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Suppose, for example, that your program contains a statement that
reads the diameter of a particular tank_car, t. If you
work with readers, you need to make no change to that statement
to accommodate the switch from a radius-based class definition to
a diameter-based class definition:
... t.read_diameter ( ) ...
On the other hand, if you do not work with the member variables in
tank_car objects using constructors, readers, and writers
only, you have to go through your entire program, modifying
myriad statements:
... t.radius ... --> ... .5 * t.diameter ...
Thus, constructors, readers, and writers isolate you from the effects of your efficiency-motivated switch from a radius-based class definition to a diameter-based class definition.