std::underlying_type - cppreference.com
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      std::underlying_type

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      <tbody> </tbody>
      Definido no cabeçalho <type_traits>
      template< class T > struct underlying_type;
      		 (desde C++11)
      Define um membro type typedef de tipo que é o tipo subjacente para a enumeração T.
      Original:
      Defines a member typedef type of type that is the underlying type for the enumeration T.
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      Tipos de membro

      Notas

      Cada tipo de enumeração tem um tipo subjacente, que pode ser
      Original:
      Each enumeration type has an underlying type, which can be
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      1. Especificado explicitamente (com escopo e enumerações sem escopo)
      Original:
      1. Specified explicitly (both scoped and unscoped enumerations)
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      2. Omitido, caso em que é int para enumerações de escopo ou uma implementação do tipo definido pelo integral capaz de representar todos os valores da enum (para enumerações sem escopo)
      Original:
      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)
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      Exemplo

      #include <iostream>
#include <type_traits>

enum e1 {};
enum class e2: int {};

int main() {
    bool e1_type = std::is_same<
        unsigned
       ,typename std::underlying_type<e1>::type
    >::value; 

    bool e2_type = std::is_same<
        int
       ,typename std::underlying_type<e2>::type
    >::value;
   
    std::cout
    << "underlying type for 'e1' is " << (e1_type?"unsigned":"non-unsigned") << '\n'
    << "underlying type for 'e2' is " << (e2_type?"int":"non-int") << '\n';
}

      Saída: 

      underlying type for 'e1' is unsigned
underlying type for 'e2' is int