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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\class QCache
\inmodule QtCore
\brief The QCache class is a template class that provides a cache.
\ingroup tools
\ingroup shared
\reentrant
QCache\<Key, T\> defines a cache that stores objects of type T
associated with keys of type Key. For example, here's the
definition of a cache that stores objects of type Employee
associated with an integer key:
\snippet code/doc_src_qcache.cpp 0
Here's how to insert an object in the cache:
\snippet code/doc_src_qcache.cpp 1
The advantage of using QCache over some other key-based data
structure (such as QMap or QHash) is that QCache automatically
takes ownership of the objects that are inserted into the cache and
deletes them to make room for new objects, if necessary. When
inserting an object into the cache, you can specify a \e{cost},
which should bear some approximate relationship to the amount of
memory taken by the object. When the sum of all objects' costs
(totalCost()) exceeds the cache's limit (maxCost()), QCache starts
deleting objects in the cache to keep under the limit, starting with
less recently accessed objects.
By default, QCache's maxCost() is 100. You can specify a
different value in the QCache constructor:
\snippet code/doc_src_qcache.cpp 2
Each time you call insert(), you can specify a cost as third
argument (after the key and a pointer to the object to insert).
After the call, the inserted object is owned by the QCache, which
may delete it at any time to make room for other objects.
To look up objects in the cache, use object() or
operator[](). This function looks up an object by its key, and
returns either a pointer to the cached object (which is owned by
the cache) or \nullptr.
If you want to remove an object from the cache for a particular key,
call remove(). This will also delete the object. If you want to
remove an object from the cache without the QCache deleting it, use
take().
\sa QPixmapCache, QHash, QMap
*/
/*! \fn template <class Key, class T> QCache<Key, T>::QCache(qsizetype maxCost = 100)
Constructs a cache whose contents will never have a total cost
greater than \a maxCost.
*/
/*! \fn template <class Key, class T> QCache<Key, T>::~QCache()
Destroys the cache. Deletes all the objects in the cache.
*/
/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::maxCost() const
Returns the maximum allowed total cost of the cache.
\sa setMaxCost(), totalCost()
*/
/*! \fn template <class Key, class T> void QCache<Key, T>::setMaxCost(qsizetype cost)
Sets the maximum allowed total cost of the cache to \a cost. If
the current total cost is greater than \a cost, some objects are
deleted immediately.
\sa maxCost(), totalCost()
*/
/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::totalCost() const
Returns the total cost of the objects in the cache.
This value is normally below maxCost(), but QCache makes an
exception for Qt's \l{implicitly shared} classes. If a cached
object shares its internal data with another instance, QCache may
keep the object lying around, possibly contributing to making
totalCost() larger than maxCost().
\sa setMaxCost()
*/
/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::size() const
Returns the number of objects in the cache.
\sa isEmpty()
*/
/*! \fn template <class Key, class T> qsizetype QCache<Key, T>::count() const
Same as size().
*/
/*! \fn template <class Key, class T> bool QCache<Key, T>::isEmpty() const
Returns \c true if the cache contains no objects; otherwise
returns \c false.
\sa size()
*/
/*! \fn template <class Key, class T> QList<Key> QCache<Key, T>::keys() const
Returns a list of the keys in the cache.
*/
/*! \fn template <class Key, class T> void QCache<Key, T>::clear();
Deletes all the objects in the cache.
\sa remove(), take()
*/
/*! \fn template <class Key, class T> bool QCache<Key, T>::insert(const Key &key, T *object, qsizetype cost = 1)
Inserts \a object into the cache with key \a key and
associated cost \a cost. Any object with the same key already in
the cache will be removed.
After this call, \a object is owned by the QCache and may be
deleted at any time. In particular, if \a cost is greater than
maxCost(), the object will be deleted immediately.
The function returns \c true if the object was inserted into the
cache; otherwise it returns \c false.
\sa take(), remove()
*/
/*! \fn template <class Key, class T> T *QCache<Key, T>::object(const Key &key) const
Returns the object associated with key \a key, or \nullptr if the key does
not exist in the cache.
\warning The returned object is owned by QCache and may be
deleted at any time.
\sa take(), remove()
*/
/*! \fn template <class Key, class T> bool QCache<Key, T>::contains(const Key &key) const
Returns \c true if the cache contains an object associated with key \a
key; otherwise returns \c false.
\sa take(), remove()
*/
/*! \fn template <class Key, class T> T *QCache<Key, T>::operator[](const Key &key) const
Returns the object associated with key \a key, or \nullptr if the key does
not exist in the cache.
This is the same as object().
\warning The returned object is owned by QCache and may be
deleted at any time.
*/
/*! \fn template <class Key, class T> bool QCache<Key, T>::remove(const Key &key)
Deletes the object associated with key \a key. Returns \c true if the
object was found in the cache; otherwise returns \c false.
\sa take(), clear()
*/
/*! \fn template <class Key, class T> T *QCache<Key, T>::take(const Key &key)
Takes the object associated with key \a key out of the cache
without deleting it. Returns a pointer to the object taken out, or
0 if the key does not exist in the cache.
The ownership of the returned object is passed to the caller.
\sa remove()
*/
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