qt3/doc/qcache.doc

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** QCache and QCacheIterator class documentation
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/*****************************************************************************
  QCache documentation
 *****************************************************************************/

/*!
    \class QCache
    \brief The QCache class is a template class that provides a cache based on QString keys.

    \ingroup collection
    \ingroup tools

    \important autoDelete setAutoDelete

    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 \l QString keys, which
    are Unicode strings. If you want to use non-Unicode, plain 8-bit
    \c char* keys, use the QAsciiCache template. A QCache has the
    same performance as a QAsciiCache.

    \sa QCacheIterator, QAsciiCache, QIntCache
*/

/*!
  \fn QCache::QCache( const QCache<type> &c )

  \internal

  Do not use. A QCache cannot be copied. Calls qFatal() in debug version.
*/


/*!
    \fn QCache::QCache( int maxCost, int size, bool caseSensitive )

    Constructs a cache whose contents will never have a total cost
    greater than \a maxCost and which is expected to contain less than
    \a size items.

    \a 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 \a
    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 \a 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.
*/

/*!
    \fn QCache::~QCache()

    Removes all items from the cache and destroys it. All iterators
    that access this cache will be reset.
*/

/*!
  \fn QCache<type>& QCache::operator=( const QCache<type> &c )

  \internal

  Do not use. A QCache cannot be copied. Calls qFatal() in debug version.
*/

/*!
    \fn int QCache::maxCost() const

    Returns the maximum allowed total cost of the cache.

    \sa setMaxCost() totalCost()
*/

/*!
    \fn int QCache::totalCost() const

    Returns the total cost of the items in the cache. This is an
    integer in the range 0 to maxCost().

    \sa setMaxCost()
*/

/*!
    \fn void QCache::setMaxCost( int m )

    Sets the maximum allowed total cost of the cache to \a m. If the
    current total cost is greater than \a m, some items are deleted
    immediately.

    \sa maxCost() totalCost()
*/

/*!
    \fn uint QCache::count() const

    Returns the number of items in the cache.

    \sa totalCost()
*/

/*!
    \fn 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.
*/

/*!
    \fn bool QCache::isEmpty() const

    Returns TRUE if the cache is empty; otherwise returns FALSE.
*/

/*!
    \fn bool QCache::insert( const QString &k, const type *d, int c, int p )

    Inserts the item \a d into the cache with key \a k and associated
    cost, \a 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 \a 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
    \a d yourself. Additionally, be very careful about using \a 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.
*/

/*!
    \fn bool QCache::remove( const QString &k )

    Removes the item associated with \a 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.

    \sa take(), clear()
*/

/*!
    \fn type *QCache::take( const QString &k )

    Takes the item associated with \a 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.

    \sa remove(), clear()
*/

/*!
    \fn void QCache::clear()

    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.

    \sa remove() take()
*/

/*!
    \fn type *QCache::find( const QString &k, bool ref ) const

    Returns the item associated with key \a k, or 0 if the key does
    not exist in the cache. If \a 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.
*/

/*!
    \fn type *QCache::operator[]( const QString &k ) const

    Returns the item associated with key \a k, or 0 if \a 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 ).

    \sa find()
*/

/*!
    \fn 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.
*/

/*****************************************************************************
  QCacheIterator documentation
 *****************************************************************************/

/*!
    \class QCacheIterator qcache.h
    \brief The QCacheIterator class provides an iterator for QCache collections.

    \ingroup collection
    \ingroup tools

    Note that the traversal order is arbitrary; you are not guaranteed
    any particular order. If new objects are inserted into the cache
    while the iterator is active, the iterator may or may not see
    them.

    Multiple iterators are completely independent, even when they
    operate on the same QCache. QCache updates all iterators that
    refer an item when that item is removed.

    QCacheIterator provides an operator++(), and an operator+=() to
    traverse the cache. The current() and currentKey() functions are
    used to access the current cache item and its key. The atFirst()
    and atLast() return TRUE if the iterator points to the first or
    last item in the cache respectively. The isEmpty() function
    returns TRUE if the cache is empty, and count() returns the number
    of items in the cache.

    Note that atFirst() and atLast() refer to the iterator's arbitrary
    ordering, not to the cache's internal least recently used list.

    \sa QCache
*/

/*!
    \fn QCacheIterator::QCacheIterator( const QCache<type> &cache )

    Constructs an iterator for \a cache. The current iterator item is
    set to point to the first item in the \a cache.
*/

/*!
    \fn QCacheIterator::QCacheIterator (const QCacheIterator<type> & ci)

    Constructs an iterator for the same cache as \a ci. The new
    iterator starts at the same item as ci.current(), but moves
    independently from there on.
*/

/*!
    \fn QCacheIterator<type>& QCacheIterator::operator=( const QCacheIterator<type> &ci )

    Makes this an iterator for the same cache as \a ci. The new
    iterator starts at the same item as ci.current(), but moves
    independently thereafter.
*/

/*!
    \fn uint QCacheIterator::count() const

    Returns the number of items in the cache on which this iterator
    operates.

    \sa isEmpty()
*/

/*!
    \fn bool QCacheIterator::isEmpty() const

    Returns TRUE if the cache is empty, i.e. count() == 0; otherwise
    it returns FALSE.
    
    \sa count()
*/

/*!
    \fn bool QCacheIterator::atFirst() const

    Returns TRUE if the iterator points to the first item in the
    cache; otherwise returns FALSE. Note that this refers to the
    iterator's arbitrary ordering, not to the cache's internal least
    recently used list.

    \sa toFirst(), atLast()
*/

/*!
    \fn bool QCacheIterator::atLast() const

    Returns TRUE if the iterator points to the last item in the cache;
    otherwise returns FALSE. Note that this refers to the iterator's
    arbitrary ordering, not to the cache's internal least recently
    used list.

    \sa toLast(), atFirst()
*/

/*!
    \fn type *QCacheIterator::toFirst()

    Sets the iterator to point to the first item in the cache and
    returns a pointer to the item.

    Sets the iterator to 0 and returns 0 if the cache is empty.

    \sa toLast() isEmpty()
*/

/*!
    \fn type *QCacheIterator::toLast()

    Sets the iterator to point to the last item in the cache and
    returns a pointer to the item.

    Sets the iterator to 0 and returns 0 if the cache is empty.

    \sa toFirst() isEmpty()
*/

/*!
    \fn QCacheIterator::operator type *() const

    Cast operator. Returns a pointer to the current iterator item.
    Same as current().
*/

/*!
    \fn type *QCacheIterator::current() const

    Returns a pointer to the current iterator item.
*/

/*!
    \fn QString QCacheIterator::currentKey() const

    Returns the key for the current iterator item.
*/

/*!
    \fn type *QCacheIterator::operator()()

    Makes the succeeding item current and returns the original current
    item.

    If the current iterator item was the last item in the cache or if
    it was 0, 0 is returned.
*/

/*!
    \fn type *QCacheIterator::operator+=( uint jump )

    Returns the item \a jump positions after the current item, or 0 if
    it is beyond the last item. Makes this the current item.
*/

/*!
    \fn type *QCacheIterator::operator-=( uint jump )

    Returns the item \a jump positions before the current item, or 0
    if it is before the first item. Makes this the current item.
*/

/*!
    \fn type *QCacheIterator::operator++()

    Prefix++ makes the iterator point to the item just after current()
    and makes that the new current item for the iterator. If current()
    was the last item, operator++() returns 0.
*/

/*!
    \fn type *QCacheIterator::operator--()

    Prefix-- makes the iterator point to the item just before
    current() and makes that the new current item for the iterator. If
    current() was the first item, operator--() returns 0.
*/