QContiguousCache Class Reference
[QtCore module]

Detailed Description

The QContiguousCache class is a template class that provides a contiguous cache.

The QContiguousCache class provides an efficient way of caching items for display in a user interface view. Unlike QCache, it adds a restriction that elements within the cache are contiguous. This has the advantage of matching how user interface views most commonly request data, as a set of rows localized around the current scrolled position. This restriction allows the cache to consume less memory and processor cycles than QCache. The QContiguousCache class also can provide an upper bound on memory usage via setCapacity().

The simplest way of using a contiguous cache is to use the append() and prepend().

 MyRecord record(int row) const
 {
     Q_ASSERT(row >= 0 && row < count());

     while(row > cache.lastIndex())
         cache.append(slowFetchRecord(cache.lastIndex()+1));
     while(row < cache.firstIndex())
         cache.prepend(slowFetchRecord(cache.firstIndex()-1));

     return cache.at(row);
 }

If the cache is full then the item at the opposite end of the cache from where the new item is appended or prepended will be removed.

This usage can be further optimized by using the insert() function in the case where the requested row is a long way from the currently cached items. If there is a gap between where the new item is inserted and the currently cached items then the existing cached items are first removed to retain the contiguous nature of the cache. Hence it is important to take some care then when using insert() in order to avoid unwanted clearing of the cache.

The range of valid indexes for the QContiguousCache class are from 0 to INT_MAX. Calling prepend() such that the first index would become less than 0 or append() such that the last index would become greater than INT_MAX can result in the indexes of the cache being invalid. When the cache indexes are invalid it is important to call normalizeIndexes() before calling any of containsIndex(), firstIndex(), lastIndex(), at() or operator[](). Calling these functions when the cache has invalid indexes will result in undefined behavior. The indexes can be checked by using areIndexesValid()

In most cases the indexes will not exceed 0 to INT_MAX, and normalizeIndexes() will not need to be used.

See the Contiguous Cache example.

Member Function Documentation

 QContiguousCache<int> cache(10);
 cache.insert(INT_MAX, 1); // cache contains one value and has valid indexes, INT_MAX to INT_MAX
 cache.append(2); // cache contains two values but does not have valid indexes.
 cache.normalizeIndexes(); // cache has two values, 1 and 2.  New first index will be in the range of 0 to capacity().