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One virtual function can shadow another, just as an ordinary member
function can shadow another. Thus, the display_short_name virtual
functions in the engine, box_car, tank_car, and
caboose classes shadow the display_short_name virtual member
function in the railroad_car class.
Assuming that you never intend to create any individual class objects
at the railroad_car level, the only reason to define the
display_short_name function in the railroad_car
class definition is to inform C++ that lower-level
display_short_name virtual functions are defined.
On the other hand, if you fail to define a
display_short_name function, either deliberately or
accidentally, in, say, the engine class definition, then
there will be nothing to shadow the display_short_name
function defined in the railroad_car class definition
when you attempt to display the short name of an engine.
Accordingly, your program will display rrc.