Base enum class inheritance

Question

Is there a pattern where I can inherit enum from another enum in C++??

Something like that:

enum eBase 
{
   one=1, two, three
};


enum eDerived: public eBase
{
   four=4, five, six
};

Answer 1

Well, if you'll define enum with the same name in derived class and start it from last item of correspondent enum in base class, you'll receive almost what you want - inherited enum. Look at this code:

class Base
{
public:
enum ErrorType
  {
  GeneralError,
  NoMemory,
  FileNotFound,
  LastItem
  }
}

class Inherited: public Base
{
enum ErrorType
  {
  SocketError = Base::LastItem,
  NotEnoughBandwidth,
  }
}

Answer 2

Not possible. There is no inheritance with enums.

You can instead use classes with named const ints.

Example:

class Colors
{
public:
  static const int RED = 1;
  static const int GREEN = 2;
};

class RGB : public Colors
{
  static const int BLUE = 10;
};


class FourColors : public Colors
{
public:
  static const int ORANGE = 100;
  static const int PURPLE = 101;
};

Answer 3

As stated by bayda, enum's don't (and/or shouldn't) have functionality, so I've taken the following approach to your quandary by adapting Mykola Golubyev's response:

typedef struct
{
    enum
    {
        ONE = 1,
        TWO,
        LAST
    };
}BaseEnum;

typedef struct : public BaseEnum
{
    enum
    {
        THREE = BaseEnum::LAST,
        FOUR,
        FIVE
    };
}DerivedEnum;

Answer 4

How about this? Ok an instance is created for every possible value, but besides that its very flexible. Are there any downsides?

.h:

class BaseEnum
{
public:
  static const BaseEnum ONE;
  static const BaseEnum TWO;

  bool operator==(const BaseEnum& other);

protected:
  BaseEnum() : i(maxI++) {}
  const int i;
  static int maxI;
};

class DerivedEnum : public BaseEnum
{
public:
  static const DerivedEnum THREE;
};

.cpp:

int BaseEnum::maxI = 0;

bool BaseEnum::operator==(const BaseEnum& other) {
  return i == other.i;
}

const BaseEnum BaseEnum::ONE;
const BaseEnum BaseEnum::TWO;
const DerivedEnum DerivedEnum::THREE;

Usage:

BaseEnum e = DerivedEnum::THREE;

if (e == DerivedEnum::THREE) {
    std::cerr << "equal" << std::endl;
}

Answer 5

Unfortunately it is not possible in C++14. I hope we will have such a language feature in C++17. As you already got few workarounds for your problem I won't provide a solution.

I would like to point out that the wording should be "extension" not "inheritance". The extension allows for more values (as you're jumping from 3 to 6 values in your example) whereas inheritance means putting more constraints to a given base class so the set of possibilities shrinks. Therefore, potential casting would work exactly opposite from inheritance. You can cast derived class to the base class and not vice-verse with class inheritance. But when having extensions you "should" be able to cast the base class to its extension and not vice-verse. I am saying "should" because, as I said such a language feature still doesn't exist.

Answer 6

Impossible.
But you can define the enum anonymously in a class, then add additional enum constants in derived classes.