How can I specialize a class for enums of underlying type int?

Question

#include <type_traits>

enum class MyEnum
{
    Hello
};

template <typename T, typename Enable = void>
class MyClass
{
public:
    MyClass(T obj) : e(obj)
    {}

private:
    T e;
};

template <typename T>
class MyClass <T, 
    typename std::enable_if< std::is_enum<T>::value 
                             && std::is_same<typename std::underlying_type<T>::type, 
                                             int>::value>::type > 
{
public:
    MyClass(T obj) : e(obj)
    {}

private:
    T e;
};

int main()
{
    MyClass<MyEnum> c(MyEnum::Hello);
    MyClass<int> c1(1); //does not compile due to std::underlying_type
    return 0;
}

I would like to be able to specialize MyClass for enums of underlying type 'int'. I can't do that using std::underlying_type because it is only specific to enums. Any ideas on how I can proceed?

Answer 1

A simple enough solution to this quirk is to insulate std::underlying_type behind your own SFINAE-friendly trait:

template <class T, class = void>
struct underlying_type {};

template <class T>
struct underlying_type<
    T,
    typename std::enable_if<std::is_enum<T>::value>::type
> {
    using type = typename std::underlying_type<T>::type;
};

Then your specialization can be written as:

template <typename T>
class MyClass<
    T, 
    typename std::enable_if<std::is_same<
        typename underlying_type<T>::type,
        int
    >::value>::type
> {
    // ...
};

Answer 2

With one extra (delayed) indirection with conditional<..>::type::type:

template <typename T> struct Identity { using type = T; };

template <typename E>
class MyClass <E, 
    typename std::enable_if<
        std::is_enum<E>::value 
        && std::is_same<
            int,
            typename std::conditional<
                std::is_enum<E>::value,
                std::underlying_type<E>,
                Identity<E>
            >::type::type
        >::value
    >::type
>
{
    // ...
};

We really does std::underlying_type<E>::type only for enum.

Demo