QAsciiCache Class Reference
The QAsciiCache class is a template class that provides a cache based on char* keys.
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#include <qasciicache.h>
Inherits QGCache.
List of all member functions.
Public Members
Detailed Description
The QAsciiCache class is a template class that provides a cache based on
char* keys.
QAsciiCache is implemented as a template class. Define a template
instance QAsciiCache<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 char* keys. QAsciiCache cannot handle
Unicode keys, instead use the QCache template, which uses QString
keys. 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 QAsciiCache is to delete the item if auto-deletion is enabled.
There is a QAsciiCacheIterator which may be used to traverse the
items in the cache in arbitrary order.
See also QAsciiCacheIterator, QCache, QIntCache and Collection Classes
Member Function Documentation
QAsciiCache::QAsciiCache ( int maxCost=100, int size=17, bool caseSensitive=TRUE, bool copyKeys=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.
- copyKeys specifies whether to copy the key strings.
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.
Setting copyKeys to TRUE will make the dictionary copy the key
when an item is inserted. Setting it to FALSE will make the dictionary
only use the pointer to the key.
QAsciiCache::~QAsciiCache ()
Removes all items from the cache and destroys it.
All iterators that access this cache will be reset.
void QAsciiCache::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 QAsciiCache::count () const [virtual]
Returns the number of items in the cache.
See also totalCost().
Reimplemented from QCollection.
type * QAsciiCache::find ( const char * 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 QAsciiCache::insert ( const char * 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,
QAsciiCache 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 QAsciiCache::isEmpty () const
Returns TRUE if the cache is empty, or TRUE if there is at least one
object in it.
int QAsciiCache::maxCost () const
Returns the maximum allowed total cost of the cache.
See also setMaxCost() and totalCost().
type * QAsciiCache::operator[] ( const char * 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 QAsciiCache::remove ( const char * 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 QAsciiCache::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 removed
immediately.
See also maxCost() and totalCost().
uint QAsciiCache::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 QAsciiCache::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 * QAsciiCache::take ( const char * 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 QAsciiCache::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().
QAsciiCache::QAsciiCache ( const QAsciiCache<type> & c )
For internal use only.
QAsciiCache<type>& QAsciiCache::operator= ( const QAsciiCache<type> & c )
For internal use only.
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