source: trunk/src/corelib/tools/qcache.qdoc@ 846

Last change on this file since 846 was 846, checked in by Dmitry A. Kuminov, 14 years ago

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1/****************************************************************************
2**
3** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
4** All rights reserved.
5** Contact: Nokia Corporation ([email protected])
6**
7** This file is part of the documentation of the Qt Toolkit.
8**
9** $QT_BEGIN_LICENSE:FDL$
10** Commercial Usage
11** Licensees holding valid Qt Commercial licenses may use this file in
12** accordance with the Qt Commercial License Agreement provided with the
13** Software or, alternatively, in accordance with the terms contained in a
14** written agreement between you and Nokia.
15**
16** GNU Free Documentation License
17** Alternatively, this file may be used under the terms of the GNU Free
18** Documentation License version 1.3 as published by the Free Software
19** Foundation and appearing in the file included in the packaging of this
20** file.
21**
22** If you have questions regarding the use of this file, please contact
23** Nokia at [email protected].
24** $QT_END_LICENSE$
25**
26****************************************************************************/
27
28/*!
29 \class QCache
30 \brief The QCache class is a template class that provides a cache.
31
32 \ingroup tools
33 \ingroup shared
34
35 \reentrant
36
37 QCache\<Key, T\> defines a cache that stores objects of type T
38 associated with keys of type Key. For example, here's the
39 definition of a cache that stores objects of type Employee
40 associated with an integer key:
41
42 \snippet doc/src/snippets/code/doc_src_qcache.qdoc 0
43
44 Here's how to insert an object in the cache:
45
46 \snippet doc/src/snippets/code/doc_src_qcache.qdoc 1
47
48 The advantage of using QCache over some other key-based data
49 structure (such as QMap or QHash) is that QCache automatically
50 takes ownership of the objects that are inserted into the cache and
51 deletes them to make room for new objects, if necessary. When
52 inserting an object into the cache, you can specify a \e{cost},
53 which should bear some approximate relationship to the amount of
54 memory taken by the object. When the sum of all objects' costs
55 (totalCost()) exceeds the cache's limit (maxCost()), QCache starts
56 deleting objects in the cache to keep under the limit, starting with
57 less recently accessed objects.
58
59 By default, QCache's maxCost() is 100. You can specify a
60 different value in the QCache constructor:
61
62 \snippet doc/src/snippets/code/doc_src_qcache.qdoc 2
63
64 Each time you call insert(), you can specify a cost as third
65 argument (after the key and a pointer to the object to insert).
66 After the call, the inserted object is owned by the QCache, which
67 may delete it at any time to make room for other objects.
68
69 To look up objects in the cache, use object() or
70 operator[](). This function looks up an object by its key, and
71 returns either a pointer to the cached object (which is owned by
72 the cache) or 0.
73
74 If you want to remove an object from the cache for a particular key,
75 call remove(). This will also delete the object. If you want to
76 remove an object from the cache without the QCache deleting it, use
77 take().
78
79 \sa QPixmapCache, QHash, QMap
80*/
81
82/*! \fn QCache::QCache(int maxCost = 100)
83
84 Constructs a cache whose contents will never have a total cost
85 greater than \a maxCost.
86*/
87
88/*! \fn QCache::~QCache()
89
90 Destroys the cache. Deletes all the objects in the cache.
91*/
92
93/*! \fn int QCache::maxCost() const
94
95 Returns the maximum allowed total cost of the cache.
96
97 \sa setMaxCost(), totalCost()
98*/
99
100/*! \fn void QCache::setMaxCost(int cost)
101
102 Sets the maximum allowed total cost of the cache to \a cost. If
103 the current total cost is greater than \a cost, some objects are
104 deleted immediately.
105
106 \sa maxCost(), totalCost()
107*/
108
109/*! \fn int QCache::totalCost() const
110
111 Returns the total cost of the objects in the cache.
112
113 This value is normally below maxCost(), but QCache makes an
114 exception for Qt's \l{implicitly shared} classes. If a cached
115 object shares its internal data with another instance, QCache may
116 keep the object lying around, possibly contributing to making
117 totalCost() larger than maxCost().
118
119 \sa setMaxCost()
120*/
121
122/*! \fn int QCache::size() const
123
124 Returns the number of objects in the cache.
125
126 \sa isEmpty()
127*/
128
129/*! \fn int QCache::count() const
130
131 Same as size().
132*/
133
134/*! \fn bool QCache::isEmpty() const
135
136 Returns true if the cache contains no objects; otherwise
137 returns false.
138
139 \sa size()
140*/
141
142/*! \fn QList<Key> QCache::keys() const
143
144 Returns a list of the keys in the cache.
145*/
146
147/*! \fn void QCache::clear();
148
149 Deletes all the objects in the cache.
150
151 \sa remove(), take()
152*/
153
154
155/*! \fn bool QCache::insert(const Key &key, T *object, int cost = 1)
156
157 Inserts \a object into the cache with key \a key and
158 associated cost \a cost. Any object with the same key already in
159 the cache will be removed.
160
161 After this call, \a object is owned by the QCache and may be
162 deleted at any time. In particular, if \a cost is greater than
163 maxCost(), the object will be deleted immediately.
164
165 The function returns true if the object was inserted into the
166 cache; otherwise it returns false.
167
168 \sa take(), remove()
169*/
170
171/*! \fn T *QCache::object(const Key &key) const
172
173 Returns the object associated with key \a key, or 0 if the key does
174 not exist in the cache.
175
176 \warning The returned object is owned by QCache and may be
177 deleted at any time.
178
179 \sa take(), remove()
180*/
181
182/*! \fn bool QCache::contains(const Key &key) const
183
184 Returns true if the cache contains an object associated with key \a
185 key; otherwise returns false.
186
187 \sa take(), remove()
188*/
189
190/*! \fn T *QCache::operator[](const Key &key) const
191
192 Returns the object associated with key \a key, or 0 if the key does
193 not exist in the cache.
194
195 This is the same as object().
196
197 \warning The returned object is owned by QCache and may be
198 deleted at any time.
199*/
200
201/*! \fn bool QCache::remove(const Key &key)
202
203 Deletes the object associated with key \a key. Returns true if the
204 object was found in the cache; otherwise returns false.
205
206 \sa take(), clear()
207*/
208
209/*! \fn T *QCache::take(const Key &key)
210
211 Takes the object associated with key \a key out of the cache
212 without deleting it. Returns a pointer to the object taken out, or
213 0 if the key does not exist in the cache.
214
215 The ownership of the returned object is passed to the caller.
216
217 \sa remove()
218*/
219
220/*!
221 \fn QCache::QCache(int maxCost, int dummy)
222
223 Use QCache(int) instead.
224*/
225
226/*!
227 \fn T *QCache::find(const Key &key) const
228
229 Use object() instead.
230*/
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