Can an enum class be converted to the underlying type?

Question

I think you can use std::underlying_type to know the underlying type, and then use cast:

#include <type_traits> //for std::underlying_type

typedef std::underlying_type<my_fields>::type utype;

utype a = static_cast<utype>(my_fields::field);

With this, you don't have to assume the underlying type, or you don't have to mention it in the definition of the enum class like enum class my_fields : int { .... } or so.

You can even write a generic convert function that should be able to convert any enum class to its underlying integral type:

template<typename E>
constexpr auto to_integral(E e) -> typename std::underlying_type<E>::type 
{
   return static_cast<typename std::underlying_type<E>::type>(e);
}

then use it:

auto value = to_integral(my_fields::field);

auto redValue = to_integral(Color::Red);//where Color is an enum class!

And since the function is declared to be constexpr, you can use it where constant expression is required:

int a[to_integral(my_fields::field)]; //declaring an array

std::array<int, to_integral(my_fields::field)> b; //better!

You cannot convert it implicitly, but an explicit cast is possible:

enum class my_fields : unsigned { field = 1 };

// ...

unsigned x = my_fields::field; // ERROR!
unsigned x = static_cast<unsigned>(my_fields::field); // OK

Also mind the fact, that the semicolon should be after the closed curly brace in your enum's definition, not before.

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

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

    template<typename EnumType,typename UnaryFunction>
    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<EnumType>::value,"Enum type required");

        auto enumIntegralValue = getIntegralEnumValue(enumValue);
        integralWritingFunction(enumIntegralValue);
        enumValue = toEnum<EnumType>(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

With C++23 you'll finally get a library function for this:

std::to_underlying

It is already implemented in the standard libraries of GCC 11, Clang 13, and MSVC 19.30 (aka 2022 17.0).

Until you're able to use C++23 I recommend you (re)name any custom implementation to to_underlying and place it between a #if !defined(__cpp_lib_to_underlying) #endif block, which is the associated feature test macro. This way you can simply ditch the code at some point in the future when C++23 becomes available for you.