SFINAE 安全な underlying_type

SFINAE-safe_underlying_type.cpp

#include <iostream>
#include <type_traits>

template <typename T, typename = typename std::is_enum<T>::type>
struct safe_underlying_type { using type = T; };

template <typename T>
struct safe_underlying_type<T, std::true_type> { using type = std::underlying_type_t<T>; };

template <typename T>
using safe_underlying_type_t = typename safe_underlying_type<T>::type;

template <typename T>
constexpr safe_underlying_type_t<T> underlying_cast(T t) noexcept { return static_cast<safe_underlying_type_t<T>>(t); }

enum class foo : int { hoge, fuga };
enum class bar : char { hoge, fuga };
struct baz { int value; };

int main()
{
    static_assert(!std::is_same<safe_underlying_type_t<foo>, int>::value, "foo is enum:int");
    static_assert(!std::is_same<safe_underlying_type_t<bar>, char>::value, "bar is enum:char");
    static_assert(std::is_same<safe_underlying_type_t<baz>, int>::value, "baz isn't enum");
    
    baz a{123};
    baz b = underlying_cast(a);
    
    std::cout << a.value << std::endl;
    std::cout << b.value << std::endl;
    
    return 0;
}

Add underlying_cast.