Question about QLists

mzimmers

Hi, all -

I've been watching some Qt tutorial videos and recently came across QLists. I added some telltales at the end, just for fun, and found the results a little surprising. Here's the code:

#include <QCoreApplication>
#include <QList>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QList<int> list;
    QMutableListIterator<int> iter(list); // must be mutable if you want to modify list.

    for (int i = 0; i < 10; i++)
    {
        list.append(i);
    }

    while (iter.hasNext())
    {
        qDebug() << iter.next();
    }

    iter.toFront();
    while(iter.hasNext())
    {
        int i = iter.next();
        if (i == 5)
        {
            iter.remove();
        }
    }

    // I just put this in to see the results.
    foreach (int n, list)
    {
        qDebug() << n << list.value(n) ;
    }

    exit(0);
    return a.exec();
}

And this is the output I get:

I'm curious about the final output line. As n = 9 (ordinal 10th), wouldn't using it as an argument to value() produce an overrun, or at least a runtime error?

VRonin

@mzimmers said in Question about QLists:

wouldn't using it as an argument to value() produce an overrun, or at least a runtime error?

no, value() explicitly prevent runtime errors, it just returns a default constructed item if your index overruns, you can specify what to return from value() in case of overrun with the second parameter list.value(n,42);. at() on the other hand will cause a runtime error if your index is out of bounds.
Ref.
http://doc.qt.io/qt-5/qlist.html#value
http://doc.qt.io/qt-5/qlist.html#at

P.S.
Did you notice 6 is missing from the output? ;)

this:

foreach (int n, list)
    {
        qDebug() << n << list.value(n) ;
    }

doesn't really make sense

mzimmers

Thanks, VRonin. And yes, I noticed that 6 is missing from the 2nd column. I did expect that, though.

I realize the foreach doesn't make any sense, but I was just experimenting. I'm not only new to Qt, but I'm relatively new to using a lot of the containers.

Thanks again.