How to set non-selectable default text on QComboBox? - c++

Question

Using a regular QComboBox populated with items, if currentIndex is set to -1, the widget is empty. It would be very useful to instead have an initial descriptive text visible in the combo box(e.g. "--Select Country--", "--Choose Topic--", etc.) which is not shown in the dropdown list.
I couldn't find anything in the documentation, nor any previous questions with answers.

Answer 1

It doesn't appear that case was anticipated in the Combo Box API. But with the underlying model flexibility it seems you should be able to add your --Select Country-- as a first "legitimate" item, and then keep it from being user selectable:
QStandardItemModel* model =
qobject_cast<QStandardItemModel*>(comboBox->model());
QModelIndex firstIndex = model->index(0, comboBox->modelColumn(),
comboBox->rootModelIndex());
QStandardItem* firstItem = model->itemFromIndex(firstIndex);
firstItem->setSelectable(false);
Depending on what precise behavior you want, you might want to use setEnabled instead. Or I'd personally prefer it if it was just a different color item that I could set it back to:
Qt, How do I change the text color of one item of a QComboBox? (C++)
(I don't like it when I click on something and then get trapped to where I can't get back where I was, even if it's a nothing-selected-yet-state!)

Answer 2

One way you can do something similar is to set a placeholder:
comboBox->setPlaceholderText(QStringLiteral("--Select Country--"));
comboBox->setCurrentIndex(-1);
This way you have a default that cannot be selected.

Answer 3

Leaving my solution here from PyQt5. Create a proxy model and shift all the rows down one, and return a default value at row 0.
class NullRowProxyModel(QAbstractProxyModel):
"""Creates an empty row at the top for null selections on combo boxes
"""
def __init__(self, src, text='---', parent=None):
super(NullRowProxyModel, self).__init__(parent)
self._text = text
self.setSourceModel(src)
def mapToSource(self, proxyIndex: QModelIndex) -> QModelIndex:
if self.sourceModel():
return self.sourceModel().index(proxyIndex.row()-1, proxyIndex.column())
else:
return QModelIndex()
def mapFromSource(self, sourceIndex: QModelIndex) -> QModelIndex:
return self.index(sourceIndex.row()+1, sourceIndex.column())
def data(self, proxyIndex: QModelIndex, role=Qt.DisplayRole) -> typing.Any:
if proxyIndex.row() == 0 and role == Qt.DisplayRole:
return self._text
elif proxyIndex.row() == 0 and role == Qt.EditRole:
return None
else:
return super(NullRowProxyModel, self).data(proxyIndex, role)
def index(self, row: int, column: int, parent: QModelIndex = ...) -> QModelIndex:
return self.createIndex(row, column)
def parent(self, child: QModelIndex) -> QModelIndex:
return QModelIndex()
def rowCount(self, parent: QModelIndex = ...) -> int:
return self.sourceModel().rowCount()+1 if self.sourceModel() else 0
def columnCount(self, parent: QModelIndex = ...) -> int:
return self.sourceModel().columnCount() if self.sourceModel() else 0
def headerData(self, section: int, orientation: Qt.Orientation, role: int = ...) -> typing.Any:
if not self.sourceModel():
return None
if orientation == Qt.Vertical:
return self.sourceModel().headerData(section-1, orientation, role)
else:
return self.sourceModel().headerData(section, orientation, role)