Attach custom object to QStandardItem in Qt

Question

I'm using QTreeView to show some data to the user. What I want is to attach an actual object to each node represented using QStandardItem.

To save the object reference into QStandardItem:

QStandardItem *child = new QStandardItem(s);
child->setFlags(child->flags() & ~Qt::ItemIsEditable);
child->setData(QVariant(QVariant::UserType, i), Qt::UserRole + 10);

To access the actual object when it is clicked in the UI:

void MyOtherClass::handleTreeViewSelectionChanged(const QModelIndex &i)
{
     MyClass* o = i.data(Qt::UserRole + 10).value<MyClass*>();
     // do other stuff with o
}

The above call just returns NULL. Anybody knows how to deal with such a requirement?

I found absolutely nothing useful on the web.

Any help would be highly appreciated.

Answer 1

To store your item in the QStandardItem you need to make sure that you've registered your type with QMetaType. For example, you might have this definition:

class MyType
{
public:
    MyType() : m_data(0) {}
    int someMethod() const { return m_data; }

private:
    int m_data;
};

Q_DECLARE_METATYPE(MyType*);  // notice that I've declared this for a POINTER to your type

Then you would store it into a QVariant like so:

MyType *object = new MyType;
QVariant variant;
variant.setValue(object);

Given a properly registered metatype for your type, you can now do something like this with your QStandardItems:

MyType *object = new MyType;
QStandardItemModel model;
QStandardItem *parentItem = model.invisibleRootItem();
QStandardItem *item = new QStandardItem;
item->setData(QVariant::fromValue(myType));  // this defaults to Qt::UserRole + 1
parentItem->appendRow(item);

And then later extract it:

void MyOtherClass::handleTreeViewSelectionChanged(const QModelIndex &i)
{
     MyType* o = i.data(Qt::UserRole + 1).value<MyType*>();
     // do other stuff with o
}