C++ Primer Plus

Using a State Member

The Vector class stores both the rectangular coordinates and the polar coordinates for a vector. It uses a member called mode to control which form the constructor, the reset() method, and the overloaded operator<<() function use, with the enumerations RECT representing the rectangular mode (the default) and POL the polar mode. Such a member is termed a state member because it describes the state an object is in. To see what this means, look at the code for the constructor:

Vector::Vector(double n1, double n2, Mode form){    mode = form;    if (form == RECT)    {         x = n1;         y = n2;         set_mag();         set_ang();    }    else if (form == POL)    {         mag = n1;         ang = n2 / Rad_to_deg;         set_x(); ...