How to write a Java-enum-like class with multiple data fields in C++?

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();

Answer 2

With C++11's introduction of constexpr. There's yet another way to implement typed enums. One that works practically the same as normal enums (is stored as an int variable and can be used in a switch statement), but also allows them to have member functions.

In the header file you would put:

class Planet {
    int index;
public:
    static constexpr int length() {return 8;}
    Planet() : index(0) {}
    constexpr explicit Planet(int index) : index(index) {}
    constexpr operator int() const { return index; }

    double mass() const;
    double radius() const;

    double surfaceGravity() const;
};
constexpr Planet PLANET_MERCURY(0);
constexpr Planet PLANET_VENUS(1);
constexpr Planet PLANET_EARTH(2);
// etc.

And in the source file:

static double G = 6.67300E-11;

double Planet::mass() {
    switch(index) {
        case PLANET_MERCURY: return 3.303e+23;
        case PLANET_VENUS: return 4.869e+24;
        case PLANET_EARTH: return 5.976e+24;
        // Etc.
    }
}

double Planet::radius() {
    // Similar to mass.
}

double Planet::surfaceGravity() {
    return G * mass() / (radius() * radius());
}

Which can then be used as:

double gravityOnMercury = PLANET_MERCURY.SurfaceGravity();

Unfortunately, the enum entries cannot be defined as static constants within the class body. They must be initialized upon declaration, because they are constexpr, but inside the class, the class is not yet a complete type and thus cannot be instantiated.

Answer 3

May be this is what you want --

#include<iostream>

using namespace std;

class Planet {
    double mass,radius;

    Planet(double m, double r) : mass(m) : radius(r) {}

public:
    static const Planet MERCURY;

    void show(){
        cout<<mass<<","<<radius<<endl;
    }
} ;
const Planet Planet::MERCURY = Planet(1.0,1.2);

int main(){
    Planet p = Planet::MERCURY;
    p.show();
}

This is just a small code, Im sure you can modify this to suit your needs..

Answer 4

This is ugly, verbose, and generally a dumb way to go. But I figured I'd post a complete code example by way of explanation. For extra points it's actually possible to define a compile-time expanded iteration over the solar planets by tweaking the template specializations just a tad.

#include <string>
#include <sstream>
#include <iostream>
#include <cstdlib>

class Planet {
 public:
   static const double G = 6.67300E-11;

   Planet(const ::std::string &name, double mass, double radius)
        : name_(name), mass_(mass), radius_(radius)
      {}
   const ::std::string &name() const { return name_; }
   double surfaceGravity() const {
      return G * mass_ / (radius_ * radius_);
   }
   double surfaceWeight(double otherMass) const {
      return otherMass * surfaceGravity();
   }

 private:
   const ::std::string name_;
   const double mass_;
   const double radius_;
};

enum SolarPlanets {
   MERCURY,
   VENUS,
   EARTH,
   MARS,
   JUPITER,
   SATURN,
   URANUS,
   NEPTUNE
};

template <SolarPlanets planet>
class SolarPlanet : public Planet {
};

template <>
class SolarPlanet<MERCURY> : public Planet {
 public:
   SolarPlanet() : Planet("MERCURY", 3.303e+23, 2.4397e6) {}
};

template <>
class SolarPlanet<VENUS> : public Planet {
 public:
   SolarPlanet() : Planet("VENUS", 4.869e+24, 6.0518e6) {}
};

template <>
class SolarPlanet<EARTH> : public Planet {
 public:
   SolarPlanet() : Planet("EARTH", 5.976e+24, 6.37814e6) {}
};

template <>
class SolarPlanet<MARS> : public Planet {
 public:
   SolarPlanet() : Planet("MARS", 6.421e+23, 3.3972e6) {}
};

template <>
class SolarPlanet<JUPITER> : public Planet {
 public:
   SolarPlanet() : Planet("JUPITER", 1.9e+27, 7.1492e7 ) {}
};

template <>
class SolarPlanet<SATURN> : public Planet {
 public:
   SolarPlanet() : Planet("SATURN", 5.688e+26, 6.0268e7) {}
};

template <>
class SolarPlanet<URANUS> : public Planet {
 public:
   SolarPlanet() : Planet("URANUS", 8.686e+25, 2.5559e7) {}
};

template <>
class SolarPlanet<NEPTUNE> : public Planet {
 public:
   SolarPlanet() : Planet("NEPTUNE", 1.024e+26, 2.4746e7) {}
};

void printTerranWeightOnPlanet(
   ::std::ostream &os, double terran_mass, const Planet &p
   )
{
   const double mass = terran_mass / SolarPlanet<EARTH>().surfaceGravity();
   os << "Your weight on " << p.name() << " is " << p.surfaceWeight(mass) << '\n';
}

int main(int argc, const char *argv[])
{
   if (argc != 2) {
      ::std::cerr << "Usage: " << argv[0] << " <earth_weight>\n";
      return 1;
   }
   const double earthweight = ::std::atof(argv[1]);
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<MERCURY>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<VENUS>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<EARTH>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<MARS>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<JUPITER>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<SATURN>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<URANUS>());
   printTerranWeightOnPlanet(::std::cout, earthweight, SolarPlanet<NEPTUNE>());
   return 0;
}