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 attr

Answer 1

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.