Another thing, use base 16 when convert from QString into int in hexChanged (because you will get empty string if you write FF or any "letter" in the hex lineEdit)
Qt Code: 
  1. void ByteConverterDialog::hexChanged(const QString& newValue)
  2. {
  3.     bool ok;
  4.     int num = newValue.toInt(&ok, 16);              // Here you need to use base 16
  5.  
  6.     if(ok)
  7.     {
  8.         decEdit->setText(QString::number(num));
  9.         binEdit->setText(QString::number(num,2));
  10.     }
  11.     else
  12.     {
  13.         decEdit->setText("");
  14.         binEdit->setText("");
  15.     }
  16. }
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