You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
141 lines
6.7 KiB
HTML
141 lines
6.7 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 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. -->
|
||
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Fast enumeration details (Using the GNU Compiler Collection (GCC))</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Fast enumeration details (Using the GNU Compiler Collection (GCC))">
|
||
|
|
<meta name="keywords" content="Fast enumeration details (Using the GNU Compiler Collection (GCC))">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Fast-enumeration.html#Fast-enumeration" rel="up" title="Fast enumeration">
|
||
|
|
<link href="Fast-enumeration-protocol.html#Fast-enumeration-protocol" rel="next" title="Fast enumeration protocol">
|
||
|
|
<link href="c99_002dlike-fast-enumeration-syntax.html#c99_002dlike-fast-enumeration-syntax" rel="prev" title="c99-like fast enumeration syntax">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Fast-enumeration-details"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="Fast-Enumeration-Details"></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>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|