Question

Coming from a Java background, I find C++’s enums very lame. I wanted to know how to write Java-like enums (the ones in which the enum values are objects, and can have attributes and methods) in C++.

For example, translate the following Java code (a part of it, sufficient to demonstrate the technique) to C++ :

public enum Planet {
    MERCURY (3.303e+23, 2.4397e6),
    VENUS   (4.869e+24, 6.0518e6),
    EARTH   (5.976e+24, 6.37814e6),
    MARS    (6.421e+23, 3.3972e6),
    JUPITER (1.9e+27,   7.1492e7),
    SATURN  (5.688e+26, 6.0268e7),
    URANUS  (8.686e+25, 2.5559e7),
    NEPTUNE (1.024e+26, 2.4746e7);

    private final double mass;   // in kilograms
    private final double radius; // in meters
    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }
    private double mass()   { return mass; }
    private double radius() { return radius; }

    // universal gravitational constant  (m3 kg-1 s-2)
    public static final double G = 6.67300E-11;

    double surfaceGravity() {
        return G * mass / (radius * radius);
    }
    double surfaceWeight(double otherMass) {
        return otherMass * surfaceGravity();
    }
    public static void main(String[] args) {
        if (args.length != 1) {
            System.err.println("Usage:  java Planet <earth_weight>");
            System.exit(-1);
        }
        double earthWeight = Double.parseDouble(args[0]);
        double mass = earthWeight/EARTH.surfaceGravity();
        for (Planet p : Planet.values())
           System.out.printf("Your weight on %s is %f%n",
                             p, p.surfaceWeight(mass));
    }
}

Any help would be greatly appreciated!

Thanks!

Answer 1

One way to simulate Java enums is to create a class with a private constructor that instantiates copies of itself as static variables:

class Planet {  
  public: 
    // Enum value DECLARATIONS - they are defined later 
    static const Planet MERCURY;  
    static const Planet VENUS;  
    // ... 

  private: 
    double mass;   // in kilograms  
    double radius; // in meters  

  private: 
    Planet(double mass, double radius) {  
        this->mass = mass;  
        this->radius = radius;  
    } 

  public: 
    // Properties and methods go here 
}; 

// Enum value DEFINITIONS 
// The initialization occurs in the scope of the class,  
// so the private Planet constructor can be used. 
const Planet Planet::MERCURY = Planet(3.303e+23, 2.4397e6);  
const Planet Planet::VENUS = Planet(4.869e+24, 6.0518e6);  
// ... 

Then you can use the enums like this:

double gravityOnMercury = Planet::MERCURY.SurfaceGravity();