I have two questions please.
first, I dont understand how this cast from qt's examples (Tree Model Completer) works:

Qt Code: 
  1. QAbstractItemModel *completionModel = completer->completionModel();
  2. QAbstractProxyModel *proxy = qobject_cast<QAbstractProxyModel *>(completionModel);
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according to qt's class references, QAbstractItemModel doesnt inherit from QAbstractProxyModel.
although indeed completer->completionModel() is a QStandardItemModel, but it makes no deference (it inherits only from QAbstractItemModel that in turn doesnt inherit from QAbstractProxyModel).

complete code of that member function is this:

Qt Code: 
  1. void MainWindow::highlight(const QModelIndex &index)
  2. {
  3.     QAbstractItemModel *completionModel = completer->completionModel();
  4.     QAbstractProxyModel *proxy = qobject_cast<QAbstractProxyModel *>(completionModel);
  5.     if (!proxy)
  6.         return;
  7.     QModelIndex sourceIndex = proxy->mapToSource(index);
  8.     treeView->selectionModel()->select(sourceIndex, QItemSelectionModel::ClearAndSelect | QItemSelectionModel::Rows);
  9.     treeView->scrollTo(index);
  10. }
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another question is why do we use a QAbstractProxyModel here?
this modified code works too:

Qt Code: 
  1. void MainWindow::highlight(const QModelIndex &index)
  2. {
  3. /*     QAbstractItemModel *completionModel = completer->completionModel();
  4.     QAbstractProxyModel *proxy = qobject_cast<QAbstractProxyModel *>(completionModel);
  5.     if (!proxy)
  6.         return;
  7.     QModelIndex sourceIndex = proxy->mapToSource(index); */
  8.     treeView->selectionModel()->select(/* sourceIndex */ index, QItemSelectionModel::ClearAndSelect | QItemSelectionModel::Rows);
  9.     treeView->scrollTo(index);
  10. }
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qt 4.4.3