Skip to main content

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
Labels:

Comments

  1. qtgeek19 January 2014 at 18:39

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

    ReplyDelete

Post a Comment

Popular posts from this blog

GDB Automatic Deadlock Detector

Have you ever had a problem with detection deadlock between threads in your C/C++ application? Would you like to do that automatically? Try this GDB Automatic Deadlock Detector from my github: GDB Automatic Deadlock Detector Picture source: http://csunplugged.org/wp-content/uploads/2015/03/deadlock.jpg1286488735
Post a Comment
Read more

STL - count and count_if algorithms

One of the basic and most useful STL algorithms is algorithm which can be used to count number of elements within selected container according to specified criteria. In order to do that we can use std::count or std::count_if algorithm. std::count (firstElementIterator, lastElementIterator, elementForSearch) - is function which will go through container using firstElementIterator and lastElementIterator and return number of container elements which value is equal elementForSearch std::count_if (firstElementIterator, lastElementIterator, UnaryPredicateFunction) - is function which examine range from firstElementIterator to lastElementIterator and return number of container elements which fulfill UnaryPredicateFunction criteria. UnaryPredicateFunction is function having following signature: bool functionName(const Type& a) . So, count_if returns number of elements where UnaryPredicateFunction returns true for. For better understanding let's take a
Post a Comment
Read more

Advanced C++ - Stack unwinding

Stack unwinding is normally a concept of removing function entries from call stack (also known as Execution stack, Control stack, Function stack or Run-time stack). Call Stack is a stack data structure that stores active functions' addresses and helps in supporting function call/return mechanism. Every time when a function is called, an entry is made into Call stack which contains the return address of the calling function where the control needs to return after the execution of called function. This entry is called by various names like stack frame , activation frame or activation record. With respect to exception handling , stack Unwinding is a process of linearly searching function call stack to reach exception handler. When an exception occurs, if it is not handled in current function where it is thrown, the function Call Stack is unwound until the control reaches try block and then passes to catch block at the end of try block to handle exception. Also, in this proc
Post a Comment
Read more