Can an enum class be converted to the underlying type?

Question

Is there a way to convert an enum class field to the underlying type? I thought this would be automatic, but apparently not.

enum class my_field         


        

Answer 1

As others have pointed out there is no implicit cast, but you can use an explicit static_cast. I use the following helper functions in my code to convert to and from an enum type and its underlying class.

    template
    constexpr inline decltype(auto) getIntegralEnumValue(EnumType enumValue)
    {
        static_assert(std::is_enum::value,"Enum type required");
        using EnumValueType = std::underlying_type_t;
        return static_cast(enumValue);
    }

    template
    constexpr inline EnumType toEnum(IntegralType value)
    {
        static_assert(std::is_enum::value,"Enum type required");
        static_assert(std::is_integral::value, "Integer required");
        return static_cast(value);
    }

    template
    constexpr inline void setIntegralEnumValue(EnumType& enumValue, UnaryFunction integralWritingFunction)
    {
        // Since using reinterpret_cast on reference to underlying enum type is UB must declare underlying type value and write to it and then cast it to enum type
        // See discussion on https://stackoverflow.com/questions/19476818/is-it-safe-to-reinterpret-cast-an-enum-class-variable-to-a-reference-of-the-unde

        static_assert(std::is_enum::value,"Enum type required");

        auto enumIntegralValue = getIntegralEnumValue(enumValue);
        integralWritingFunction(enumIntegralValue);
        enumValue = toEnum(enumIntegralValue);
    }

Usage code

enum class MyEnum {
   first = 1,
   second
};

MyEnum myEnum = MyEnum::first;
std::cout << getIntegralEnumValue(myEnum); // prints 1

MyEnum convertedEnum = toEnum(1);

setIntegralEnumValue(convertedEnum,[](auto& integralValue) { ++integralValue; });
std::cout << getIntegralEnumValue(convertedEnum); // prints 2