QCache Class Reference
The QCache class is a template class that provides a cache based on QString keys.
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#include <qcache.h>
Inherits QGCache.
List of all member functions.
Public Members
Detailed 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).
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 Documentation
QCache::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.
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.
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.
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.
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