Program for demonstrating performance gains by using loop initialization and QList::reserve(int).

QListLoopPerformanceDemo

#include <QCoreApplication>
#include <QDateTime>
#include <QList>
#include <QObject>
#include <QTimer>
#include <QDebug>

QList<QObject*> buildObjectList()
{
    qint64 size = 10000000;
    QList<QObject*> objectList;
    for (int i = 0; i < size; i++)
    {
        objectList << new QTimer();
    }
    return objectList;
}

QList<QObject*> buildObjectListWithReserve()
{
    qint64 size = 10000000;
    QList<QObject*> objectList;
    objectList.reserve(size);
    for (int i = 0; i < size; i++)
    {
        objectList << new QTimer();
    }
    return objectList;
}

void timeBuildList()
{
    qint64 tStart = QDateTime::currentMSecsSinceEpoch();
    QList<QObject*> list = buildObjectList();
    qint64 tEnd = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "building list w/o reserve took" << (tEnd - tStart) << "ms";
    qDeleteAll(list);
}

void timeBuildListWithReserve()
{
    qint64 tStart = QDateTime::currentMSecsSinceEpoch();
    QList<QObject*> list = buildObjectListWithReserve();
    qint64 tEnd = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "building list w/ reserve took" << (tEnd - tStart) << "ms";
    qDeleteAll(list);
}

void timeReverseStackOverflowOneLiner()
{
    QList<QObject*> list = buildObjectListWithReserve();
    qint64 tStart = QDateTime::currentMSecsSinceEpoch();
    for(int k = 0; k < (list.size()/2); k++) list.swap(k,list.size()-(1+k));
    qint64 tEnd = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "reversing list stack overflow took" << (tEnd - tStart) << "ms";
    qDeleteAll(list);
}

void timeReverseStackOverflowOneLinerWithMax()
{
    QList<QObject*> list = buildObjectListWithReserve();
    qint64 tStart = QDateTime::currentMSecsSinceEpoch();
    for(int k = 0, max = (list.size()/2); k < max; k++) list.swap(k,list.size()-(1+k));
    qint64 tEnd = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "reversing list stack overflow with max took" << (tEnd - tStart) << "ms";
    qDeleteAll(list);
}

void timeReverseStackOverflowOneLinerWithMaxAndSize()
{
    QList<QObject*> list = buildObjectListWithReserve();
    qint64 tStart = QDateTime::currentMSecsSinceEpoch();
    for(int k=0, s=list.size(), max=(s/2); k<max; k++) list.swap(k,s-(1+k));
    qint64 tEnd = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "reversing list stack overflow with max and size took" << (tEnd - tStart) << "ms";
    qDeleteAll(list);
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    timeBuildList();
    timeBuildListWithReserve();
    timeReverseStackOverflowOneLiner();
    timeReverseStackOverflowOneLinerWithMax();
    timeReverseStackOverflowOneLinerWithMaxAndSize();

    return a.exec();
}