generic_tostring_6.cpp

//big credits to this guy, for a hints: https://stackoverflow.com/a/16824239/4175394
template <typename Type>
struct HasToString {
private:
    template<typename T>
    static constexpr auto check(T*) -> typename std::is_same<
                                                             decltype(std::declval<T>().toString()), 
                                                             std::string
                                                            >::type;

    template<typename>
    static constexpr std::false_type check(...);

    using type = decltype(check<Type>(nullptr));

public:
    static constexpr bool value = type::value;
};