C++ – Can an Enum Class Be Converted to the Underlying Type?

c++c++11

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_fields : unsigned { field = 1 };

unsigned a = my_fields::field;

That assignment is being rejected by GCC. error: cannot convert 'my_fields' to 'unsigned int' in assignment.

Best Answer

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!

Related Solutions

C++ – Manually Define Conversion for Enum Class

No, it's not.

Actually, an enum class is no class at all. The class keyword is only used because suddenly changing the unscoped enum to a scoped enum would have mean reworking all enums codes. So the committee decided that to distinguish between new-style and old-style enums, the new ones would be tagged with class, because it's a keyword already so no enum could have been named class in C++. They could have picked another, it would not have made much more sense anyway.

However, despite the class keyword they are still regular enums in that only enumerators (and potentially values assigned to them) are allowed within the brackets.

C++ – Conversion from int to Enum Class Type

Yes, you can convert both ways: int to enum class and enum class to int. This example should be self explanatory:

enum class Color{Red = 1, Yellow = 2, Green = 3, Blue = 4};
std::cout << static_cast<int>(Color::Green) << std::endl; // 3
// more flexible static_cast - See Tony's comment below
std::cout << static_cast<std::underlying_type_t<Color>>(Color::Green) << std::endl; // 3
std::cout << (Color::Green == static_cast<Color>(3)) << std::endl; // 1
std::cout << (Color::Green == static_cast<Color>(2)) << std::endl; // 0

You can try it yourself here.

[EDIT] Since C++23, we'll have available std::to_underlying (at <utility>), what will allow us to write:

std::cout << std::to_underlying(Color::Green) << std::endl; // 3

Best Answer

Related Solutions

C++ – Manually Define Conversion for Enum Class

C++ – Conversion from int to Enum Class Type

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