QCache(3qt)QCache(3qt)NAMEQCache - Template class that provides a cache based on
#include <qcache.h>
Inherits QGCache.
Public Members
QCache ( const QCache<type> & c ) (internal)
QCache ( int maxCost=100, int size=17, bool
caseSensitive=TRUE )
~QCache ()
QCache<type>& operator= ( const QCache<type> & c )
(internal)
int maxCost () const
int totalCost () const
void setMaxCost ( int m )
virtual uint count () const
uint size () const
bool isEmpty () const
virtual void clear ()
bool insert ( const QString & k, const type * d, int c=1,
int p=0 )
bool remove ( const QString & k )
type* take ( const QString & k )
type* find ( const QString & k, bool ref=TRUE ) const
type* operator[] ( const QString & k ) const
void statistics () const
DESCRIPTION
The QCache class is a template class that provides a cache
based on QString keys.
A cache is a least recently used (LRU) list of cache
items. Each cache item has a key and a certain cost. The
sum of item costs, totalCost(), never exceeds the maximum
cache cost, maxCost(). If inserting a new item would cause
the total cost to exceed the maximum cost, the least
recently used items in the cache are removed.
QCache is a template class. QCache<X> defines a cache that
operates on pointers to X, or X*.
Apart from insert(), by far the most important function is
find() (which also exists as operator[]()). This function
looks up an item, returns it, and by default marks it as
being the most recently used item.
There are also methods to remove() or take() an object
from the cache. Calling setAutoDelete(TRUE) for a cache
tells it to delete items that are removed. The default is
to not delete items when then are removed (i.e. remove()
and take() are equivalent).
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When inserting an item into the cache, only the pointer is
copied, not the item itself. This is called a shallow
copy. It is possible to make the dictionary copy all of
the item's data (known as a deep copy) when an item is
inserted. insert() calls the virtual function
QCollection::newItem() for the item to be inserted.
Inherit a dictionary and reimplement it if you want deep
copies.
When removing a cache item, the virtual function
QCollection::deleteItem() is called. The default
implementation deletes the item if auto-deletion is
enabled, and does nothing otherwise.
There is a QCacheIterator which may be used to traverse
the items in the cache in arbitrary order.
In QCache, the cache items are accessed via QString keys,
which are Unicode strings. If you want to use non-Unicode,
plain 8-bit char* keys, use the QAsciiCache template. A
QCache has the same performace as a QAsciiCache.
See also QCacheIterator, QAsciiCache, QIntCache and
Collection Classes
MEMBER FUNCTION DOCUMENTATIONQCache::QCache ( int maxCost=100, int size=17, bool
caseSensitive=TRUE )
Constructs a cache with the following properties:
Arguments:
maxCost is the maximum allowed total cost.
size is the size of the internal hash array.
caseSensitive specifies whether to use case sensitive
lookup or not. Each inserted item is associated with a
cost. When inserting a new item, if the total cost of all
items in the cache will exceeds maxCost, the cache will
start throwing out the older (recently least used) items
until there is room enough for the new item to be
inserted.
Setting size to a suitably large prime number (equal to or
greater than the expected number of entries) makes the
hash distribution better and hence the loopup faster.
Setting caseSensitive to TRUE will treat "abc" and "Abc"
as different keys. Setting it to FALSE will make the
dictionary ignore case. Case insensitive comparison
includes the whole Unicode alphabeth.
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QCache(3qt)QCache(3qt)QCache::~QCache ()
Removes all items from the cache and destroys it. All
iterators that access this cache will be reset.
void QCache::clear () [virtual]
Removes all items from the cache, and deletes them if
auto-deletion has been enabled.
All cache iterators that operate this on cache are reset.
See also remove() and take().
Reimplemented from QCollection.
uint QCache::count () const [virtual]
Returns the number of items in the cache.
See also totalCost().
Reimplemented from QCollection.
type * QCache::find ( const QString & k, bool ref=TRUE ) const
Returns the item associated with k, or null if the key
does not exist in the cache. If ref is TRUE (the default),
the item is moved to the front of the LRU list.
If there are two or more items with equal keys, then the
one that was inserted last is returned.
bool QCache::insert ( const QString & k, const type * d, int c=1,
int p=0 )
Inserts the item d into the cache with key k and cost c.
Returns TRUE if it is successful and FALSE if it fails.
The cache's size is limited, and if the total cost is too
high, QCache will remove old, least-used items until there
is room for this new item.
The parameter p is internal and should be left at the
default value (0).
Warning: If this function returns FALSE, you must delete d
yourself. Additionally, be very careful about using d
after calling this function, as any other insertions into
the cache, from anywhere in the application, or within Qt
itself, could cause the object to be discarded from the
cache, and the pointer to become invalid.
bool QCache::isEmpty () const
Returns TRUE if the cache is empty, or FALSE if there is
at least one object in it.
int QCache::maxCost () const
Returns the maximum allowed total cost of the cache.
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QCache(3qt)QCache(3qt)
See also setMaxCost() and totalCost().
type * QCache::operator[] ( const QString & k ) const
Returns the item associated with k, or null if k does not
exist in the cache, and moves the item to the front of the
LRU list.
If there are two or more items with equal keys, then the
one that was inserted last is returned.
This is the same as find( k, TRUE ).
See also find().
bool QCache::remove ( const QString & k )
Removes the item associated with k, and returns TRUE if
the item was present in the cache or FALSE if it was not.
The item is deleted if auto-deletion has been enabled,
i.e. you have called setAutoDelete(TRUE).
If there are two or more items with equal keys, then the
one that was inserted last is is removed.
All iterators that refer to the removed item are set to
point to the next item in the cache's traversal order.
See also take() and clear().
void QCache::setMaxCost ( int m )
Sets the maximum allowed total cost of the cache to m. If
the current total cost is above m, some items are deleted
immediately.
See also maxCost() and totalCost().
uint QCache::size () const
Returns the size of the hash array used to implement the
cache. This should be a bit bigger than count() is likely
to be.
void QCache::statistics () const
A debug-only utility function. Prints out cache usage,
hit/miss, and distribution information using qDebug().
This function does nothing in the release library.
type * QCache::take ( const QString & k )
Takes the item associated with k out of the cache without
deleting it, and returns a pointer to the item taken out,
or null if the key does not exist in the cache.
If there are two or more items with equal keys, then the
one that was inserted last is taken.
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QCache(3qt)QCache(3qt)
All iterators that refer to the taken item are set to
point to the next item in the cache's traversal order.
See also remove() and clear().
int QCache::totalCost () const
Returns the total cost of the items in the cache. This is
an integer in the range 0 to maxCost().
See also setMaxCost().
QCache::QCache ( const QCache<;type> & c )
For internal use only.
QCache<;type>& QCache::operator= ( const QCache<type> & c )
For internal use only.
SEE ALSO
http://doc.trolltech.com/qcache.html
http://www.trolltech.com/faq/tech.html
COPYRIGHT
Copyright 1992-2001 Trolltech AS,
http://www.trolltech.com. See the license file included
in the distribution for a complete license statement.
AUTHOR
Generated automatically from the source code.
BUGS
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http://doc.trolltech.com/bughowto.html. Good bug reports
make our job much simpler. Thank you.
In case of content or formattting problems with this
manual page, please report them to qt-bugs@trolltech.com.
Please include the name of the manual page (qcache.3qt)
and the Qt version (2.3.1).
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