indexWidget() unexpectedly returns a NULL pointer

I am attempting to create a model/view application in Qt 4.7.1. I am a very new Qt developer.

Summary of what I am attempting to do:

I have a treeview that is organized as a rectangular table of rows and columns. One column of items contains a button. By default this button is to be transparent and disabled. A given button is to become visible and enabled when the mouse is hovering over its row.

The approach I am pursuing is to

1. find the model index for the cell that the mouse is hovering over, and
2. obtain a pointer to the widget associated with the widget, and
3. using this pointer manipulate the visibility of the button within said widget.

I cannot get a valid pointer to the widget.

my current code looks like this:

void HistoryTreeView::mouseMoveEvent(QMouseEvent *event)
{
   QAbstractItemModel *m(model());

   // Only do something when a model is set.
   if (m)
   {
      QModelIndex index = indexAt(even开发者_运维百科t->pos());
      if (index.isValid())
      {
         // if the mouse has moved to another row
         if (index.row() != m_currentRow)
         {
           m_currentRow = index.row();

           QMessageBox::information( this, "HistoryTreeView", QString("index(%1)").arg(index.row()));

           QWidget * item = indexWidget(index);
           Q_ASSERT(item != NULL  );
        }
     }
     else  // model is invalid
     {
         m_currentRow = -1;
     }
  }

  QTreeView::mouseMoveEvent(event);
}

The symptoms:

I expected the call to indexWidget() to return a valid pointer to the widget the mouse is over. Instead it unexpectedly returns a NULL pointer.

Commentary:

The variable named 'index' is acting as I expected because the QMessageBox shows the correct row value. Consequently I do not think there is anything wrong with the value I am providing to indexWidget().

This is just debug code. It is missing things like code that selects the column that holds the buttons.

---

OK, Here is the nature of my error as I understand it.

I had incorrectly understood that every item in a view is its own widget. I now understand that the view itself is a widget, but that individual items within the view are not widgets, per se.

Because I had misunderstood that view items were widgets I believed that could:

1. obtain an index from a given element in a model,
2. use indexWidget() to obtain a Widget * to the view item associated with the model element
3. and then use this pointer to manipulate the view item as though it was a widget.

indexWidget() simply returned a NULL because view items are not widgets.