I'm a bit unclear on your question. Is the problem that you have a QListWidgetItem* instead of a PlaceHolder*?
Try:
void MyWidget::handleDoubleClickSlot(QListWidgetItem* item)
{
PlaceHolder* placeHolderItem = qobject_cast<PlaceHolder*>(item);
if (placeHolderItem == NULL)
{
// oops, not a PlaceHolder item
return;
}
placeHolderItem->myAwesomePlaceHolderMethod();
}
As shown, it will return NULL if the item is somehow not a PlaceHolder object.