| <html lang="en"> |
| <head> |
| <title>Fast enumeration details - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Fast-enumeration.html#Fast-enumeration" title="Fast enumeration"> |
| <link rel="prev" href="c99_002dlike-fast-enumeration-syntax.html#c99_002dlike-fast-enumeration-syntax" title="c99-like fast enumeration syntax"> |
| <link rel="next" href="Fast-enumeration-protocol.html#Fast-enumeration-protocol" title="Fast enumeration protocol"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988-2013 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Fast-enumeration-details"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Fast-enumeration-protocol.html#Fast-enumeration-protocol">Fast enumeration protocol</a>, |
| Previous: <a rel="previous" accesskey="p" href="c99_002dlike-fast-enumeration-syntax.html#c99_002dlike-fast-enumeration-syntax">c99-like fast enumeration syntax</a>, |
| Up: <a rel="up" accesskey="u" href="Fast-enumeration.html#Fast-enumeration">Fast enumeration</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">8.9.3 Fast enumeration details</h4> |
| |
| <p>Here is a more technical description with the gory details. Consider the code |
| |
| <pre class="smallexample"> for (<var>object expression</var> in <var>collection expression</var>) |
| { |
| <var>statements</var> |
| } |
| </pre> |
| <p>here is what happens when you run it: |
| |
| <ul> |
| <li><var>collection expression</var> 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>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>if there are no objects in the collection, then |
| <var>object expression</var> is set to <code>nil</code> and the loop |
| immediately terminates. |
| |
| <li>if there are objects in the collection, then for each object in the |
| collection (in the order they are returned) <var>object expression</var> |
| is set to the object, then <var>statements</var> are executed. |
| |
| <li><var>statements</var> 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>when the iteration ends because there are no more objects to iterate |
| over, <var>object expression</var> 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 <var>object expression</var> will remain |
| set to the last object that was iterated over) or because it iterated |
| over all the objects (in which case <var>object expression</var> will be |
| set to <code>nil</code>). |
| |
| <li><var>statements</var> 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. |
| |
| </ul> |
| |
| <!-- ================================ --> |
| </body></html> |
| |
