514: Mainline

In Chapter 14, you learned that class definitions can have a private part as well as a public part. More specifically, you learned that you can access a class's private member variables only through member functions that belong to the same class.

If, for example, you decide that a cylinder's dimensions never change once the cylinder is built, you can reflect that constraint in the cylinder class definition by moving the dimensions out of the public interface into the private part of the definition:

class box {
  public:
    // Default constructor:
    box ( ) { }
    // Argument-bearing constructor:
    box (double h, double w, double l) {
      height = h; width = w; length = l;
    }
    // Volume member function:
    double volume ( ) {return height * width * length;}
  private: double height, width, length;  
}; 

Given this box class definition, only the argument-bearing constructor defined in the box class can write values into the three dimension member variables. Similarly, only the volume member function can reference values in those member variables.

Because no other access to the member variables is allowed, no member function of box_car can write directly into or read directly from the dimension member variables; all writing and reading must go through the argument-bearing constructor and volume functions defined in the box class's public interface.