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, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). 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Consider the code 61 62<pre class="smallexample"> for (<var>object expression</var> in <var>collection expression</var>) 63 { 64 <var>statements</var> 65 } 66</pre> 67 <p>here is what happens when you run it: 68 69 <ul> 70<li><var>collection expression</var> is evaluated exactly once and the 71result is used as the collection object to iterate over. This means 72it is safe to write code such as <code>for (object in [NSDictionary 73keyEnumerator]) ...</code>. 74 75 <li>the iteration is implemented by the compiler by repeatedly getting 76batches of objects from the collection object using the fast 77enumeration protocol (see below), then iterating over all objects in 78the batch. This is faster than a normal enumeration where objects are 79retrieved one by one (hence the name “fast enumeration”). 80 81 <li>if there are no objects in the collection, then 82<var>object expression</var> is set to <code>nil</code> and the loop 83immediately terminates. 84 85 <li>if there are objects in the collection, then for each object in the 86collection (in the order they are returned) <var>object expression</var> 87is set to the object, then <var>statements</var> are executed. 88 89 <li><var>statements</var> can contain <code>break</code> and <code>continue</code> 90commands, which will abort the iteration or skip to the next loop 91iteration as expected. 92 93 <li>when the iteration ends because there are no more objects to iterate 94over, <var>object expression</var> is set to <code>nil</code>. This allows 95you to determine whether the iteration finished because a <code>break</code> 96command was used (in which case <var>object expression</var> will remain 97set to the last object that was iterated over) or because it iterated 98over all the objects (in which case <var>object expression</var> will be 99set to <code>nil</code>). 100 101 <li><var>statements</var> must not make any changes to the collection 102object; if they do, it is a hard error and the fast enumeration 103terminates by invoking <code>objc_enumerationMutation</code>, a runtime 104function that normally aborts the program but which can be customized 105by Foundation libraries via <code>objc_set_mutation_handler</code> to do 106something different, such as raising an exception. 107 108 </ul> 109 110<!-- ================================ --> 111 </body></html> 112 113