Qt - problem appending to QList of QList

Question

I'm trying to append items to a QList at runtime, but I'm running on a error message. Basically what I'm trying to do is to make a QList of QLists and add a few customClass objects to each of the inner lists. Here's my code:

widget.h:

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();

public slots:
    static QList<QList<customClass> > testlist(){
        QList<QList<customClass> > mylist;
        for(int w=0 ; w<5 ; w++){
            mylist.append(QList<customClass>());
        }
        for(int z=0 ; z<mylist.size() ; z++){
            for(int x=0 ; x<10 ; x++){
                customClass co = customClass();
                mylist.at(z).append(co);
            }
        }
        return mylist;
    }
};

customclass.h:

class customClass
{
public:
    customClass(){
        this->varInt = 1;
        this->varQString = "hello world";
    }
    int varInt;
    QString varQString;
};

main.cpp:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    QList<QList<customClass> > list;
    list = w.testlist();
    w.show();
    return a.exec();
}

But when I try to compile the application, it gives off this error:

error: passing `const QList<customClass>' as `this' argument of `void List<T>::append(const T&) [with T = customClass]' discards qualifiers

I also tried inserting the objects using foreach:

foreach(QList<customClass> list, mylist){
    for(int x=0 ; x<10 ; x++){
        list.append(customClass());
    }
}

The error was gone, but the customClass objects weren't appended, I could verify that by using a debugging loop in main that showed the inner QLists sizes as zero. What am I doing wrong?

Answer 1

Qt Reference says:

const T & at ( int i ) const

but not:

T& at ( int i )

so there's no non-const version of at. You have to use operator[] instead.

So change it to:

mylist[z].append(co);

and it will work. I even tested it.

I think the foreach version doesn't work, because in foreach(QList<customClass> list, mylist) QList<customClass> list is not reference. But foreach(QList<customClass>& list, mylist) doesn't compile. So you'll have to use the normal for(...) version.

Answer 2

The error must be reported on the following line:

    for(int z=0 ; z<mylist.size() ; z++){
        for(int x=0 ; x<10 ; x++){
            customClass co = customClass();
            mylist.at(z).append(co);           // Error is here
        }
    }

QList::at(int); returns a const reference to the object at index i.

You should use QList::operator[](int i); that return a non const reference.

Answer 3

foreach( T i, container ) creates a copy, if you modify it, you modify the copy, not the original. Thus one should always use foreach( const T& i, container) instead.