Extending enums in C++?

Question

Is there a way in C++ to extend/\"inherit\" enums?

I.E:

enum Enum {A,B,C};
enum EnumEx : public Enum {D,E,F};

or at least define a

Answer 1

I had this problem in some projects that ran on small hardware devices I design. There is a common project that holds a number of services. Some of these services use enums as parameters to get additional type checking and safety. I needed to be able to extend these enums in the projects that use these services.

As other people have mentioned c++ doesn't allow you to extend enums. You can however emulate enums using a namespace and a template that has all the benefits of enum class.

enum class has the following benefits:

1. Converts to a known integer type.
2. Is a value type
3. Is constexpr by default and takes up no valuable RAM on small processors
4. Is scoped and accessible by enum::value
5. Works in case statements
6. Provides type safety when used as a parameter and needs to be explicitly cast

Now if you define a class as an enum you can't create constexpr instances of the enum in the class declaration, because the class is not yet complete and it leads to a compile error. Also even if this worked you could not extend the value set of enums easily later in another file/sub project .

Now namespaces have no such problem but they don't provide type safety.

The answer is to first create a templated base class which allows enums of different base sizes so we don't waste what we don't use.

template 
class EnumClass {
  private:
    TYPE value_;
  public:
    explicit constexpr EnumClass(TYPE value) :
        value_(value){
    }
    constexpr EnumClass() = default;
    ~EnumClass() = default;
    constexpr explicit EnumClass(const EnumClass &) = default;
    constexpr EnumClass &operator=(const EnumClass &) = default;

    constexpr operator TYPE() const {return    value_;}
    constexpr TYPE value() const {return value_;}

};

Then for each enum class we want to extend and emulate we create a namespace and a Type like this:

namespace EnumName {
   class Type :public Enum {
     public:
        explicit constexpr Type(uint8_t value): Enum(value){}
        constexpr Enum() = default;
   }
   constexpr auto Value1 = Type(1); 
   constexpr auto Value2 = Type(2); 
   constexpr auto Value3 = Type(3); 
}

Then later in your code if you have included the original EnumName you can do this:

   namespace EnumName {
       constexpr auto Value4 = Type(4U); 
       constexpr auto Value5 = Type(5U); 
       constexpr auto Value6 = Type(6U); 

       constexpr std::array Set = {Value1, Value2, Value3, Value4, Value5, Value6};
    }

now you can use the Enum like this:

#include 

void fn(EnumName::Type val){
    if( val != EnumName::Value1 ){
      std::cout << val;
    }
}

int main(){
  for( auto e :EnumName::Set){
    switch(e){
      case EnumName::Value1:  
        std::cout << "a";
        break;
      case EnumName::Value4:  
        std::cout << "b";
        break;
      default:
        fn(e);
    }
  }
}

So we have a case statement, enum comparisons, parameter type safety and its all extensible. Note the set is constexpr and wont end up using valuable RAM on a small micro (placement verified on Godbolt.org. :-). As a bonus we have the ability to iterate over a set of enum values.