In Qt4/MVC,how to obtain the content of the selected item?

Hello, I wish to put my own data to the current selected item.How do I do this?
I simply know calling QModelIndex currentIndex() to get the data index of all the current selected items in the QTreeView.
But how to get the item so that I may change its content?

Code:

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QModelIndex idx = tree->currentIndex();
tree->model()->setData(idx, QVariant("blaa"));

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See QAbstractItemModel::setData() for reference.