QAbstractItemView asks model for invalid index

Question

I'm writting a program using Qt5 which works fine on Linux but on Windows I was observing strange behaviour:

When QTreeView::setModel was called, it asked model for index (QAbstractItemModel::index) with some row and col and invalid parent. It never happend on Linux, view always asked for hasChildren, rowCount etc before calling index.

I've downloaded Qt5's sources to see what's going on and I can see:

// These asserts do basic sanity checking of the model
Q_ASSERT_X(d->model->index(0,0) == d->model->index(0,0),
           "QAbstractItemView::setModel",
           "A model should return the exact same index "
           "(including its internal id/pointer) when asked for it twice in a row.");
Q_ASSERT_X(!d->model->index(0,0).parent().isValid(),
           "QAbstractItemView::setModel",
           "The parent of a top level index should be invalid");

I cannot find a single word about those sanity checks in documentation of view classes nor model classes.

Where are they defined?

Another interesting thing here is that I could deduce by observation of model/view classes I've written that top index should be invalid but I couldn't find this information directly in docs.

Answer 1

From the documentation of QAbstractItemModel::parent():

If the item has no parent, an invalid QModelIndex is returned.

This means that calling index() with an invalid QModelIndex asks for top level items.

The sanity checks you encountered may have been disabled in Linux (maybe release build?) - but your model's functionality should in no way depend on the order of function calls.

If index() is called with invalid row / column parameters (also if your model is not populated yet), return QModelIndex().