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120 lines
5.6 KiB
HTML
120 lines
5.6 KiB
HTML
4 years ago
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<title>Fast enumeration protocol - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Fast-enumeration.html#Fast-enumeration" title="Fast enumeration">
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<link rel="prev" href="Fast-enumeration-details.html#Fast-enumeration-details" title="Fast enumeration details">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Fast-enumeration-protocol"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Fast-enumeration-details.html#Fast-enumeration-details">Fast enumeration details</a>,
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Up: <a rel="up" accesskey="u" href="Fast-enumeration.html#Fast-enumeration">Fast enumeration</a>
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<hr>
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</div>
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<h4 class="subsection">8.9.4 Fast Enumeration Protocol</h4>
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<p>If you want your own collection object to be usable with fast
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enumeration, you need to have it implement the method
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<pre class="smallexample"> - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state
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objects: (id *)objects
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count: (unsigned long)len;
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</pre>
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<p>where <code>NSFastEnumerationState</code> must be defined in your code as follows:
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<pre class="smallexample"> typedef struct
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{
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unsigned long state;
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id *itemsPtr;
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unsigned long *mutationsPtr;
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unsigned long extra[5];
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} NSFastEnumerationState;
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</pre>
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<p>If no <code>NSFastEnumerationState</code> is defined in your code, the
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compiler will automatically replace <code>NSFastEnumerationState *</code>
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with <code>struct __objcFastEnumerationState *</code>, where that type is
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silently defined by the compiler in an identical way. This can be
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confusing and we recommend that you define
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<code>NSFastEnumerationState</code> (as shown above) instead.
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<p>The method is called repeatedly during a fast enumeration to retrieve
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batches of objects. Each invocation of the method should retrieve the
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next batch of objects.
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<p>The return value of the method is the number of objects in the current
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batch; this should not exceed <code>len</code>, which is the maximum size of
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a batch as requested by the caller. The batch itself is returned in
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the <code>itemsPtr</code> field of the <code>NSFastEnumerationState</code> struct.
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<p>To help with returning the objects, the <code>objects</code> array is a C
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array preallocated by the caller (on the stack) of size <code>len</code>.
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In many cases you can put the objects you want to return in that
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<code>objects</code> array, then do <code>itemsPtr = objects</code>. But you
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don't have to; if your collection already has the objects to return in
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some form of C array, it could return them from there instead.
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<p>The <code>state</code> and <code>extra</code> fields of the
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<code>NSFastEnumerationState</code> structure allows your collection object
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to keep track of the state of the enumeration. In a simple array
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implementation, <code>state</code> may keep track of the index of the last
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object that was returned, and <code>extra</code> may be unused.
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<p>The <code>mutationsPtr</code> field of the <code>NSFastEnumerationState</code> is
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used to keep track of mutations. It should point to a number; before
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working on each object, the fast enumeration loop will check that this
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number has not changed. If it has, a mutation has happened and the
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fast enumeration will abort. So, <code>mutationsPtr</code> could be set to
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point to some sort of version number of your collection, which is
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increased by one every time there is a change (for example when an
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object is added or removed). Or, if you are content with less strict
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mutation checks, it could point to the number of objects in your
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collection or some other value that can be checked to perform an
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approximate check that the collection has not been mutated.
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<p>Finally, note how we declared the <code>len</code> argument and the return
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value to be of type <code>unsigned long</code>. They could also be declared
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to be of type <code>unsigned int</code> and everything would still work.
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<!-- ========================================================================= -->
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</body></html>
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