C++17一个很冷门很有意思的新特性

template< class... >using void_t = void;

template <class, class = std::void_t<>>struct has_type : std::false_type {};
template <class T>struct has_type<T, std::void_t<typename T::type>> : std::true_type {};

template <class, class = std::void_t<>>struct has_a_member : std::false_type {};
template <class T>struct has_a_member<T, std::void_t<decltype(std::declval<T>().a)>> : std::true_type {};

template <typename, typename = void>constexpr bool is_iterable{};
template <typename T>constexpr bool is_iterable<T, std::void_t<decltype(std::declval<T>().begin()), decltype(std::declval<T>().end())>> = true;

template <class T, class = void>struct has_hello_func : std::false_type {};
template <class T>struct has_hello_func<T, std::void_t<decltype(std::declval<T>().hello())>> : std::true_type {};

struct HasType {  typedef int type;};struct NHasType {  int hello;};
struct Hasa {  int a;};struct NHasa {  int b;};
struct HasHello {  void hello();};struct NoHasHello {};
int main() {  std::cout << has_type<HasType>::value << '\n';   // 1  std::cout << has_type<NHasType>::value << '\n';  // 0
  std::cout << has_a_member<Hasa>::value << '\n';   // 1  std::cout << has_a_member<NHasa>::value << '\n';  // 0
  std::cout << has_hello_func<HasHello>::value << '\n';    // 1  std::cout << has_hello_func<NoHasHello>::value << '\n';  // 0
  std::cout << is_iterable<std::vector<double>> << '\n';  // 1  std::cout << is_iterable<double> << '\n';               // 0}

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