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1// Copyright (C) 2016 The Qt Company Ltd.
2// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
3
4/*!
5 \class QCache
6 \inmodule QtCore
7 \brief The QCache class is a template class that provides a cache.
8
9 \ingroup tools
10 \ingroup shared
11
12 \reentrant
13
14 QCache<Key, T> defines a cache that stores objects of type T
15 associated with keys of type Key. For example, here's the
16 definition of a cache that stores objects of type Employee
17 associated with an integer key:
18
19 \snippet code/doc_src_qcache.cpp 0
20
21 Here's how to insert an object in the cache:
22
23 \snippet code/doc_src_qcache.cpp 1
24
25 The advantage of using QCache over some other key-based data
26 structure (such as QMap or QHash) is that QCache automatically
27 takes ownership of the objects that are inserted into the cache and
28 deletes them to make room for new objects, if necessary. When
29 inserting an object into the cache, you can specify a \e{cost},
30 which should bear some approximate relationship to the amount of
31 memory taken by the object. When the sum of all objects' costs
32 (totalCost()) exceeds the cache's limit (maxCost()), QCache starts
33 deleting objects in the cache to keep under the limit, starting with
34 less recently accessed objects.
35
36 By default, QCache's maxCost() is 100. You can specify a
37 different value in the QCache constructor:
38
39 \snippet code/doc_src_qcache.cpp 2
40
41 Each time you call insert(), you can specify a cost as third
42 argument (after the key and a pointer to the object to insert).
43 After the call, the inserted object is owned by the QCache, which
44 may delete it at any time to make room for other objects.
45
46 To look up objects in the cache, use object() or
47 operator[](). This function looks up an object by its key, and
48 returns either a pointer to the cached object (which is owned by
49 the cache) or \nullptr.
50
51 If you want to remove an object from the cache for a particular key,
52 call remove(). This will also delete the object. If you want to
53 remove an object from the cache without the QCache deleting it, use
54 take().
55
56 \sa QPixmapCache, QHash, QMap
57*/
58
59/*! \fn template <class Key, class T> QCache<Key, T>::QCache(qsizetype maxCost = 100)
60
61 Constructs a cache whose contents will never have a total cost
62 greater than \a maxCost.
63*/
64
65/*! \fn template <class Key, class T> QCache<Key, T>::~QCache()
66
67 Destroys the cache. Deletes all the objects in the cache.
68*/
69
70/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::maxCost() const
71
72 Returns the maximum allowed total cost of the cache.
73
74 \sa setMaxCost(), totalCost()
75*/
76
77/*! \fn template <class Key, class T> void QCache<Key, T>::setMaxCost(qsizetype cost)
78
79 Sets the maximum allowed total cost of the cache to \a cost. If
80 the current total cost is greater than \a cost, some objects are
81 deleted immediately.
82
83 \sa maxCost(), totalCost()
84*/
85
86/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::totalCost() const
87
88 Returns the total cost of the objects in the cache.
89
90 This value is normally below maxCost(), but QCache makes an
91 exception for Qt's \l{implicitly shared} classes. If a cached
92 object shares its internal data with another instance, QCache may
93 keep the object lying around, possibly contributing to making
94 totalCost() larger than maxCost().
95
96 \sa setMaxCost()
97*/
98
99/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::size() const
100
101 Returns the number of objects in the cache.
102
103 \sa isEmpty()
104*/
105
106/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::count() const
107
108 Same as size().
109*/
110
111/*! \fn template <class Key, class T> bool QCache<Key, T>::isEmpty() const
112
113 Returns \c true if the cache contains no objects; otherwise
114 returns \c false.
115
116 \sa size()
117*/
118
119/*! \fn template <class Key, class T> QList<Key> QCache<Key, T>::keys() const
120
121 Returns a list of the keys in the cache.
122*/
123
124/*! \fn template <class Key, class T> void QCache<Key, T>::clear();
125
126 Deletes all the objects in the cache.
127
128 \sa remove(), take()
129*/
130
131
132/*! \fn template <class Key, class T> bool QCache<Key, T>::insert(const Key &key, T *object, qsizetype cost = 1)
133
134 Inserts \a object into the cache with key \a key and
135 associated cost \a cost. Any object with the same key already in
136 the cache will be removed.
137
138 After this call, \a object is owned by the QCache and may be
139 deleted at any time. In particular, if \a cost is greater than
140 maxCost(), the object will be deleted immediately.
141
142 The function returns \c true if the object was inserted into the
143 cache; otherwise it returns \c false.
144
145 \sa take(), remove()
146*/
147
148/*! \fn template <class Key, class T> T *QCache<Key, T>::object(const Key &key) const
149
150 Returns the object associated with key \a key, or \nullptr if the key does
151 not exist in the cache.
152
153 \warning The returned object is owned by QCache and may be
154 deleted at any time.
155
156 \sa take(), remove()
157*/
158
159/*! \fn template <class Key, class T> bool QCache<Key, T>::contains(const Key &key) const
160
161 Returns \c true if the cache contains an object associated with key \a
162 key; otherwise returns \c false.
163
164 \sa take(), remove()
165*/
166
167/*! \fn template <class Key, class T> T *QCache<Key, T>::operator[](const Key &key) const
168
169 Returns the object associated with key \a key, or \nullptr if the key does
170 not exist in the cache.
171
172 This is the same as object().
173
174 \warning The returned object is owned by QCache and may be
175 deleted at any time.
176*/
177
178/*! \fn template <class Key, class T> bool QCache<Key, T>::remove(const Key &key)
179
180 Deletes the object associated with key \a key. Returns \c true if the
181 object was found in the cache; otherwise returns \c false.
182
183 \sa take(), clear()
184*/
185
186/*! \fn template <class Key, class T> T *QCache<Key, T>::take(const Key &key)
187
188 Takes the object associated with key \a key out of the cache
189 without deleting it. Returns a pointer to the object taken out, or
190 0 if the key does not exist in the cache.
191
192 The ownership of the returned object is passed to the caller.
193
194 \sa remove()
195*/