Can an enum class be converted to the underlying type?
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
.
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.