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Using the specialized pattern, you can overload the output operator so that output statements can handle railroad-car operands, surrounding each car's short name with brackets:
ostream& operator<< (ostream& output_stream, railroad_car& r) {
output_stream << "[" << r.short_name ( ) << "]";
return output_stream;}
Note that the parameter, r, declared to be a
railroad_car must be a call-by-reference parameter, because
you want the overloaded output operator to handle objects that
belong to subclasses of the railroad_car class, such as
box_car objects.
As you learned in Chapter 37, if r were a call-by-value
parameter, and the argument were, say, a box_car object, then
only that part of the object inherited from the railroad_car class
would be copied, and there would be no way for the overloaded output
operator to know that [box], rather than [rrc], is supposed
to be displayed.