Performance Tip Optimizing Iteration
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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.
Java style iterator:
QListIterator<int> i(list); while (i.hasNext()) process(i.next());
STL (Standard Template Library) style iterator:
QList<int>::iterator i; for (i = list.begin(); i != list.end(); ++i) process(*i);
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) process(i);
- Kai Köhne's blog, Iterating efficiently