Where does QModelIndex() come from in seekRoot.parent() != QModelIndex();

In Qt help, there is an example in Model/View Tutorial - 3.2 Working with Selections. The resource codes are in Qt\Qt5.9.1\Examples\Qt-5.9.1\widgets\tutorials\modelview\7_selections.

I cannot understand what is QModelIndex() in while(seekRoot.parent() != QModelIndex()). It looks like a constructor of QModelIndex, but what's the usage here? It returns a new empty model index? Or it is a function of MainWindow? It seems impossible.

Where does it come from? And what is the return value?

void MainWindow::selectionChangedSlot(const QItemSelection & /*newSelection*/, const QItemSelection & /*oldSelection*/)
{
    //get the text of the selected item
    const QModelIndex index = treeView->selectionModel()->currentIndex();
    QString selectedText = index.data(Qt::DisplayRole).toString();
    //find out the hierarchy level of the selected item
    int hierarchyLevel=1;
    QModelIndex seekRoot = index;
    while(seekRoot.parent() != QModelIndex())
    {
        seekRoot = seekRoot.parent();
        hierarchyLevel++;
    }
    QString showString = QString("%1, Level %2").arg(selectedText)
                         .arg(hierarchyLevel);
    setWindowTitle(showString);
}

The empty QModelIndex() constructor indicates an invalid (i.e. non existing) QModelIndex:

Creates a new empty model index. This type of model index is used to indicate that the position in the model is invalid.

So seekRoot.parent() != QModelIndex() checks if seekRoot has a parent (i.e. its parent is not invalid).

It could also be written (more clearly) as seekRoot.parent().isValid() (see QModelIndex::isValid).