what does the constructor do here?

[squidge]

You can use whatever code you like in the constructor, in this case various widgets are added depending on the value to pass to the constructor.

For example, if you pass ButtonWidget::ComboBoxes, then you get ComboBoxes, if you pass ( ButtonWidget::ComboBoxes | ButtonWidget::SpinBoxes) then you'll get boith ComboBoxes and SpinBoxes, etc.

If you really wanted to you could write an entire application in your constructor, but its not best practice to do so.