using methods with parameter(s) in void_t for SFINAE

1
votes

I'm trying to build a struct is_container for using it on SFINAE template

  template<typename T, typename = void>
  struct is_container : std::false_type {};

  template<typename T>
  struct is_container<
    T,
    std::void_t<
      typename T::value_type,
      typename T::reference,
      typename T::const_reference,
      typename T::iterator,
      typename T::const_iterator,
      typename T::difference_type,
      typename T::size_type,

      decltype(std::declval<T>().begin()),
      decltype(std::declval<T>().end()),
      decltype(std::declval<T>().cbegin()),
      decltype(std::declval<T>().cend()) ,

      decltype(std::declval<T>().size()),
      decltype(std::declval<T>().max_size()),
      decltype(std::declval<T>().empty()),
      decltype(std::declval<T>().swap()) //<------ problem here
      >
    > : public std::true_type {};

  template<typename T>
  inline constexpr bool is_container_v = is_container<T>::value;

my problem is the swap function take a T& parameter and so static_assert(is_container_v<vector < int > >) failed. Is their a way to use methods with parameters in my void_t ?

1
c++sfinae

1 Answers

3
votes

You already used declval once to get something for a member function access. So just use it again to get an lvalue argument.

decltype(std::declval<T>().swap(std::declval<T&>()))