function_traits.hpp

template <typename T>
struct function_traits : public function_traits<decltype(&T::operator())>{
};

template <typename ClassType, typename ReturnType, typename... Args>
struct function_traits<ReturnType(ClassType::*)(Args...) const> {
    static constexpr auto arity = sizeof...(Args);
    typedef ReturnType result_type;
    
    template <size_t i>
    using arg = std::tuple_element_t<i, std::tuple<Args...>>;
    using args = typename std::tuple<Args...>;
};