How to implement passing a value from a Qt form to a variable?

Question

Qt

There is a form with a trivial spinBox on it.
How to extract its value from it and pass it to a variable in a child class?
Not using the header file that qt designer generates.

Answer 1

doc.qt.io/qt-5/qobject.html#findChild
doc.qt.io/qt-4.8/qspinbox.html#value-prop