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.
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QCache is a template class. QCache<X> defines a cache that operates on pointers to X, or X*.
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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.
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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 TQPtrCollection::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 TQPtrCollection::deleteItem() is called. The default implementation deletes the item if auto-deletion is enabled, and does nothing otherwise.
In QCache, the cache items are accessed via TQString keys, which are Unicode strings. If you want to use non-Unicode, plain 8-bit \fCchar*\fR keys, use the QAsciiCache template. A QCache has the same performance as a QAsciiCache.
See also QCacheIterator, QAsciiCache, QIntCache, Collection Classes, and Non-GUI Classes.
.SH MEMBER FUNCTION DOCUMENTATION
.SH "QCache::QCache ( int maxCost = 100, int size = 17, bool caseSensitive = TRUE )"
Constructs a cache whose contents will never have a total cost greater than \fImaxCost\fR and which is expected to contain less than \fIsize\fR items.
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\fIsize\fR 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.
.PP
Each inserted item has an associated cost. When inserting a new item, if the total cost of all items in the cache will exceed \fImaxCost\fR, the cache will start throwing out the older (least recently used) items until there is enough room for the new item to be inserted.
.PP
If \fIcaseSensitive\fR 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.
.SH "QCache::~QCache ()"
Removes all items from the cache and destroys it. All iterators that access this cache will be reset.
Returns the item associated with key \fIk\fR, or 0 if the key does not exist in the cache. If \fIref\fR is TRUE (the default), the item is moved to the front of the least recently used list.
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If there are two or more items with equal keys, the one that was inserted last is returned.
Inserts the item \fId\fR into the cache with key \fIk\fR and associated cost, \fIc\fR. Returns TRUE if it is successfully inserted; otherwise returns FALSE.
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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.
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The parameter \fIp\fR is internal and should be left at the default value (0).
\fBWarning:\fR If this function returns FALSE (which could happen, e.g. if the cost of this item alone exceeds maxCost()) you must delete \fId\fR yourself. Additionally, be very careful about using \fId\fR after calling this function because any other insertions into the cache, from anywhere in the application or within TQt itself, could cause the object to be discarded from the cache and the pointer to become invalid.
Returns the item associated with key \fIk\fR, or 0 if \fIk\fR does not exist in the cache, and moves the item to the front of the least recently used list.
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If there are two or more items with equal keys, the one that was inserted last is returned.
Sets the collection to auto-delete its contents if \fIenable\fR is TRUE and to never delete them if \fIenable\fR is FALSE.
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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.
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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.
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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.
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See also autoDelete().
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Examples:
.)l grapher/grapher.cpp, scribble/scribble.cpp, and table/bigtable/main.cpp.
.SH "void QCache::setMaxCost ( int m )"
Sets the maximum allowed total cost of the cache to \fIm\fR. If the current total cost is greater than \fIm\fR, some items are deleted immediately.
.PP
See also maxCost() and totalCost().
.SH "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.
A debug-only utility function. Prints out cache usage, hit/miss, and distribution information using tqDebug(). This function does nothing in the release library.
Takes the item associated with \fIk\fR 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.
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If there are two or more items with equal keys, the one that was inserted last is taken.
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All iterators that refer to the taken item are set to point to the next item in the cache's traversal order.
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See also remove() and clear().
.SH "int QCache::totalCost () const"
Returns the total cost of the items in the cache. This is an integer in the range 0 to maxCost().
