std::underlying_type(3)

NAME
std::underlying_type - std::underlying_type

Synopsis
   Defined in header <type_traits>
   template< class T > (since C++11)
   struct underlying_type;
   If T is a complete enumeration (enum) type, provides a member typedef type that
   names the underlying type of T.
   Otherwise, the behavior is undefined. (until C++20)
   Otherwise, if T is not an enumeration type, there is no member type.
   Otherwise (T is an incomplete enumeration type), the program is (since C++20)
   ill-formed.
   The behavior of a program that adds specializations for underlying_type is
   undefined.

Member types
   Name Definition
   type the underlying type of T

Helper types
   template< class T > (since C++14)
   using underlying_type_t = typename underlying_type<T>::type;

Notes
   Each enumeration type has an underlying type, which can be
   1. Specified explicitly (both scoped and unscoped enumerations)
   2. Omitted, in which case it is int for scoped enumerations or an
   implementation-defined integral type capable of representing all values of the enum
   (for unscoped enumerations)

Example
// Run this code
 #include <iostream>
 #include <type_traits>
 enum e1 {};
 enum class e2 {};
 enum class e3: unsigned {};
 enum class e4: int {};
 int main() {
   constexpr bool e1_t = std::is_same_v< std::underlying_type_t<e1>, int >;
   constexpr bool e2_t = std::is_same_v< std::underlying_type_t<e2>, int >;
   constexpr bool e3_t = std::is_same_v< std::underlying_type_t<e3>, int >;
   constexpr bool e4_t = std::is_same_v< std::underlying_type_t<e4>, int >;
   std::cout
     << "underlying type for 'e1' is " << (e1_t ? "int" : "non-int") << '\n'
     << "underlying type for 'e2' is " << (e2_t ? "int" : "non-int") << '\n'
     << "underlying type for 'e3' is " << (e3_t ? "int" : "non-int") << '\n'
     << "underlying type for 'e4' is " << (e4_t ? "int" : "non-int") << '\n'
     ;
 }

Possible output:
 underlying type for 'e1' is non-int
 underlying type for 'e2' is int
 underlying type for 'e3' is non-int
 underlying type for 'e4' is int
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2396 C++11 incomplete enumeration types were complete enumeration type
                       allowed required

See also
   is_enum checks if a type is an enumeration type
   (C++11) (class template)
   is_scoped_enum checks if a type is a scoped enumeration type
   (C++23) (class template)
   to_underlying converts an enumeration to its underlying type
   (C++23) (function template)