如何在C++中编写具有多个数据字段的类Java-enum类？

Question

来自Java背景,我发现C++的枚举非常蹩脚.我想知道如何在C++中编写类似Java的枚举(枚举值是对象,可以有属性和方法).

例如,将以下Java代码(其中一部分,足以演示该技术)转换为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));
    }
}

任何帮助将不胜感激!

谢谢!

Answer 1

模拟Java枚举的一种方法是创建一个带有私有构造函数的类,该构造函数将自身的副本实例化为静态变量:

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);  
// ... 

然后你可以使用这样的枚举:

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

Answer 2

随着C++ 11的推出constexpr.还有另一种实现类型枚举的方法.一个与普通枚举实际上相同的(存储为int变量并可以在switch语句中使用),但也允许它们具有成员函数.

在头文件中,您将放置:

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.

并在源文件中:

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());
}

然后可以用作:

double gravityOnMercury = PLANET_MERCURY.SurfaceGravity();

不幸的是,枚举条目不能在类体中定义为静态常量.它们必须在声明时初始化,因为它们是constexpr,但在类中,类不是一个完整的类型,因此无法实例化.