QTableView massive model data updates issue

Hi,

Just learning QT as an option for replacing existing WPF application which shows horizontally scrolled set of mid size grids (10 grids, 100 rows x 20 cols) with custom cell contents (some numbers, arrows, signs ...). Grid contents are refreshed about 2-3 times per second, almost all cells. Updates are done through model.

The issue is the following, application crashes randomly and I think it is related to QVariant object manipulation, check/set/get variant from model. Here is the sample code that is crashing after running for some time (10 sec to 2-3 mins):

Here is the code:

Code:

---

#include <QApplication>
#include <QtCore>
#include <QWidget>
#include <QStandardItemModel>
#include <QTableView>
#include <QScrollArea>
#include <QHBoxLayout>
#include <QHeaderView>
#include <QSizePolicy>
#include <QMainWindow>
#include <iostream>
#include <QtGui>
#include <stdlib.h>
 
using namespace std;
 
class MyObject {
 
};
 
Q_DECLARE_METATYPE(MyObject)
 
 
class PaintDelegate : public QStyledItemDelegate {
 
public:
    PaintDelegate(QWidget *parent = 0) : QStyledItemDelegate(parent) {}
 
    void paint(QPainter *painter, const QStyleOptionViewItem &option,
                         const QModelIndex &index) const
    {
        if (qVariantCanConvert<MyObject>(index.data(Qt::UserRole + 1))) {
            MyObject myObj = qVariantValue<MyObject>(index.data(Qt::UserRole + 1));
 
            painter->save();
            painter->translate(option.rect.x() + 5, option.rect.y() + 10);
            painter->drawText(0, 0, QString("test %1").arg(rand()%1000));
            painter->restore();
 
        } else {
            QStyledItemDelegate::paint(painter, option, index);
        }
    }
 
    QSize sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const
    {
        qVariantCanConvert<MyObject>(index.data(Qt::UserRole + 1));
        return QSize(50, 10);
    }
};
 
int main(int argc, char** argv)
{
    QApplication app( argc, argv );
 
    QStandardItemModel *model = new QStandardItemModel();
    QStandardItem * item = new QStandardItem("item 1");
    item->setData(qVariantFromValue(MyObject()));
    model->setItem(0, 0, item);
    QTableView * tableView2 = new QTableView();
    tableView2->setModel(model);
    tableView2->setItemDelegate(new PaintDelegate);
    tableView2->horizontalHeader()->setResizeMode(QHeaderView::ResizeToContents);
    tableView2->show();
 
    extern void updateModel(QStandardItemModel*);
    QtConcurrent::run(updateModel, model);
 
    return app.exec();
}
 
void updateModel(QStandardItemModel* model){
    for (;;){
        model->item(0, 0)->setData(qVariantFromValue(MyObject()));
        usleep(1000);
    }
}

---

Here is the error:

Code:

---

*** glibc detected *** /home/bob/Documents/TableViewTest-build-desktop/TableViewTest: free(): invalid pointer: 0x00007f57d000a190 ***
======= Backtrace: =========
/lib/libc.so.6(+0x774b6)[0x7f57ea7cd4b6]
/lib/libc.so.6(cfree+0x73)[0x7f57ea7d3c83]
/home/bob/Documents/TableViewTest-build-desktop/TableViewTest[0x4031e8]
/usr/lib/libQtCore.so.4(_ZN9QMetaType7destroyEiPv+0x62)[0x7f57eb6089a2]
/usr/lib/libQtCore.so.4(+0x18bfe2)[0x7f57eb620fe2]
/home/bob/Documents/TableViewTest-build-desktop/TableViewTest[0x402db7]
/usr/lib/libQtGui.so.4(+0x73f04d)[0x7f57ec07004d]
/usr/lib/libQtGui.so.4(_ZN10QTableView10paintEventEP11QPaintEvent+0x913)[0x7f57ec071753]
/usr/lib/libQtGui.so.4(_ZN7QWidget5eventEP6QEvent+0x59e)[0x7f57ebb3ec2e]
/usr/lib/libQtGui.so.4(_ZN6QFrame5eventEP6QEvent+0x26)[0x7f57ebf0a5e6]
/usr/lib/libQtGui.so.4(_ZN17QAbstractItemView13viewportEventEP6QEvent+0x3eb)[0x7f57ec043a8b]
/usr/lib/libQtCore.so.4(_ZN23QCoreApplicationPrivate29sendThroughObjectEventFiltersEP7QObjectP6QEvent+0x87)[0x7f57eb5fd507]
/usr/lib/libQtGui.so.4(_ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent+0x7c)[0x7f57ebae8fac]
/usr/lib/libQtGui.so.4(_ZN12QApplication6notifyEP7QObjectP6QEvent+0x14d)[0x7f57ebaeeaed]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication14notifyInternalEP7QObjectP6QEvent+0x8c)[0x7f57eb5fdcdc]
/usr/lib/libQtGui.so.4(_ZN14QWidgetPrivate10drawWidgetEP12QPaintDeviceRK7QRegionRK6QPointiP8QPainterP19QWidgetBackingStore+0x3bd)[0x7f57ebb4567d]
/usr/lib/libQtGui.so.4(+0x3e83a6)[0x7f57ebd193a6]
/usr/lib/libQtGui.so.4(_ZN14QWidgetPrivate16syncBackingStoreEv+0x80)[0x7f57ebb38730]
/usr/lib/libQtGui.so.4(_ZN7QWidget5eventEP6QEvent+0xc65)[0x7f57ebb3f2f5]
/usr/lib/libQtGui.so.4(_ZN6QFrame5eventEP6QEvent+0x26)[0x7f57ebf0a5e6]
/usr/lib/libQtGui.so.4(_ZN19QAbstractScrollArea5eventEP6QEvent+0x8b)[0x7f57ebf9871b]
/usr/lib/libQtGui.so.4(_ZN17QAbstractItemView5eventEP6QEvent+0x9b)[0x7f57ec03c51b]
/usr/lib/libQtGui.so.4(_ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent+0xac)[0x7f57ebae8fdc]
/usr/lib/libQtGui.so.4(_ZN12QApplication6notifyEP7QObjectP6QEvent+0x14d)[0x7f57ebaeeaed]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication14notifyInternalEP7QObjectP6QEvent+0x8c)[0x7f57eb5fdcdc]
/usr/lib/libQtCore.so.4(_ZN23QCoreApplicationPrivate16sendPostedEventsEP7QObjectiP11QThreadData+0x2d2)[0x7f57eb600c22]
/usr/lib/libQtCore.so.4(+0x195653)[0x7f57eb62a653]
/lib/libglib-2.0.so.0(g_main_context_dispatch+0x1f2)[0x7f57e9deb342]
/lib/libglib-2.0.so.0(+0x442a8)[0x7f57e9def2a8]
/lib/libglib-2.0.so.0(g_main_context_iteration+0x6c)[0x7f57e9def45c]
/usr/lib/libQtCore.so.4(_ZN20QEventDispatcherGlib13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE+0x73)[0x7f57eb62a193]
/usr/lib/libQtGui.so.4(+0x26aa4e)[0x7f57ebb9ba4e]
/usr/lib/libQtCore.so.4(_ZN10QEventLoop13processEventsE6QFlagsINS_17ProcessEventsFlagEE+0x32)[0x7f57eb5fca02]
/usr/lib/libQtCore.so.4(_ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE+0xdc)[0x7f57eb5fcdec]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication4execEv+0xbb)[0x7f57eb600ebb]
/home/bob/Documents/TableViewTest-build-desktop/TableViewTest[0x4025e6]
/lib/libc.so.6(__libc_start_main+0xfe)[0x7f57ea774d8e]
/home/bob/Documents/TableViewTest-build-desktop/TableViewTest[0x4022c9]

---

Please advice or correct me if I am using View/Model approach incorrectly.

Simplified version of the code which gives the same error:

Code:

---

#include <QApplication>
#include <QtCore>
#include <QStandardItemModel>
#include <QTableView>
#include <QtGui>
 
using namespace std;
 
class MyObject {
 
};
 
Q_DECLARE_METATYPE(MyObject)
 
 
class PaintDelegate : public QStyledItemDelegate {
 
public:
    PaintDelegate(QWidget *parent = 0) : QStyledItemDelegate(parent) {}
 
    void paint(QPainter *painter, const QStyleOptionViewItem &option,
                         const QModelIndex &index) const
    {
        if (qVariantCanConvert<MyObject>(index.data(Qt::UserRole + 1))) {
            MyObject myObj = qVariantValue<MyObject>(index.data(Qt::UserRole + 1));
        } else {
            QStyledItemDelegate::paint(painter, option, index);
        }
    }
 
};
 
int main(int argc, char** argv)
{
    QApplication app( argc, argv );
 
    QStandardItemModel *model = new QStandardItemModel();
    QStandardItem * item = new QStandardItem("item 1");
    item->setData(qVariantFromValue(MyObject()));
    model->setItem(0, 0, item);
    QTableView * tableView2 = new QTableView();
    tableView2->setModel(model);
    tableView2->setItemDelegate(new PaintDelegate);
    tableView2->show();
 
    extern void updateModel(QStandardItemModel*);
    QtConcurrent::run(updateModel, model);
 
    return app.exec();
}
 
void updateModel(QStandardItemModel* model){
    for (;;){
        model->item(0, 0)->setData(qVariantFromValue(MyObject()));
    }
}

---

The original code didn't fail here on 32-bit Linux after a few minutes of running, the new code crashes immediately probably because the delay is no longer in your update loop. It could be a thread synchronisation issue. Have you considered locking? Have you tried using a QTimer and dispensing with the separate thread?

Are you intending to paint anything if the item contains a MyObject?

Hi, Thanks for the try.

This is just an example to show the error. If this is a defect inside qt maybe it should be logged as such so someone could fix it.

In my real app I used two QTimers and a synchronized temporary store (QSet) to accumulate changes. First QTimer puts data into the store (high frequency) another updates UI (less frequently). This approach has not crashed yet, maybe because I am not touching model at such a high rate.

I have 63bit Linux. The example above crashed once on 32bit windows virtual box.