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Accordingly, you must somehow arrange for your compiled program
to determine which display_short_name function to use,
rather than having the C++ compiler decide, in advance, at
compile time.
You could easily imagine many ways to convey such a message to
C++. The way the designers of C++ decided to convey the
message is to place a special symbol, virtual, in front of
the definition of display_short_name in the
railroad_car class:
class railroad_car {
public: // Constructor:
railroad_car ( ) { }
// Virtual display function:
virtual void display_short_name ( ) { }
};
Thus, to mark a member function for selection at run time, rather than at
compile time, you preface its definition with virtual. Such a
function is said to be a
virtual member function.
Virtual member functions are called virtual because the information required to determine which function to use is not available—hence is only virtual—at compile time.