199: Mainline

If you work with the member variables in tank_car objects using constructors, readers, and writers only, you only need to change what happens in member functions:

class tank_car { 
  public: 
    double diameter, length; 
    tank_car ( ) {diameter = 7.0; length = 40.0;} 
    tank_car (double r, double l) {diameter = r * 2.0; length = l;} 
    double read_radius ( ) {return diameter / 2.0;} 
    void write_radius (double r) {diameter = r * 2.0;} 
    double read_diameter ( ) {return diameter;} 
    void write_diameter (double d) {diameter = d;} 
    double read_length ( ) {return length;} 
    void write_length (double l) {length = l;} 
    double volume ( ) {return .25 * pi * diameter * diameter * length;} 
};