Qt QAbstractItemModel - item removal crashes - c++

Question

I'm writing an application using Qt classes where I have a hierarchical structure and I need to display it in a tree view (I'm using a QTreeView widget). The data itself look like this:
class StatisticsEntry
{
// additional data management methods
...
bool setData(int column, const QVariant &value)
{
if (column < 0 || column >= _data.size())
return false;
_data[column] = value;
return true;
}
private:
// each item has a parent item except the top-most one
StatisticsEntry *_parent;
// each item may have child items which are accessible through the following member
QList<StatisticsEntry*> _children;
// each item is defined by a set of vallues stored in the following vector
QVector<QVariant> _data;
}
I have a class called it StatisticsModel that implements QAbstractItemModel - use it to manipulate and present the data stored in the StatisticsEntry tree.
The class has a method called addStatisticsData which I use to push StatisticsEntry records into the model. The method roughly looks like this:
QModelIndex StatisticsModel::addStatisticsData(const QString &title, const QString &description, const QModelIndex &parent)
{
int row = rowCount(parent);
if (!insertRow(row, parent))
return QModelIndex();
// Get new item index
QModelIndex child = index(row, 0, parent);
// set item data
setTitle(child, title);
setDescription(child, description);
return child;
}
SetTitle and setDescription methods are identical - here's the setTitle one:
void StatisticsModel::setTitle(const QModelIndex &index, const QString& title)
{
QModelIndex columnIndex = this->index(index.row(), StatColumn::Title, index.parent());
this->setData(columnIndex, title, Qt::EditRole);
}
The setData method is as follows:
bool StatisticsModel::setData(const QModelIndex &index, const QVariant &value, int role)
{
if (role != Qt::EditRole)
return false;
int column = index.column();
StatisticsEntry *item = getItem(index);
if (!item->setData(column, value))
return false;
emit dataChanged(index, index);
return true;
}
What is left is the getItem method:
StatisticsEntry *StatisticsModel::getItem(const QModelIndex &index) const
{
if (index.isValid())
{
StatisticsEntry *item = static_cast<StatisticsEntry*>(index.internalPointer());
if (item)
return item;
}
return _rootItem;
}
That is about all there is when speaking of adding new and modifying existing entries. In my application I have implemented QSortFilterProxyModel as well - nothing special, just the lessThan method. I use the proxy model to provide a sort feature in the QTreeView that displays the data. There is the following code that connects the models to the treeview widget:
in the header of the main window:
...
StatisticsModel* _statisticsModel;
StatisticsProxyModel* _statisticsProxyModel;
...
in the constructor of the main widow
...
_statisticsModel = new StatisticsModel(ths);
_statisticsProxyModel = new StatisticsProxyModel(htis);
...
_statisticsProxyModel->setSourceModel(_statisticsModel);
ui->statisticsTreeView->setModel(_statisticsProxyModel);
...
The application also has a button that allows me to remove the selected item from the model - I'm currently only testing with the QTreeView::selectionModel()->currentIndex, no multi-selections for the time being.
I have the following code for the StatisticsModel::removeRows method
bool StatisticsModel::removeRows(int position, int rows, const QModelIndex &parent)
{
StatisticsEntry *parentItem = getItem(parent);
bool success1 = true;
bool success2 = true;
beginRemoveRows(parent, position, position + rows - 1);
for (int i = position + rows - 1; i >= position; i--)
{
QModelIndex child = index(i, 0, parent);
QString title = this->title(child); // the title method is the getter method that matches the setTitle one
success1 = success1 && removeRows(0, rowCount(child), child); //rowCount is implemented to return the number of items stored in StatisticsEntry::_children list for the specified parent index
success2 = success2 && parentItem->removeChild(i); // deletes an entry from the StatisticsEntry::_children list
}
endRemoveRows();
return success1 && success2;
}
The problem is that sometimes when I remove an item using QAbstractItemModel::removeRow method I get an exception and the stack trace looks like:
StatisticsModel::parent StatisticsModel.cpp 307 0x13e7bf8
QModelIndex::parent qabstractitemmodel.h 393 0x72e57265
QSortFilterProxyModelPrivate::source_to_proxy qsortfilterproxymodel.cpp 386 0x711963e2
QSortFilterProxyModel::mapFromSource qsortfilterproxymodel.cpp 2514 0x7119d28b
QSortFilterProxyModel::parent qsortfilterproxymodel.cpp 1660 0x7119a32c
QModelIndex::parent qabstractitemmodel.h 393 0x72e57265
QPersistentModelIndex::parent qabstractitemmodel.cpp 347 0x72e58b86
QItemSelectionRange::isValid qitemselectionmodel.h 132 0x70e3f62b
QItemSelection::merge qitemselectionmodel.cpp 466 0x711503d1
QItemSelectionModelPrivate::finalize qitemselectionmodel_p.h 92 0x7115809a
QItemSelectionModelPrivate::_q_rowsAboutToBeRemoved qitemselectionmodel.cpp 623 0x71151132
QItemSelectionModel::qt_static_metacall moc_qitemselectionmodel.cpp 113 0x711561c2
QMetaObject::activate qobject.cpp 3547 0x72e8d9a4
QAbstractItemModel::rowsAboutToBeRemoved moc_qabstractitemmodel.cpp 204 0x72f08a76
QAbstractItemModel::beginRemoveRows qabstractitemmodel.cpp 2471 0x72e5c53f
QSortFilterProxyModelPrivate::remove_proxy_interval qsortfilterproxymodel.cpp 558 0x71196ce7
QSortFilterProxyModelPrivate::remove_source_items qsortfilterproxymodel.cpp 540 0x71196c7f
QSortFilterProxyModelPrivate::source_items_about_to_be_removed qsortfilterproxymodel.cpp 841 0x71197c77
QSortFilterProxyModelPrivate::_q_sourceRowsAboutToBeRemoved qsortfilterproxymodel.cpp 1291 0x711995cc
QSortFilterProxyModel::qt_static_metacall moc_qsortfilterproxymodel.cpp 115 0x7119d506
QMetaCallEvent::placeMetaCall qobject.cpp 525 0x72e883fd
QObject::event qobject.cpp 1195 0x72e894ba
QApplicationPrivate::notify_helper qapplication.cpp 4550 0x709d710e
QApplication::notify qapplication.cpp 3932 0x709d4d87
QCoreApplication::notifyInternal qcoreapplication.cpp 876 0x72e6b091
Oddly enough this seems to happen after all immediate method calls concerning the item removal are already over. It seems that the proxy model is looking for model indices that should not be present anymore (or so I think). The StatisticsModel::parent method is as follows:
QModelIndex StatisticsModel::parent(const QModelIndex &index) const
{
if (!index.isValid())
return QModelIndex();
StatisticsEntry *childItem = getItem(index);
StatisticsEntry *parentItem = childItem->parent();
if (NULL != parentItem)
return createIndex(parentItem->childNumber(), 0, parentItem);
return QModelIndex();
}
When the exception happens the values associated with the childItem and parentItem variables from the above method seem invalid - either the pointers themselves point to non-accessible memory or the member QLists either have no entries or their entries give Memory access violation.
It may be that the parent method is not correct, but then how to fetch the parent index - the qabstractItemModel documentation discourages the usage of QModelIndex::parent in that method for it will create an infinite recursion.
Any help would be appreciated,

Answer 1

When doing your StatisticsModel::removeRows, You are nesting begingRemoveRows/endRemoveRows, which, AFAIK, does not work. You should do:
bool StatisticsModel::removeRows(int position, int rows, const QModelIndex &parent)
{
StatisticsEntry *parentItem = getItem(parent);
bool success1 = true;
bool success2 = true;
for (int i = position + rows - 1; i >= position; i--)
{
QModelIndex child = index(i, 0, parent);
QString title = this->title(child); // the title method is the getter method that matches the setTitle one
success1 = success1 && removeRows(0, rowCount(child), child); //rowCount is implemented to return the number of items stored in StatisticsEntry::_children list for the specified parent index
beginRemoveRows(parent, i, i);
success2 = success2 && parentItem->removeChild(i); // deletes an entry from the StatisticsEntry::_children list
endRemoveRows();
}
return success1 && success2;
}