Wrapping a QStringListModel in a QAbstractItemModel renders a blank list - c++

Question

I want to start off making my own models for Qt list views, and I thought that I would start by wrapping a QStringListModel in my own QAbstractItemModel, then render it in a list view. However, it only renders a blank white square, instead of the list I expect. I don't really know what could be happening, given that all I'm doing is delegating all calls to the QStringListModel. Perhaps there's some aspects of the QStringListModel that are called by the QListView that are not mandated by the QAbstractItemModel pure virtual methods? Or maybe it's related to storage of the QStringList somehow?
My attempt is below. The header:
class DelegatingItemModel: public QAbstractItemModel {
public:
DelegatingItemModel();
QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
int columnCount(const QModelIndex &parent = QModelIndex()) const override;
int rowCount(const QModelIndex &parent = QModelIndex()) const override;
QModelIndex parent(const QModelIndex &index) const override;
QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const override;
private:
QAbstractItemModel* innerModel;
};
This is the implementation:
#include "delegating_item_model.hh"
DelegatingItemModel::DelegatingItemModel() {
QStringList available = {"foo", "bar", "baz"};
this->innerModel = new QStringListModel(available);
}
QVariant DelegatingItemModel::data(const QModelIndex &index, int role) const {
return innerModel->data(index, role);
}
int DelegatingItemModel::columnCount(const QModelIndex &parent) const {
return innerModel->columnCount(parent);
}
int DelegatingItemModel::rowCount(const QModelIndex &parent) const {
return innerModel->rowCount(parent);
}
QModelIndex DelegatingItemModel::parent(const QModelIndex &index) const {
return innerModel->parent(index);
}
QModelIndex DelegatingItemModel::index(int row, int column, const QModelIndex &parent) const {
return innerModel->index(row, column, parent);
}
And this is the entry point:
int main(int argc, char** argv) {
qDebug() << "Starting up";
QApplication app(argc, argv);
QMainWindow mainWindow;
QListView* listView = new QListView;
DelegatingItemModel* theModel = new DelegatingItemModel;
listView->setModel(theModel);
mainWindow.setCentralWidget(listView);
mainWindow.show();
return app.exec();
}

Answer 1

Your view will get data from the model only if the given index is linked to its model. If you print a trace in the data() method, you will see that it's never called.
So, you cannot return a new index created by your inner list model because it will be linked to the list and not your own model. For example:
QModelIndex DelegatingItemModel::index(int row, int column, const QModelIndex &parent) const {
//return innerModel->index(row, column, parent);
if (parent.isValid()) // It's a list. Not a tree
return QModelIndex();
return createIndex(row, column); // Create a index for your own model.
}
To be full compliant, you should convert the index in data():
QVariant DelegatingItemModel::data(const QModelIndex &index, int role) const {
QModelIndex const innerIndex(innerModel->index(index.row(), index.column()));
return innerModel->data(innerIndex, role);
}