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Maintaining multiple copies of member variables and member functions makes software development and maintenance difficult as you try to correct bugs, to add features, to improve performance, and to change behavior. Adding multiple programmers and multiple years to the mix turns mere difficulty into certain failure.
Of course, with just two new member variables and one new member function
about which to worry, you could cope. In a more complex example,
every kind of container, from railroad car to cereal box, would
have a percentage_loaded member variable, along with a variety of
other member variables, capturing information dealing with such
characteristics as durability and visual appeal. Similarly,
every kind of railroad car would have not only a
year_built member variable and an age member function, but
also a variety of other member variables and member functions dealing with
such characteristics as their weight, number, owner, and destination.