Qtreeview colored border - c++

Question

I have a QTreeView that is filled using an AbstractItemModel. I want to highlight some entries by showing a red border based on an internal state.
Currently my code looks like this:
QVariant MyAbstractItemModel::data(QModelIndex const& index, int role)
...
else if (Qt::BackgroundRole == role)
{
if (someMethod(index))
return QColor(255,0,0);
return QVariant();
} ...
Obviously this codes sets the background color to red and not the border color.
How can I set the border color of the item?

Answer 1

So thanks to SaZ the solution is the QStyledItemDelegate.
My code looks now like this:
QVariant MyAbstractItemModel::data(QmodelIndex const&, int role)
{
...
else if (Qt::UserRole == role)
{
return someMethod(index);
}
...
}
and I have a delegate:
class MyDelegate
: public QStyledItemDelegate
{
virtual void paint(QPainter* painter, QStyleOptionViewItem const& option, QModelIndex const& index) const override
{
QStyledItemDelegate::paint(painter, option, index);
QStyleOptionViewItemV4 opt = option;
initStyleOption(&opt, index);
if (index.data(Qt::UserRole))
{
QRect rect(opt.rect.x(), opt.rect.y(), opt.rect.width(), opt.rect.height());
painter.save();
painter.setPen(Qt::red);
painter.drawRect(rect);
painter->restore();
}
}
}
...
MyDelegate _delegate;
_ui.myTreeView->setItemDelegate(&_delegate);