QModelIndex() - parent() and SIGSEV

[MasterQ]

Hi,

I am calling the parent() method of a QModelIndex and get a segmentation fault. Yes, the index do not have a parent. I then would expect to get back the default but not a crash.

        auto backendIndex = QSqlTableModel::createIndex(meItem->backendRow(), meIndex.column());
        qDebug() << "backendindex" << toString(backendIndex);

with

QString toString(QModelIndex const &index) {
    auto row = index.row();
    auto column = index.column();
    auto ret = QString("r: %1 c: %2").arg(row).arg(column);
    try {
        auto p = index.parent(); //<== crashes here
        if (index.parent().column() != -1)
            ret += QString(" p:(%1)").arg(toString(index.parent()));
    } catch (...) {
    }
    return ret;
}

Why does parent() crashes?

[MasterQ]

Sry, I didn't think about this opportunity. Of course the trace! I assume it's Monday ;-)

The trace is going back to QObject::parent, while the QObject is NULL. I do not have catched the case of NULL in my code.

Thanks for pointing me to the stack trace. File closed!

[jsulm]

@MasterQ Please post the stack trace after the crash

[MasterQ]

Sry, I didn't think about this opportunity. Of course the trace! I assume it's Monday ;-)

The trace is going back to QObject::parent, while the QObject is NULL. I do not have catched the case of NULL in my code.

Thanks for pointing me to the stack trace. File closed!

[MasterQ]

the database of the forum seems to be irritated. My last posts appear twice. first time before the post I am answering and second time after.

[JonB]

@MasterQ said in QModelIndex() - parent() and SIGSEV:

The trace is going back to QObject::parent, while the QObject is NULL

What QObject, and what NULL or nullptr? In your code neither index nor index.parent can be nullptr, they are not pointers, and nor are they QObjects. An invalid QModelIndex certianly still has valid row() & column(), not sure if it guarantees that parent() is also an invalid QModelIndex so unsure how index.parent().column() might behave.

[MasterQ]

QModelndex::parent() internally calls parent() from its model

    QModelIndex ProjectsBackendModel::parent(const QModelIndex &meIndex) const {

        if (!meIndex.isValid())
            return {};

        auto *meItem = static_cast<ProjectsViewTreeItem *>(meIndex.internalPointer());

        ProjectsViewTreeItem *parentItem = meItem ? meItem->parent() : m_root; // <= correct
        //ProjectsViewTreeItem *parentItem = meItem->parent(); // wrong, ignoring potential nullptr of meItem

        auto ret = parentItem != m_root
                       ? createIndex(parentItem->frontendRow(), 0, parentItem)
                       : QModelIndex{};
        return ret;
    }

internalPointer of meIndex was nullptr , so meIndex->parent() was crashing. The type of meItem (ProjectsViewTreeItem) is inherited from QObject.