Iterating over a QMap with for

Question

I\'ve a QMap object and I am trying to write its content to a file.

QMap extensions;
//.. 

for(auto e : extensions)
{
  fou         


        

Answer 1

For people interested in optimizations, I have tried several approaches, did some micro benchmarks, and I can conclude that STL style approach is significantly faster.

I have tried adding integers with these methods :

• QMap::values()
• Java style iterator (as advised in the documentation)
• STL style iterator (as advised in the documentation too)

And I have compared it with summing integers of a QList/QVector

Results :

Reference vector :   244  ms
Reference list :     1239  ms

QMap::values() :     6504  ms
Java style iterator :    6199  ms
STL style iterator :     2343  ms

Code for those interested :

#include 
#include 
#include 
#include 
#include 

void testQMap(){
    QMap map;
    QVector vec;
    QList list;

    int nbIterations = 100;
    int size = 1000000;
    volatile int sum = 0;

    for(int i = 0; i values = map.values();
        for(int k : values){
            sum += k;
        }
    }
    qint64 end0 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "QMap::values() : \t" << (end0-start0) << " ms";


    // Java style iterator
    qint64 start1 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i it(map);
        while (it.hasNext()) {
            it.next();
            sum += it.value();
        }
    }
    qint64 end1 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Java style iterator : \t" << (end1-start1) << " ms";


    // STL style iterator
    qint64 start2 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i::const_iterator it = map.constBegin();
        auto end = map.constEnd();
        while (it != end) {
            sum += it.value();
            ++it;
        }
    }
    qint64 end2 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "STL style iterator : \t" << (end2-start2) << " ms";


    qint64 start3 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i

Edit July 2017 : I ran this code again on my new laptop (Qt 5.9, i7-7560U) and got some interesting changes

Reference vector :   155  ms 
Reference list :     157  ms
QMap::values():      1874  ms 
Java style iterator: 1156  ms 
STL style iterator:  1143  ms

STL style and Java style have very similar performances in this benchmark