Performance Tip Optimizing Iteration: Difference between revisions
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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. | 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. | 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. | ||
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== Additional reading == | |||
''' [http://labs.trolltech.com/blogs/2009/01/23/iterating-efficiently/ Iterating efficiently] - Qt Labs blog post | ''' [http://labs.trolltech.com/blogs/2009/01/23/iterating-efficiently/ Iterating efficiently] - Qt Labs blog post | ||
Revision as of 16:07, 5 March 2015
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[toc align_right="yes" depth="2"]
Performance Tip: Optimizing Iteration
Here are two tips to help produce code that makes the most out of Qt internal performance.
Iterator selection
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())
process(i.next());
Java style iterator
QList<int>::iterator i;
for (i = list.begin(); i != list.end(); ++i)
process('''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)
process(i);
Additional reading
Iterating efficiently - Qt Labs blog post