by invoking <code>objc_enumerationMutation</code>, a runtime |
| function that normally aborts the program but which can be customized |
| by Foundation libraries via <code>objc_set_mutation_handler</code> to do |
| something different, such as raising an exception. |
| |
| </li></ul> |
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