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Performance Tip Optimizing Iteration: Difference between revisions
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[[Category:Learning]] | [[Category:Learning]] | ||
[[Category:HowTo]] | [[Category:HowTo]] | ||
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Here are two tips to help produce code that makes the most out of Qt internal performance. | Here are two tips to help produce code that makes the most out of Qt internal performance. | ||
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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. | 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. | ||
<code>QListIterator<int> i(list); | Java style iterator: | ||
<code> | |||
QListIterator<int> i(list); | |||
while (i.hasNext()) | while (i.hasNext()) | ||
process(i.next()); | |||
</code> | </code> | ||
STL (Standard Template Library) style iterator: | |||
<code> | |||
<code>QList<int>::iterator i; | QList<int>::iterator i; | ||
for (i = list.begin(); i != list.end(); ++i) | for (i = list.begin(); i != list.end(); ++i) | ||
process(*i); | |||
</code> | </code> | ||
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 + | |||
== Careful with foreach == | == Careful with foreach == | ||
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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. | ||
<code> | |||
<code>foreach (const QString &i, list) | foreach (const QString &i, list) | ||
process(i); | |||
</code> | </code> | ||
== Additional reading == | == Additional reading == | ||
' | # Kai Köhne's blog, [https://blog.qt.io/blog/2009/01/23/iterating-efficiently/ Iterating efficiently] | ||
Latest revision as of 21:18, 28 June 2015
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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.
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);
Additional reading
- Kai Köhne's blog, Iterating efficiently