segfault with QMessageBox in QAbstractItemModel::hasChildren

[eric.frederich]

It appears if I display something inside the implementation of hasChildren on a QAbstractItemModel I get a segfault.
This is easily reproducible with the simpletreemodel example included in the source tarball.
Simply add the following method to treemodel.cpp (and corresponding method to the header as well).
You can click through the first round of messages. After that when you mouse over one of the tree items it will give you another prompt. After clicking it you'll segfault.

#include <string>
#include <sstream>
 
// ...
 
bool TreeModel::hasChildren(const QModelIndex &parent) const
{
    std::ostringstream oss;
    if (!parent.isValid()) {
        oss << "hasChildren root" << std::endl;
    } else {
        oss << "hasChildren (" << parent.row() << "," << parent.column() << ")" << std::endl;
    }
    oss << QAbstractItemModel::hasChildren(parent) << std::endl;
 
    QMessageBox::information((QWidget*)QAbstractItemModel::parent(), "TITLE", QString::fromStdString(oss.str()));
    return QAbstractItemModel::hasChildren(parent);
 
}

#include <string>
#include <sstream>

// ...

bool TreeModel::hasChildren(const QModelIndex &parent) const
{
    std::ostringstream oss;
    if (!parent.isValid()) {
        oss << "hasChildren root" << std::endl;
    } else {
        oss << "hasChildren (" << parent.row() << "," << parent.column() << ")" << std::endl;
    }
    oss << QAbstractItemModel::hasChildren(parent) << std::endl;

    QMessageBox::information((QWidget*)QAbstractItemModel::parent(), "TITLE", QString::fromStdString(oss.str()));
    return QAbstractItemModel::hasChildren(parent);
    
}

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Since I'm sure it will be asked, let me explain why I'm doing this.
I have an application which connects to a server but can also work off-line.
While connected a user can drag an item from the server to their local cache.
This does a shallow copy.
If the user then closes and re-opens the application they will see their local copy of that top level item and 1 level of items under it.
Expanding nodes any further than what the client has cached locally will cause a login dialog to appear to log into the server.

What should I do?
I'm open to suggestions.

[anda_skoa]

The model index might have become invalid.

Basically model indexes are only considere valid as long as the model doesn't change or something else has happend, e.g. events have been processed.
The latter happens inside the message box.

You could try storing the index in a QPersitantModelIndex before you go into the nested event loop.

Ideally of course the model wouldn't use a message box with a nested event loop at all.

Cheers,
_

[eric.frederich]

Thanks for the reply.

I don't think a QPersistentModelIndex helps here.
There is only a single line after the QMessageBox where I call...

return QAbstractItemModel::hasChildren(parent);

return QAbstractItemModel::hasChildren(parent);

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If I were to store the result of this call as a boolean variable at the top of the method and just return it, then I'm not using the index at all after the QMessageBox and it still segfaults.

bool TreeModel::hasChildren(const QModelIndex &parent) const
{
    bool ret = QAbstractItemModel::hasChildren(parent);
    QPersistentModelIndex tmp = QPersistentModelIndex(parent);
    std::ostringstream oss;
    if (!parent.isValid()) {
        oss << "hasChildren root" << std::endl;
    } else {
        oss << "hasChildren (" << parent.row() << "," << parent.column() << ")" << std::endl;
    }
    oss << QAbstractItemModel::hasChildren(parent) << std::endl;
 
    QMessageBox::information((QWidget*)QAbstractItemModel::parent(), "TITLE", QString::fromStdString(oss.str()));
    return ret;
}

bool TreeModel::hasChildren(const QModelIndex &parent) const
{
    bool ret = QAbstractItemModel::hasChildren(parent);
    QPersistentModelIndex tmp = QPersistentModelIndex(parent);
    std::ostringstream oss;
    if (!parent.isValid()) {
        oss << "hasChildren root" << std::endl;
    } else {
        oss << "hasChildren (" << parent.row() << "," << parent.column() << ")" << std::endl;
    }
    oss << QAbstractItemModel::hasChildren(parent) << std::endl;

    QMessageBox::information((QWidget*)QAbstractItemModel::parent(), "TITLE", QString::fromStdString(oss.str()));
    return ret;
}

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[anda_skoa]

Hmm, what does the backtrace say where it crashes?

Is the parent really a QWidget? A C style cast is always a chance for trouble.

Cheers,
_