ok, the problem is in Disp:: openDatumForm in this line
Qt Code: 
  1. ...
  2. if(datumform.exec() == QDialog::Accepted)
  3. ...
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when you close this dialog by Alt+F4 then the result of exec will be QDialog::Rejected and you pass this block
Qt Code: 
  1. {
  2.    DatumVal = Datumno_Val;
  3.    XVal = Xdatum_Val;
  4.    YVal = Ydatum_Val;
  5.    ZVal = Zdatum_Val;
  6.    datuminitial();  
  7. }
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there is two ways:
add a "OK" button in that dialog and connect signal QPushButton::clicked with QDialog::accept and leave this code unchanged
Qt Code: 
  1. void Disp::openDatumForm()
  2. {
  3. 	DatumForm datumform(this);
  4. 	datumform.move(35, 90);
  5. 	if(datumform.exec() == QDialog::Accepted) {
  6. 		DatumVal = Datumno_Val;
  7. 		XVal = Xdatum_Val;
  8. 		YVal = Ydatum_Val;
  9. 		ZVal = Zdatum_Val;
  10. 		datuminitial();
  11. 	}
  12. }
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the second ways is: to use code like this
Qt Code: 
  1. void Disp::openDatumForm()
  2. {
  3. 	DatumForm datumform(this);
  4. 	datumform.move(35, 90);
  5. 	datumform.exec();
  6. 	DatumVal = Datumno_Val;
  7. 	XVal = Xdatum_Val;
  8. 	YVal = Ydatum_Val;
  9. 	ZVal = Zdatum_Val;
  10. 	datuminitial();
  11. }
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in this case there is no need to add extra button in the dialog, but you also lose checking of the dialog acception or rejection.