The QCache class is a template class that provides a cache based on QString keys.
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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
they 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 cache copy all of the item's data (known as a deep copy) when an item is inserted. insert() calls the virtual
function QPtrCollection::newItem() for the item to be inserted.
Inherit a cache and reimplement newItem() if you want deep copies.
When removing a cache item, the virtual function
QPtrCollection::deleteItem() is called. The default
implementation deletes the item if auto-deletion is enabled, and
does nothing otherwise.
There is a QCacheIterator that can 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, Collection Classes, and Non-GUI Classes.
Member Function Documentation
QCache::QCache ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )
Constructs a cache whose contents will never have a total cost
greater than maxCost and which is expected to contain less than
size items.
size is actually the size of an internal hash array; it's
usually best to make it a prime number and at least 50% bigger
than the largest expected number of items in the cache.
Each inserted item has an associated cost. When inserting a new
item, if the total cost of all items in the cache will exceed maxCost, the cache will start throwing out the older (least
recently used) items until there is enough room for the new item
to be inserted.
If caseSensitive is TRUE (the default), the cache keys are case
sensitive; if it is FALSE, they are case-insensitive.
Case-insensitive comparison considers all Unicode letters.
QCache::~QCache ()
Removes all items from the cache and destroys it. All iterators
that access this cache will be reset.
bool QPtrCollection::autoDelete () const
Returns the setting of the auto-delete option. The default is FALSE.
See also setAutoDelete().
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 QPtrCollection.
uint QCache::count () const [virtual]
Returns the number of items in the cache.
See also totalCost().
Reimplemented from QPtrCollection.
type * QCache::find ( const QString & k, bool ref = TRUE ) const
Returns the item associated with key k, or 0 if the key does
not exist in the cache. If ref is TRUE (the default), the item
is moved to the front of the least recently used list.
If there are two or more items with equal keys, 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 associated
cost, c. Returns TRUE if it is successfully inserted; otherwise
returns FALSE.
The cache's size is limited, and if the total cost is too high,
QCache will remove old, least recently 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 (which could happen, e.g.
if the cost of this item alone exceeds maxCost()) you must delete
d yourself. Additionally, be very careful about using d
after calling this function because 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; otherwise returns FALSE.
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 key k, or 0 if k does not
exist in the cache, and moves the item to the front of the least
recently used list.
If there are two or more items with equal keys, 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; otherwise returns FALSE.
The item is deleted if auto-deletion has been enabled, i.e., if
you have called setAutoDelete(TRUE).
If there are two or more items with equal keys, the one that was
inserted last 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 QPtrCollection::setAutoDelete ( bool enable )
Sets the collection to auto-delete its contents if enable is
TRUE and to never delete them if enable is FALSE.
If auto-deleting is turned on, all the items in a collection are
deleted when the collection itself is deleted. This is convenient
if the collection has the only pointer to the items.
The default setting is FALSE, for safety. If you turn it on, be
careful about copying the collection - you might find yourself
with two collections deleting the same items.
Note that the auto-delete setting may also affect other functions
in subclasses. For example, a subclass that has a remove()
function will remove the item from its data structure, and if
auto-delete is enabled, will also delete the item.
See also autoDelete().
Examples: grapher/grapher.cpp, scribble/scribble.cpp, and table/bigtable/main.cpp.
void QCache::setMaxCost ( int m )
Sets the maximum allowed total cost of the cache to m. If the
current total cost is greater than 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 0
if the key does not exist in the cache.
If there are two or more items with equal keys, 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().
This file is part of the Qt toolkit.
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