1<html lang="en"> 2<head> 3<title>Fast enumeration details - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Fast-enumeration.html#Fast-enumeration" title="Fast enumeration"> 9<link rel="prev" href="c99_002dlike-fast-enumeration-syntax.html#c99_002dlike-fast-enumeration-syntax" title="c99-like fast enumeration syntax"> 10<link rel="next" href="Fast-enumeration-protocol.html#Fast-enumeration-protocol" title="Fast enumeration protocol"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Funding Free Software'', the Front-Cover 19Texts being (a) (see below), and with the Back-Cover Texts being (b) 20(see below). 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Consider the code 59 60<pre class="smallexample"> for (<var>object expression</var> in <var>collection expression</var>) 61 { 62 <var>statements</var> 63 } 64</pre> 65 <p>here is what happens when you run it: 66 67 <ul> 68<li><var>collection expression</var> is evaluated exactly once and the 69result is used as the collection object to iterate over. This means 70it is safe to write code such as <code>for (object in [NSDictionary 71keyEnumerator]) ...</code>. 72 73 <li>the iteration is implemented by the compiler by repeatedly getting 74batches of objects from the collection object using the fast 75enumeration protocol (see below), then iterating over all objects in 76the batch. This is faster than a normal enumeration where objects are 77retrieved one by one (hence the name “fast enumeration”). 78 79 <li>if there are no objects in the collection, then 80<var>object expression</var> is set to <code>nil</code> and the loop 81immediately terminates. 82 83 <li>if there are objects in the collection, then for each object in the 84collection (in the order they are returned) <var>object expression</var> 85is set to the object, then <var>statements</var> are executed. 86 87 <li><var>statements</var> can contain <code>break</code> and <code>continue</code> 88commands, which will abort the iteration or skip to the next loop 89iteration as expected. 90 91 <li>when the iteration ends because there are no more objects to iterate 92over, <var>object expression</var> is set to <code>nil</code>. This allows 93you to determine whether the iteration finished because a <code>break</code> 94command was used (in which case <var>object expression</var> will remain 95set to the last object that was iterated over) or because it iterated 96over all the objects (in which case <var>object expression</var> will be 97set to <code>nil</code>). 98 99 <li><var>statements</var> must not make any changes to the collection 100object; if they do, it is a hard error and the fast enumeration 101terminates by invoking <code>objc_enumerationMutation</code>, a runtime 102function that normally aborts the program but which can be customized 103by Foundation libraries via <code>objc_set_mutation_handler</code> to do 104something different, such as raising an exception. 105 106 </ul> 107 108<!-- ================================ --> 109 </body></html> 110 111