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+<div class="header">
+<p>
+Previous: <a href="Fast-enumeration-details.html#Fast-enumeration-details" accesskey="p" rel="previous">Fast enumeration details</a>, Up: <a href="Fast-enumeration.html#Fast-enumeration" accesskey="u" rel="up">Fast enumeration</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
+</div>
+<hr>
+<a name="Fast-Enumeration-Protocol"></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&rsquo;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="previous">Fast enumeration details</a>, Up: <a href="Fast-enumeration.html#Fast-enumeration" accesskey="u" rel="up">Fast enumeration</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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