Table of Content
Performance Tip: Optimizing Iteration
Here are two tips to help produce code that makes the most out of Qt internal performance.
Qt allows you to use both Java-style and STL-style iterators to step through your structures. Your selection of iterator style has no performance impact, but your selection of iterator operator does.
- QListIterator<int> i(list);
- while (i.hasNext())
Java style iterator
- QList<int>::iterator i;
- for (i = list.begin(); i != list.end(); ++i)
STL (standard template library) style iterator
When using STL-style iterators with a list containing complex items, execution is faster if you use the ++i operator instead of the i++ operator. The i++ will force your loop to work on a copy of the item i.
Careful with foreach
Benchmarking Qt applications indicates there is always a performance penalty to using a foreach loop as opposed to a for loop with an iterator. However, you can greatly reduce the performance penalty if you use a const iterator in your foreach loop. This can often make the performance penalty negligible, though it is never zero.
- foreach (const QString &i, list)
- Iterating efficiently [labs.trolltech.com] – Qt Labs blog post
Some of this material originally appeared in Maximizing Performance with Qt [qt.nokia.com] white paper.