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QT - foreach algoriithm with const references performance improvement

Today I would like to show you optimal way of using foreach QT algorithm. I will show you why we should pass elements of foreach algorithm by const reference instead of passing them by value.

Let me explain it on the below example:
Output of this example is:
In point I we are creating 3 objects of MyClass class and push them to myClasses QList element.

In point II we are using QT foreach algorithm to invoke getValue() method for each object from myClasses list. As you can see on output text for that part of code we are invoking copy constructor before and destructor after invoking getValue() function. It is because we are passing each myClasses list element to foreach algorithm by value. Therefore we are copying that element at the beginning of foreach loop step and removing them (destructing) at the end. This is inefficient solution, especially when class of object being copied is big. It decreases performance. of our application.

Solution for that inefficiency is passing elements to foreach QT algorithm by const reference, as shown on point III. Thanks to that we are doing our operations on existing objects from myClasses list without copying. As you can see on the output text we are not invoking constructor and destructor foreach element here, so performance is better.

Code of this example you can find on our GitHub account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/qt/foreachRefCount
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Comments

  1. qtgeek19 January 2014 at 18:39

    Good explanation. Good to know this advantage of const references in QT foreach algorithm.

    ReplyDelete

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