| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 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 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Using the GNU Compiler Collection (GCC): Fast enumeration protocol</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Fast enumeration protocol"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Fast enumeration protocol"> |
| <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="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime" rel="next" title="Messaging with the GNU Objective-C runtime"> |
| <link href="Fast-enumeration-details.html#Fast-enumeration-details" rel="prev" title="Fast enumeration details"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| 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.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Fast-enumeration-protocol"></a> |
| <div class="header"> |
| <p> |
| Previous: <a href="Fast-enumeration-details.html#Fast-enumeration-details" accesskey="p" rel="prev">Fast enumeration details</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-protocol-1"></a> |
| <h4 class="subsection">8.9.4 Fast enumeration protocol</h4> |
| |
| <p>If you want your own collection object to be usable with fast |
| enumeration, you need to have it implement the method |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">- (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state |
| objects: (id *)objects |
| count: (unsigned long)len; |
| </pre></div> |
| |
| <p>where <code>NSFastEnumerationState</code> must be defined in your code as follows: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">typedef struct |
| { |
| unsigned long state; |
| id *itemsPtr; |
| unsigned long *mutationsPtr; |
| unsigned long extra[5]; |
| } NSFastEnumerationState; |
| </pre></div> |
| |
| <p>If no <code>NSFastEnumerationState</code> is defined in your code, the |
| compiler will automatically replace <code>NSFastEnumerationState *</code> |
| with <code>struct __objcFastEnumerationState *</code>, where that type is |
| silently defined by the compiler in an identical way. This can be |
| confusing and we recommend that you define |
| <code>NSFastEnumerationState</code> (as shown above) instead. |
| </p> |
| <p>The method is called repeatedly during a fast enumeration to retrieve |
| batches of objects. Each invocation of the method should retrieve the |
| next batch of objects. |
| </p> |
| <p>The return value of the method is the number of objects in the current |
| batch; this should not exceed <code>len</code>, which is the maximum size of |
| a batch as requested by the caller. The batch itself is returned in |
| the <code>itemsPtr</code> field of the <code>NSFastEnumerationState</code> struct. |
| </p> |
| <p>To help with returning the objects, the <code>objects</code> array is a C |
| array preallocated by the caller (on the stack) of size <code>len</code>. |
| In many cases you can put the objects you want to return in that |
| <code>objects</code> array, then do <code>itemsPtr = objects</code>. But you |
| don’t have to; if your collection already has the objects to return in |
| some form of C array, it could return them from there instead. |
| </p> |
| <p>The <code>state</code> and <code>extra</code> fields of the |
| <code>NSFastEnumerationState</code> structure allows your collection object |
| to keep track of the state of the enumeration. In a simple array |
| implementation, <code>state</code> may keep track of the index of the last |
| object that was returned, and <code>extra</code> may be unused. |
| </p> |
| <p>The <code>mutationsPtr</code> field of the <code>NSFastEnumerationState</code> is |
| used to keep track of mutations. It should point to a number; before |
| working on each object, the fast enumeration loop will check that this |
| number has not changed. If it has, a mutation has happened and the |
| fast enumeration will abort. So, <code>mutationsPtr</code> could be set to |
| point to some sort of version number of your collection, which is |
| increased by one every time there is a change (for example when an |
| object is added or removed). Or, if you are content with less strict |
| mutation checks, it could point to the number of objects in your |
| collection or some other value that can be checked to perform an |
| approximate check that the collection has not been mutated. |
| </p> |
| <p>Finally, note how we declared the <code>len</code> argument and the return |
| value to be of type <code>unsigned long</code>. They could also be declared |
| to be of type <code>unsigned int</code> and everything would still work. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Previous: <a href="Fast-enumeration-details.html#Fast-enumeration-details" accesskey="p" rel="prev">Fast enumeration details</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> |