QTableView drag and drop crashing program

[Rodrigo B.]

I've created a minimal QTableView example using QAbstractTableModel with drag and drop.

It works fine except that dragging a selection to the area surrounding the table crashes the program:

Any idea of why this is happening?

I list my implementation of QAbstractTableModel below. The full project is at GitHub.

#include "mainwindow.h"

#include <QAbstractTableModel>
#include <qabstractitemmodel.h>
#include "tablemodel.h"

using std::vector;


QModelIndex TableModel::index(int row, int column, const QModelIndex &parent) const {
    Q_UNUSED(parent);
    return createIndex(row, column);
}

QModelIndex TableModel::parent(const QModelIndex &child) const {
    Q_UNUSED(child);
    return QModelIndex();
}

QVariant TableModel::data(const QModelIndex &index, int role) const {
    if (role == Qt::DisplayRole)
        return dataList[index.row()][index.column()];
    return QVariant();
}

bool TableModel::setData(const QModelIndex &index, const QVariant &value, int role) {
    Q_UNUSED(role);
    dataList[index.row()][index.column()] = value;
    emit dataChanged(index, index);
    return true;
}

int TableModel::rowCount(const QModelIndex &parent) const {
    Q_UNUSED(parent);
    return dataList.size();
}

int TableModel::columnCount(const QModelIndex &parent) const {
    Q_UNUSED(parent);
    return 3;
}

Qt::ItemFlags TableModel::flags(const QModelIndex &index) const {
    Q_UNUSED(index);
    return Qt::ItemIsSelectable | Qt::ItemIsEditable | Qt::ItemIsEnabled | Qt::ItemIsDragEnabled | Qt::ItemIsDropEnabled;
}

Qt::DropActions TableModel::supportedDropActions() const {
    return Qt::CopyAction | Qt::MoveAction;
}

bool TableModel::insertRows(int row, int count, const QModelIndex &parent) {
    if (count > 0) {
        beginInsertRows(parent, row, row + count + 1);
        for (int i = 0; i != count; i++)
            dataList.emplace(dataList.begin() + row, columnCount());
        endInsertRows();
        emit dataChanged(index(row, 0), index(row + count - 1, columnCount()));
        emit layoutChanged();
        return true;
    }
    else {
        return false;
    }
}

bool TableModel::removeRows(int row, int count, const QModelIndex &parent) {
    if (count > 0) {
        beginRemoveRows(parent, row, row + count + 1);
        dataList.erase(dataList.begin() + row, dataList.begin() + row + count);
        endRemoveRows();
        emit dataChanged(index(row, 0), index(row + count - 1, columnCount()));
        emit layoutChanged();
        return true;
    }
    else {
        return false;
    }
}

[mrjj]

Hi
Good call with a runnable project. makes it more appealing to look into.

It seems to crash in emplace as row is -1 when you drop outside the rows and
datalist.begin() -1 will then point to something invalid.

Also in the base class
void QAbstractItemModel::beginInsertRows(const QModelIndex &parent, int first, int last)
{
Q_ASSERT(first >= 0);

which is also shown when run. ( as row is -1 so first is not > 0)

So it seems thats the issue. as

bool TableModel::insertRows(int row, int count, const QModelIndex &parent) {

if (row < 0)  return false; ////<<<<< skip if minus

    if (count > 0) {
        beginInsertRows(parent, row, row + count + 1);
        for (int i = 0; i != count; i++)
            dataList.emplace(dataList.begin() + row, columnCount());
        endInsertRows();
        emit dataChanged(index(row, 0), index(row + count - 1, columnCount()));
        emit layoutChanged();
        return true;
    }
    else {
        return false;
    }
}

makes it not crash when dragging there so i think you just need to handle that case and append it to your data list.

[JonB]

@Rodrigo-B
Does the stack trace when it crashes reveal anything?

[mrjj]

Hi
Good call with a runnable project. makes it more appealing to look into.

It seems to crash in emplace as row is -1 when you drop outside the rows and
datalist.begin() -1 will then point to something invalid.

Also in the base class
void QAbstractItemModel::beginInsertRows(const QModelIndex &parent, int first, int last)
{
Q_ASSERT(first >= 0);

which is also shown when run. ( as row is -1 so first is not > 0)

So it seems thats the issue. as

bool TableModel::insertRows(int row, int count, const QModelIndex &parent) {

if (row < 0)  return false; ////<<<<< skip if minus

    if (count > 0) {
        beginInsertRows(parent, row, row + count + 1);
        for (int i = 0; i != count; i++)
            dataList.emplace(dataList.begin() + row, columnCount());
        endInsertRows();
        emit dataChanged(index(row, 0), index(row + count - 1, columnCount()));
        emit layoutChanged();
        return true;
    }
    else {
        return false;
    }
}

makes it not crash when dragging there so i think you just need to handle that case and append it to your data list.

[Christian Ehrlicher]

@mrjj said in QTableView drag and drop crashing program:

beginInsertRows(parent, row, row + count + 1);

This is wrong: https://doc.qt.io/qt-5/qabstractitemmodel.html#beginInsertRows

[Rodrigo B.]

@JonB ,
It seems @mrjj figured it out, but to answer your question, I only see the information below. Perhaps there is a way to configure Qt Creator to show more but I haven't seen it (I am a newbie with it):

12:34:28: Starting /Users/rodrigobraz/Documents/Qt Projects/build-tabledraganddrop-Desktop_Qt_5_15_2_clang_64bit-Debug/qtableviewdraganddrop.app/Contents/MacOS/qtableviewdraganddrop ...
12:34:32: The program has unexpectedly finished.
12:34:32: The process was ended forcefully.
12:34:32: /Users/rodrigobraz/Documents/Qt Projects/build-tabledraganddrop-Desktop_Qt_5_15_2_clang_64bit-Debug/qtableviewdraganddrop.app/Contents/MacOS/qtableviewdraganddrop crashed.

[JonB]

@Rodrigo-B
For next time: you are supposed to run your code via Debug in Creator. Running under a debugger, it should stop when it crashed, and allow you to view where in your code the problem emanated.

[Rodrigo B.]

@mrjj That's a great answer, thank you very much!

And of course I now see I could have debugged just like you did... lesson learned! :-)

[Rodrigo B.]

@Christian-Ehrlicher Thank you, you're right! I had meant to type "-1".

[Rodrigo B.]

Great tip, thank you, @JonB!