Qt logo

QCache Class Reference


The QCache class is a template class that provides a cache based on QString keys. More...

#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.

QCache is implemented as a template class. Define a template instance QCache<X> to create a cache that operates on pointers to X, or X*.

A cache is a least recently used (LRU) list of cache items. 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.

Each cache item has a cost. The sum of item costs, totalCost(), will not exceed 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.

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. It's default implementation in QCache is to delete the item if auto-deletion is enabled.

There is a QCacheIterator which may be used to traverse the items in the cache in arbitrary order.

See also: QCacheIterator, QAsciiCache, QIntCache and Collection Classes


Member Function Documentation

QCache::QCache(constQCache<type>&c)

For internal use only.

QCache::QCache(intmaxCost=100, intsize=17, boolcaseSensitive=TRUE)

Constructs a cache with the following properties:

Arguments:

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.

voidQCache::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.

uintQCache::count()const [virtual]

Returns the number of items in the cache.

See also: totalCost().

Reimplemented from QCollection.

type*QCache::find(constQString&k, boolref=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.

boolQCache::insert(constQString&k, consttype*d, intc=1, intp=0)

Inserts the item d into the cache with key k, cost and priority p, and 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.

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.

boolQCache::isEmpty()const

Returns TRUE if the cache is empty, or TRUE if there is at least one object in it.

intQCache::maxCost()const

Returns the maximum allowed total cost of the cache.

See also: setMaxCost() and totalCost().

QCache<type>&QCache::operator=(constQCache<type>&c)

For internal use only.

type*QCache::operator[](constQString&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().

boolQCache::remove(constQString&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().

voidQCache::setMaxCost(intm)

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().

uintQCache::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.

type*QCache::take(constQString&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().

intQCache::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().


Search the documentation, FAQ, qt-interest archive and more (uses www.troll.no):


This file is part of the Qt toolkit, copyright © 1995-99 Troll Tech, all rights reserved.


Copyright 1999 Troll TechTrademarks
Qt version 2.0.2