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