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Next, to ensure access to those display_capacity functions, you need
to install a virtual version of display_capacity in the
railroad_car class:
class railroad_car {
public: railroad_car ( ) { }
virtual void display_short_name ( ) { }
virtual void display_capacity ( ) { }
};
Although this version of the display_capacity does no displaying,
you accomplish two objectives by defining it. First, because this version
is marked virtual, C++ knows that railroad_car pointers may be
directed at run time to class objects that require versions of
display_capacity found in railroad_car subclasses.
Second, assuming that there are no definitions for display_capacity
in the engine and caboose classes, inheritance ensures that
the version found in the railroad_car class is the version used when
engines or cabooses are encountered. Thus, the do-nothing version of
display_capacity found in the railroad_car class acts as a
safety net for class objects that otherwise would not have a
display_capacity member function.
*----------------*
| railroad_car |
*----------------*
^ ^ ^ ^
*----------------------------------------* | | |
| *----------------------* | |
| | *--------------* |
| | | |
*----------* *----------* *----------* *----------*
| box_car | | tank_car | | engine | | caboose |
*----------* *----------* *----------* *----------*
--------------------------------- ---------------------------------
These classes define their own These classes inherit the version
versions of the display_capacity of the display_capacity function
function defined in railroad_car