source: trunk/doc/src/q3asciicache.qdoc@ 109

/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information ([email protected])
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Nokia.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain
** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
** package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at [email protected].
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \class Q3AsciiCache
    \brief The Q3AsciiCache class is a template class that provides a cache based on char* keys.
    \compat

    Q3AsciiCache is implemented as a template class. Define a template
    instance Q3AsciiCache\<X\> to create a cache that operates on
    pointers to X (X*).

    A cache is a least recently used (LRU) list of cache items. The
    cache items are accessed via \c char* keys. For Unicode keys use
    the Q3Cache template instead, which uses QString keys. A Q3Cache
    has the same performace as a Q3AsciiCache.

    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 \link Q3PtrCollection::setAutoDelete()
    setAutoDelete(TRUE)\endlink tells the cache 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 cache copy all of the item's data (known as a deep
    copy) when an item is inserted. insert() calls the virtual
    function Q3PtrCollection::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
    Q3PtrCollection::deleteItem() is called. Its default implementation
    in Q3AsciiCache is to delete the item if \link
    Q3PtrCollection::setAutoDelete() auto-deletion\endlink is enabled.

    There is a Q3AsciiCacheIterator which may be used to traverse the
    items in the cache in arbitrary order.

    \sa Q3AsciiCacheIterator, Q3Cache, Q3IntCache
*/

/*!
  \fn Q3AsciiCache::Q3AsciiCache( const Q3AsciiCache<type> &c )

  \internal

  Do not use. A Q3AsciiCache cannot be copied. Calls qFatal() in debug version.
*/


/*!
    \fn Q3AsciiCache::Q3AsciiCache( int maxCost, int size, bool caseSensitive, bool copyKeys )

    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 prime 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 only affects the 26 letters in
    US-ASCII. If \a copyKeys is TRUE (the default), Q3AsciiCache makes
    a copy of the cache keys, otherwise it copies just the const char
    * pointer - slightly faster if you can guarantee that the keys
    will never change, but very risky.
*/

/*!
    \fn Q3AsciiCache::~Q3AsciiCache()

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

/*!
  \fn Q3AsciiCache<type>& Q3AsciiCache::operator=( const Q3AsciiCache<type> &c )

  \internal

  Do not use. A Q3AsciiCache cannot be copied. Calls qFatal() in debug version.
*/

/*!
    \fn int Q3AsciiCache::maxCost() const

    Returns the maximum allowed total cost of the cache.

    \sa setMaxCost() totalCost()
*/

/*!
    \fn int Q3AsciiCache::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 Q3AsciiCache::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 removed
    immediately.

    \sa maxCost() totalCost()
*/

/*!
    \fn uint Q3AsciiCache::count() const

    Returns the number of items in the cache.

    \sa totalCost() size()
*/

/*!
    \fn uint Q3AsciiCache::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 Q3AsciiCache::isEmpty() const

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

/*!
    \fn bool Q3AsciiCache::insert( const char *k, const type *d, int c, int p )

    Inserts the item \a d into the cache using key \a k, and with an
    associated cost of \a c. Returns TRUE if the item is successfully
    inserted. Returns FALSE if the item is not inserted, for example,
    if the cost of the item exceeds maxCost().

    The cache's size is limited, and if the total cost is too high,
    Q3AsciiCache will remove old, least recently used items until there
    is room for this new item.

    Items with duplicate keys can be inserted.

    The parameter \a p is internal and should be left at the default
    value (0).

    \warning If this function returns FALSE, 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 Q3AsciiCache::remove( const char *k )

    Removes the item with key \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 \link Q3PtrCollection::setAutoDelete()
    setAutoDelete(TRUE)\endlink.

    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 *Q3AsciiCache::take( const char *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 Q3AsciiCache::clear()

    Removes all items from the cache, and deletes them if \link
    Q3PtrCollection::setAutoDelete() auto-deletion\endlink has been
    enabled.

    All cache iterators that operate on this cache are reset.

    \sa remove() take()
*/

/*!
    \fn type *Q3AsciiCache::find( const char *k, bool ref ) const

    Returns the item 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 *Q3AsciiCache::operator[]( const char *k ) const

    Returns the item 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 Q3AsciiCache::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.
*/

/*!
    \class Q3AsciiCacheIterator
    \brief The Q3AsciiCacheIterator class provides an iterator for Q3AsciiCache collections.
    \compat

    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 Q3AsciiCache. Q3AsciiCache updates all iterators
    that refer an item when that item is removed.

    Q3AsciiCacheIterator provides an operator++() and an operator+=()
    to traverse the cache; current() and currentKey() to access the
    current cache item and its key. It also provides atFirst() and
    atLast(), which 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 Q3AsciiCache
*/

/*!
    \fn Q3AsciiCacheIterator::Q3AsciiCacheIterator( const Q3AsciiCache<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 Q3AsciiCacheIterator::Q3AsciiCacheIterator (const Q3AsciiCacheIterator<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 Q3AsciiCacheIterator<type>& Q3AsciiCacheIterator::operator=( const Q3AsciiCacheIterator<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 Q3AsciiCacheIterator::count() const

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

    \sa isEmpty()
*/

/*!
    \fn bool Q3AsciiCacheIterator::isEmpty() const

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

/*!
    \fn bool Q3AsciiCacheIterator::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 Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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 Q3AsciiCacheIterator::operator type *() const

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

/*!
    \fn type *Q3AsciiCacheIterator::current() const

    Returns a pointer to the current iterator item.
*/

/*!
    \fn const char *Q3AsciiCacheIterator::currentKey() const

    Returns the key for the current iterator item.
*/

/*!
    \fn type *Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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 *Q3AsciiCacheIterator::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.
*/