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280 lines
14 KiB
280 lines
14 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/qcache.doc:41 -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>QCache Class</title>
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<style type="text/css"><!--
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--></style>
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<body>
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<table border="0" cellpadding="0" cellspacing="0" width="100%">
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<tr bgcolor="#E5E5E5">
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<td valign=center>
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<a href="index.html">
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<font color="#004faf">Main Classes</font></a>
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<font color="#004faf">Annotated</font></a>
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<font color="#004faf">Grouped Classes</font></a>
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<font color="#004faf">Functions</font></a>
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</td>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>QCache Class Reference</h1>
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<p>The QCache class is a template class that provides a cache based on QString keys.
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<a href="#details">More...</a>
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<p><tt>#include <<a href="qcache-h.html">qcache.h</a>></tt>
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<p>Inherits <a href="qptrcollection.html">QPtrCollection</a>.
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<p><a href="qcache-members.html">List of all member functions.</a>
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<h2>Public Members</h2>
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<ul>
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<li class=fn><a href="#QCache-2"><b>QCache</b></a> ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )</li>
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<li class=fn><a href="#~QCache"><b>~QCache</b></a> ()</li>
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<li class=fn>int <a href="#maxCost"><b>maxCost</b></a> () const</li>
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<li class=fn>int <a href="#totalCost"><b>totalCost</b></a> () const</li>
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<li class=fn>void <a href="#setMaxCost"><b>setMaxCost</b></a> ( int m )</li>
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<li class=fn>virtual uint <a href="#count"><b>count</b></a> () const</li>
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<li class=fn>uint <a href="#size"><b>size</b></a> () const</li>
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<li class=fn>bool <a href="#isEmpty"><b>isEmpty</b></a> () const</li>
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<li class=fn>virtual void <a href="#clear"><b>clear</b></a> ()</li>
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<li class=fn>bool <a href="#insert"><b>insert</b></a> ( const QString & k, const type * d, int c = 1, int p = 0 )</li>
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<li class=fn>bool <a href="#remove"><b>remove</b></a> ( const QString & k )</li>
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<li class=fn>type * <a href="#take"><b>take</b></a> ( const QString & k )</li>
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<li class=fn>type * <a href="#find"><b>find</b></a> ( const QString & k, bool ref = TRUE ) const</li>
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<li class=fn>type * <a href="#operator[]"><b>operator[]</b></a> ( const QString & k ) const</li>
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<li class=fn>void <a href="#statistics"><b>statistics</b></a> () const</li>
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</ul>
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<h2>Important Inherited Members</h2>
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<ul>
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<li class=fn>bool <a href="#autoDelete"><b>autoDelete</b></a> () const</li>
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<li class=fn>void <a href="#setAutoDelete"><b>setAutoDelete</b></a> ( bool enable )</li>
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</ul>
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<hr><a name="details"></a><h2>Detailed Description</h2>
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The QCache class is a template class that provides a cache based on <a href="qstring.html">QString</a> keys.
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<p>
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<p>
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<p> A cache is a least recently used (LRU) list of cache items. Each
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cache item has a key and a certain cost. The sum of item costs,
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<a href="#totalCost">totalCost</a>(), never exceeds the maximum cache cost, <a href="#maxCost">maxCost</a>(). If
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inserting a new item would cause the total cost to exceed the
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maximum cost, the least recently used items in the cache are
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removed.
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<p> QCache is a template class. QCache<X> defines a cache that
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operates on pointers to X, or X*.
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<p> Apart from <a href="#insert">insert</a>(), by far the most important function is <a href="#find">find</a>()
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(which also exists as <a href="#operator[]">operator[]</a>()). This function looks up an
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item, returns it, and by default marks it as being the most
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recently used item.
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<p> There are also methods to <a href="#remove">remove</a>() or <a href="#take">take</a>() an object from the
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cache. Calling <a href="qptrcollection.html#setAutoDelete">setAutoDelete</a>(TRUE) for a cache tells it to delete
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items that are removed. The default is to not delete items when
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they are removed (i.e., remove() and take() are equivalent).
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<p> When inserting an item into the cache, only the pointer is copied,
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not the item itself. This is called a <a href="shclass.html#shallow-copy">shallow copy</a>. It is possible
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to make the cache copy all of the item's data (known as a <a href="shclass.html#deep-copy">deep copy</a>) when an item is inserted. insert() calls the virtual
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function <a href="qptrcollection.html#newItem">QPtrCollection::newItem</a>() for the item to be inserted.
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Inherit a cache and reimplement <a href="qptrcollection.html#newItem">newItem</a>() if you want deep copies.
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<p> When removing a cache item, the virtual function
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<a href="qptrcollection.html#deleteItem">QPtrCollection::deleteItem</a>() is called. The default
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implementation deletes the item if auto-deletion is enabled, and
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does nothing otherwise.
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<p> There is a <a href="qcacheiterator.html">QCacheIterator</a> that can be used to traverse the items
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in the cache in arbitrary order.
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<p> In QCache, the cache items are accessed via <a href="qstring.html">QString</a> keys, which
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are Unicode strings. If you want to use non-Unicode, plain 8-bit
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<tt>char*</tt> keys, use the <a href="qasciicache.html">QAsciiCache</a> template. A QCache has the
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same performance as a QAsciiCache.
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<p> <p>See also <a href="qcacheiterator.html">QCacheIterator</a>, <a href="qasciicache.html">QAsciiCache</a>, <a href="qintcache.html">QIntCache</a>, <a href="collection.html">Collection Classes</a>, and <a href="tools.html">Non-GUI Classes</a>.
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<hr><h2>Member Function Documentation</h2>
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<h3 class=fn><a name="QCache-2"></a>QCache::QCache ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )
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</h3>
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<p> Constructs a cache whose contents will never have a total cost
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greater than <em>maxCost</em> and which is expected to contain less than
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<em>size</em> items.
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<p> <em>size</em> is actually the size of an internal hash array; it's
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usually best to make it a <a href="primes.html#prime">prime</a> number and at least 50% bigger
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than the largest expected number of items in the cache.
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<p> Each inserted item has an associated cost. When inserting a new
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item, if the total cost of all items in the cache will exceed <em>maxCost</em>, the cache will start throwing out the older (least
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recently used) items until there is enough room for the new item
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to be inserted.
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<p> If <em>caseSensitive</em> is TRUE (the default), the cache keys are case
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sensitive; if it is FALSE, they are case-insensitive.
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Case-insensitive comparison considers all Unicode letters.
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<h3 class=fn><a name="~QCache"></a>QCache::~QCache ()
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</h3>
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<p> Removes all items from the cache and destroys it. All iterators
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that access this cache will be reset.
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<h3 class=fn>bool <a name="autoDelete"></a>QPtrCollection::autoDelete () const
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</h3>
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<p> Returns the setting of the auto-delete option. The default is FALSE.
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<p> <p>See also <a href="qptrcollection.html#setAutoDelete">setAutoDelete</a>().
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<h3 class=fn>void <a name="clear"></a>QCache::clear ()<tt> [virtual]</tt>
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</h3>
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<p> Removes all items from the cache and deletes them if auto-deletion
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has been enabled.
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<p> All cache iterators that operate this on cache are reset.
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<p> <p>See also <a href="#remove">remove</a>() and <a href="#take">take</a>().
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<p>Reimplemented from <a href="qptrcollection.html#clear">QPtrCollection</a>.
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<h3 class=fn>uint <a name="count"></a>QCache::count () const<tt> [virtual]</tt>
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</h3>
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<p> Returns the number of items in the cache.
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<p> <p>See also <a href="#totalCost">totalCost</a>().
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<p>Reimplemented from <a href="qptrcollection.html#count">QPtrCollection</a>.
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<h3 class=fn>type * <a name="find"></a>QCache::find ( const <a href="qstring.html">QString</a> & k, bool ref = TRUE ) const
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</h3>
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<p> Returns the item associated with key <em>k</em>, or 0 if the key does
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not exist in the cache. If <em>ref</em> is TRUE (the default), the item
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is moved to the front of the least recently used list.
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<p> If there are two or more items with equal keys, the one that was
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inserted last is returned.
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<h3 class=fn>bool <a name="insert"></a>QCache::insert ( const <a href="qstring.html">QString</a> & k, const type * d, int c = 1, int p = 0 )
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</h3>
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<p> Inserts the item <em>d</em> into the cache with key <em>k</em> and associated
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cost, <em>c</em>. Returns TRUE if it is successfully inserted; otherwise
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returns FALSE.
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<p> The cache's size is limited, and if the total cost is too high,
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QCache will remove old, least recently used items until there is
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room for this new item.
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<p> The parameter <em>p</em> is internal and should be left at the default
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value (0).
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<p> <b>Warning:</b> If this function returns FALSE (which could happen, e.g.
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if the cost of this item alone exceeds <a href="#maxCost">maxCost</a>()) you must delete
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<em>d</em> yourself. Additionally, be very careful about using <em>d</em>
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after calling this function because any other insertions into the
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cache, from anywhere in the application or within Qt itself, could
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cause the object to be discarded from the cache and the pointer to
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become invalid.
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<h3 class=fn>bool <a name="isEmpty"></a>QCache::isEmpty () const
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</h3>
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<p> Returns TRUE if the cache is empty; otherwise returns FALSE.
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<h3 class=fn>int <a name="maxCost"></a>QCache::maxCost () const
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</h3>
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<p> Returns the maximum allowed total cost of the cache.
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<p> <p>See also <a href="#setMaxCost">setMaxCost</a>() and <a href="#totalCost">totalCost</a>().
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<h3 class=fn>type * <a name="operator[]"></a>QCache::operator[] ( const <a href="qstring.html">QString</a> & k ) const
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</h3>
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<p> Returns the item associated with key <em>k</em>, or 0 if <em>k</em> does not
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exist in the cache, and moves the item to the front of the least
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recently used list.
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<p> If there are two or more items with equal keys, the one that was
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inserted last is returned.
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<p> This is the same as <a href="#find">find</a>( k, TRUE ).
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<p> <p>See also <a href="#find">find</a>().
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<h3 class=fn>bool <a name="remove"></a>QCache::remove ( const <a href="qstring.html">QString</a> & k )
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</h3>
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<p> Removes the item associated with <em>k</em>, and returns TRUE if the
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item was present in the cache; otherwise returns FALSE.
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<p> The item is deleted if auto-deletion has been enabled, i.e., if
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you have called <a href="qptrcollection.html#setAutoDelete">setAutoDelete</a>(TRUE).
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<p> If there are two or more items with equal keys, the one that was
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inserted last is removed.
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<p> All iterators that refer to the removed item are set to point to
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the next item in the cache's traversal order.
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<p> <p>See also <a href="#take">take</a>() and <a href="#clear">clear</a>().
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<h3 class=fn>void <a name="setAutoDelete"></a>QPtrCollection::setAutoDelete ( bool enable )
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</h3>
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<p> Sets the collection to auto-delete its contents if <em>enable</em> is
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TRUE and to never delete them if <em>enable</em> is FALSE.
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<p> If auto-deleting is turned on, all the items in a collection are
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deleted when the collection itself is deleted. This is convenient
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if the collection has the only pointer to the items.
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<p> The default setting is FALSE, for safety. If you turn it on, be
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careful about copying the collection - you might find yourself
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with two collections deleting the same items.
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<p> Note that the auto-delete setting may also affect other functions
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in subclasses. For example, a subclass that has a <a href="#remove">remove</a>()
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function will remove the item from its data structure, and if
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auto-delete is enabled, will also delete the item.
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<p> <p>See also <a href="qptrcollection.html#autoDelete">autoDelete</a>().
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<p>Examples: <a href="grapher-nsplugin-example.html#x2769">grapher/grapher.cpp</a>, <a href="scribble-example.html#x924">scribble/scribble.cpp</a>, and <a href="bigtable-example.html#x1291">table/bigtable/main.cpp</a>.
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<h3 class=fn>void <a name="setMaxCost"></a>QCache::setMaxCost ( int m )
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</h3>
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<p> Sets the maximum allowed total cost of the cache to <em>m</em>. If the
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current total cost is greater than <em>m</em>, some items are deleted
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immediately.
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<p> <p>See also <a href="#maxCost">maxCost</a>() and <a href="#totalCost">totalCost</a>().
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<h3 class=fn>uint <a name="size"></a>QCache::size () const
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</h3>
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<p> Returns the size of the hash array used to implement the cache.
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This should be a bit bigger than <a href="#count">count</a>() is likely to be.
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<h3 class=fn>void <a name="statistics"></a>QCache::statistics () const
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</h3>
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<p> A debug-only utility function. Prints out cache usage, hit/miss,
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and distribution information using <a href="qapplication.html#qDebug">qDebug</a>(). This function does
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nothing in the release library.
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<h3 class=fn>type * <a name="take"></a>QCache::take ( const <a href="qstring.html">QString</a> & k )
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</h3>
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<p> Takes the item associated with <em>k</em> out of the cache without
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deleting it, and returns a pointer to the item taken out, or 0
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if the key does not exist in the cache.
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<p> If there are two or more items with equal keys, the one that was
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inserted last is taken.
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<p> All iterators that refer to the taken item are set to point to the
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next item in the cache's traversal order.
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<p> <p>See also <a href="#remove">remove</a>() and <a href="#clear">clear</a>().
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<h3 class=fn>int <a name="totalCost"></a>QCache::totalCost () const
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</h3>
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<p> Returns the total cost of the items in the cache. This is an
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integer in the range 0 to <a href="#maxCost">maxCost</a>().
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<p> <p>See also <a href="#setMaxCost">setMaxCost</a>().
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<!-- eof -->
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<hr><p>
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This file is part of the <a href="index.html">Qt toolkit</a>.
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Copyright © 1995-2007
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<a href="http://www.trolltech.com/">Trolltech</a>. All Rights Reserved.<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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</html>
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