Code for checking Values:
ui->text_result->setText("Error! " + QVariant(a).toString() + "|" + QVariant(b).toString());
when I edit the Values with the DoubleSpinBoxes, then the Value a doesn't change. (i.e. It's 0.)
I think the problem is even worse than what is mentioned above.
double a = ui->input_a->value(); does not imply that when input_a changes it automatically updates a