1QCache(3qt)                                                        QCache(3qt)
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NAME

6       QCache - Template class that provides a cache based on QString keys
7

SYNOPSIS

9       #include <qcache.h>
10
11       Inherits QPtrCollection.
12
13   Public Members
14       QCache ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )
15       ~QCache ()
16       int maxCost () const
17       int totalCost () const
18       void setMaxCost ( int m )
19       virtual uint count () const
20       uint size () const
21       bool isEmpty () const
22       virtual void clear ()
23       bool insert ( const QString & k, const type * d, int c = 1, int p = 0 )
24       bool remove ( const QString & k )
25       type * take ( const QString & k )
26       type * find ( const QString & k, bool ref = TRUE ) const
27       type * operator[] ( const QString & k ) const
28       void statistics () const
29
30   Important Inherited Members
31       bool autoDelete () const
32       void setAutoDelete ( bool enable )
33

DESCRIPTION

35       The QCache class is a template class that provides a cache based on
36       QString keys.
37
38       A cache is a least recently used (LRU) list of cache items. Each cache
39       item has a key and a certain cost. The sum of item costs, totalCost(),
40       never exceeds the maximum cache cost, maxCost(). If inserting a new
41       item would cause the total cost to exceed the maximum cost, the least
42       recently used items in the cache are removed.
43
44       QCache is a template class. QCache<X> defines a cache that operates on
45       pointers to X, or X*.
46
47       Apart from insert(), by far the most important function is find()
48       (which also exists as operator[]()). This function looks up an item,
49       returns it, and by default marks it as being the most recently used
50       item.
51
52       There are also methods to remove() or take() an object from the cache.
53       Calling setAutoDelete(TRUE) for a cache tells it to delete items that
54       are removed. The default is to not delete items when they are removed
55       (i.e., remove() and take() are equivalent).
56
57       When inserting an item into the cache, only the pointer is copied, not
58       the item itself. This is called a shallow copy. It is possible to make
59       the cache copy all of the item's data (known as a deep copy) when an
60       item is inserted. insert() calls the virtual function
61       QPtrCollection::newItem() for the item to be inserted. Inherit a cache
62       and reimplement newItem() if you want deep copies.
63
64       When removing a cache item, the virtual function
65       QPtrCollection::deleteItem() is called. The default implementation
66       deletes the item if auto-deletion is enabled, and does nothing
67       otherwise.
68
69       There is a QCacheIterator that can be used to traverse the items in the
70       cache in arbitrary order.
71
72       In QCache, the cache items are accessed via QString keys, which are
73       Unicode strings. If you want to use non-Unicode, plain 8-bit char*
74       keys, use the QAsciiCache template. A QCache has the same performance
75       as a QAsciiCache.
76
77       See also QCacheIterator, QAsciiCache, QIntCache, Collection Classes,
78       and Non-GUI Classes.
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MEMBER FUNCTION DOCUMENTATION

QCache::QCache ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )

82
83       Constructs a cache whose contents will never have a total cost greater
84       than maxCost and which is expected to contain less than size items.
85
86       size is actually the size of an internal hash array; it's usually best
87       to make it a prime number and at least 50% bigger than the largest
88       expected number of items in the cache.
89
90       Each inserted item has an associated cost. When inserting a new item,
91       if the total cost of all items in the cache will exceed maxCost, the
92       cache will start throwing out the older (least recently used) items
93       until there is enough room for the new item to be inserted.
94
95       If caseSensitive is TRUE (the default), the cache keys are case
96       sensitive; if it is FALSE, they are case-insensitive. Case-insensitive
97       comparison considers all Unicode letters.
98

QCache::~QCache ()

100       Removes all items from the cache and destroys it. All iterators that
101       access this cache will be reset.
102

bool QPtrCollection::autoDelete () const

104       Returns the setting of the auto-delete option. The default is FALSE.
105
106       See also setAutoDelete().
107

void QCache::clear () [virtual]

109       Removes all items from the cache and deletes them if auto-deletion has
110       been enabled.
111
112       All cache iterators that operate this on cache are reset.
113
114       See also remove() and take().
115
116       Reimplemented from QPtrCollection.
117

uint QCache::count () const [virtual]

119       Returns the number of items in the cache.
120
121       See also totalCost().
122
123       Reimplemented from QPtrCollection.
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type * QCache::find ( const QString & k, bool ref = TRUE ) const

126       Returns the item associated with key k, or 0 if the key does not exist
127       in the cache. If ref is TRUE (the default), the item is moved to the
128       front of the least recently used list.
129
130       If there are two or more items with equal keys, the one that was
131       inserted last is returned.
132

bool QCache::insert ( const QString & k, const type * d, int c = 1, int p = 0

134       )
135       Inserts the item d into the cache with key k and associated cost, c.
136       Returns TRUE if it is successfully inserted; otherwise returns FALSE.
137
138       The cache's size is limited, and if the total cost is too high, QCache
139       will remove old, least recently used items until there is room for this
140       new item.
141
142       The parameter p is internal and should be left at the default value
143       (0).
144
145       Warning: If this function returns FALSE (which could happen, e.g. if
146       the cost of this item alone exceeds maxCost()) you must delete d
147       yourself. Additionally, be very careful about using d after calling
148       this function because any other insertions into the cache, from
149       anywhere in the application or within Qt itself, could cause the object
150       to be discarded from the cache and the pointer to become invalid.
151

bool QCache::isEmpty () const

153       Returns TRUE if the cache is empty; otherwise returns FALSE.
154

int QCache::maxCost () const

156       Returns the maximum allowed total cost of the cache.
157
158       See also setMaxCost() and totalCost().
159

type * QCache::operator[] ( const QString & k ) const

161       Returns the item associated with key k, or 0 if k does not exist in the
162       cache, and moves the item to the front of the least recently used list.
163
164       If there are two or more items with equal keys, the one that was
165       inserted last is returned.
166
167       This is the same as find( k, TRUE ).
168
169       See also find().
170

bool QCache::remove ( const QString & k )

172       Removes the item associated with k, and returns TRUE if the item was
173       present in the cache; otherwise returns FALSE.
174
175       The item is deleted if auto-deletion has been enabled, i.e., if you
176       have called setAutoDelete(TRUE).
177
178       If there are two or more items with equal keys, the one that was
179       inserted last is removed.
180
181       All iterators that refer to the removed item are set to point to the
182       next item in the cache's traversal order.
183
184       See also take() and clear().
185

void QPtrCollection::setAutoDelete ( bool enable )

187       Sets the collection to auto-delete its contents if enable is TRUE and
188       to never delete them if enable is FALSE.
189
190       If auto-deleting is turned on, all the items in a collection are
191       deleted when the collection itself is deleted. This is convenient if
192       the collection has the only pointer to the items.
193
194       The default setting is FALSE, for safety. If you turn it on, be careful
195       about copying the collection - you might find yourself with two
196       collections deleting the same items.
197
198       Note that the auto-delete setting may also affect other functions in
199       subclasses. For example, a subclass that has a remove() function will
200       remove the item from its data structure, and if auto-delete is enabled,
201       will also delete the item.
202
203       See also autoDelete().
204
205       Examples:
206

void QCache::setMaxCost ( int m )

208       Sets the maximum allowed total cost of the cache to m. If the current
209       total cost is greater than m, some items are deleted immediately.
210
211       See also maxCost() and totalCost().
212

uint QCache::size () const

214       Returns the size of the hash array used to implement the cache. This
215       should be a bit bigger than count() is likely to be.
216

void QCache::statistics () const

218       A debug-only utility function. Prints out cache usage, hit/miss, and
219       distribution information using qDebug(). This function does nothing in
220       the release library.
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type * QCache::take ( const QString & k )

223       Takes the item associated with k out of the cache without deleting it,
224       and returns a pointer to the item taken out, or 0 if the key does not
225       exist in the cache.
226
227       If there are two or more items with equal keys, the one that was
228       inserted last is taken.
229
230       All iterators that refer to the taken item are set to point to the next
231       item in the cache's traversal order.
232
233       See also remove() and clear().
234

int QCache::totalCost () const

236       Returns the total cost of the items in the cache. This is an integer in
237       the range 0 to maxCost().
238
239       See also setMaxCost().
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241

SEE ALSO

243       http://doc.trolltech.com/qcache.html
244       http://www.trolltech.com/faq/tech.html
245
247       Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com.  See the
248       license file included in the distribution for a complete license
249       statement.
250

AUTHOR

252       Generated automatically from the source code.
253

BUGS

255       If you find a bug in Qt, please report it as described in
256       http://doc.trolltech.com/bughowto.html.  Good bug reports help us to
257       help you. Thank you.
258
259       The definitive Qt documentation is provided in HTML format; it is
260       located at $QTDIR/doc/html and can be read using Qt Assistant or with a
261       web browser. This man page is provided as a convenience for those users
262       who prefer man pages, although this format is not officially supported
263       by Trolltech.
264
265       If you find errors in this manual page, please report them to qt-
266       bugs@trolltech.com.  Please include the name of the manual page
267       (qcache.3qt) and the Qt version (3.3.8).
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271Trolltech AS                    2 February 2007                    QCache(3qt)
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