enum size and initialization of variable [duplicate] - c++

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_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.

Answer 1

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!

Answer 2

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.

Answer 3

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.

Answer 4

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

Answer 5

I find the following function underlying_cast useful when having to serialise enum values correctly.
namespace util
{
namespace detail
{
template <typename E>
using UnderlyingType = typename std::underlying_type<E>::type;
template <typename E>
using EnumTypesOnly = typename std::enable_if<std::is_enum<E>::value, E>::type;
} // namespace util.detail
template <typename E, typename = detail::EnumTypesOnly<E>>
constexpr detail::UnderlyingType<E> underlying_cast(E e) {
return static_cast<detail::UnderlyingType<E>>(e);
}
} // namespace util
enum SomeEnum : uint16_t { A, B };
void write(SomeEnum /*e*/) {
std::cout << "SomeEnum!\n";
}
void write(uint16_t /*v*/) {
std::cout << "uint16_t!\n";
}
int main(int argc, char* argv[]) {
SomeEnum e = B;
write(util::underlying_cast(e));
return 0;
}