My apologies, in my last post it should have said-: "Searching though the program I found this for "addLine", which is declared as a non virtual function.
Thanks to @Repeater, I understand a little more about how to call virtual fuction in another class.
Thanks to all who took an interest.
Actually, assuming Doc_plugin_interface is derived from Document_Interface, it's not. Because void addLine(QPointF *start, QPointF *end) is declared virtual in the base class, it will be virtual in all derived subclasses. So, even though the keyword virtual may not have been used in the Doc_plugin_interface declaration, the function was declared as a virtual function through inheritance.