Performance Tip Optimizing Iteration: Difference between revisions
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=Performance Tip: Optimizing Iteration= | [[Category:Learning]]<br />[[Category:HowTo]]<br />[[Category:Developing with Qt]]<br />[toc align_right="yes&quot; depth="2&quot;] | ||
= Performance Tip: Optimizing Iteration = | |||
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. | ||
==Iterator selection== | == 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. | |||
<code>QListIterator&lt;int&gt; i(list);<br />while (i.hasNext())<br /> process(i.next());<br /></code> | |||
''Java style iterator'' | ''Java style iterator'' | ||
''< | <code>QList&lt;int&gt;::iterator i;<br />for (i = list.begin(); i != list.end(); ++i)<br /> process('''i);<br /></code> | ||
<br />''STL (standard template library) style iterator'' | |||
When using | <br />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. | ||
<br />h2. Careful with foreach | |||
<br />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. | |||
<br /><code>foreach (const QString &i, list)<br /> process(i);<br /></code> | |||
<br />h2. Additional reading | |||
<br />''' "Iterating efficiently&quot;:http://labs.trolltech.com/blogs/2009/01/23/iterating-efficiently/ - Qt Labs blog post | |||
Revision as of 10:19, 24 February 2015
[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&lt;int&gt; i(list);<br />while (i.hasNext())<br /> process(i.next());<br />
Java style iterator
QList&lt;int&gt;::iterator i;<br />for (i = list.begin(); i != list.end(); ++i)<br /> process('''i);<br />
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.
h2. 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)<br /> process(i);<br />
h2. Additional reading
"Iterating efficiently":http://labs.trolltech.com/blogs/2009/01/23/iterating-efficiently/ - Qt Labs blog post