std::experimental::is_detected,std::experimental::detected_t,std::experimental::detected_or (3) - Linux Manuals

std::experimental::is_detected,std::experimental::detected_t,std::experimental::detected_or: std::experimental::is_detected,std::experimental::detected_t,std::experimental::detected_or

NAME

std::experimental::is_detected,std::experimental::detected_t,std::experimental::detected_or - std::experimental::is_detected,std::experimental::detected_t,std::experimental::detected_or

Synopsis

Defined in header <experimental/type_traits>
template< template<class...> class Op, class... Args > (library fundamentals TS v2)
using is_detected = /* see below */;
template< template<class...> class Op, class... Args > (library fundamentals TS v2)
using detected_t = /* see below */;
template< class Default, template<class...> class Op, class... Args > (library fundamentals TS v2)
using detected_or = /* see below */;

The alias template detected_or is an alias for an unspecified class type with two public member typedefs value_t and type, which are defined as follows:

* If the template-id Op<Args...> denotes a valid type, then value_t is an alias for std::true_type, and type is an alias for Op<Args...>;
* Otherwise, value_t is an alias for std::false_type and type is an alias for Default.

The alias template is_detected is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::value_t. It is an alias for std::true_type if the template-id Op<Args...> denotes a valid type; otherwise it is an alias for std::false_type.

The alias template detected_t is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::type. It is an alias for Op<Args...> if that template-id denotes a valid type; otherwise it is an alias for the class std::experimental::nonesuch.

Additional utilities

template< template<class...> class Op, class... Args > (library fundamentals TS v2)
constexpr bool is_detected_v = is_detected<Op, Args...>::value;
template< class Default, template<class...> class Op, class... Args > (library fundamentals TS v2)
using detected_or_t = typename detected_or<Default, Op, Args...>::type;
template <class Expected, template<class...> class Op, class... Args> (library fundamentals TS v2)
using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>;
template <class Expected, template<class...> class Op, class... Args> (library fundamentals TS v2)
constexpr bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value;
template <class To, template<class...> class Op, class... Args> (library fundamentals TS v2)
using is_detected_convertible = std::is_convertible<detected_t<Op, Args...>, To>;
template <class To, template<class...> class Op, class... Args> (library fundamentals TS v2)
constexpr bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value;

The alias template is_detected_exact checks whether detected_t<Op, Args...> is Expected.
The alias template is_detected_convertible checks whether detected_t<Op, Args...> is convertible to To.

Possible implementation

  namespace detail {
  template <class Default, class AlwaysVoid,
            template<class...> class Op, class... Args>
  struct detector {
    using value_t = std::false_type;
    using type = Default;
  };

  template <class Default, template<class...> class Op, class... Args>
  struct detector<Default, std::void_t<Op<Args...>>, Op, Args...> {
    // Note that std::void_t is a C++17 feature
    using value_t = std::true_type;
    using type = Op<Args...>;
  };

  } // namespace detail

  template <template<class...> class Op, class... Args>
  using is_detected = typename detail::detector<nonesuch, void, Op, Args...>::value_t;

  template <template<class...> class Op, class... Args>
  using detected_t = typename detail::detector<nonesuch, void, Op, Args...>::type;

  template <class Default, template<class...> class Op, class... Args>
  using detected_or = detail::detector<Default, void, Op, Args...>;

Example

// Run this code

  #include <experimental/type_traits>
  #include <cstddef>

  template<class T>
  using copy_assign_t = decltype(std::declval<T&>() = std::declval<const T&>());

  struct Meow { };
  struct Purr { void operator=(const Purr&) = delete; };

  static_assert(std::experimental::is_detected<copy_assign_t, Meow>::value,
                "Meow should be copy assignable!");
  static_assert(!std::experimental::is_detected_v<copy_assign_t, Purr>,
                "Purr should not be copy assignable!");
  static_assert(std::experimental::is_detected_exact_v<Meow&, copy_assign_t, Meow>,
                "Copy assignment of Meow should return Meow&!");

  template<class T>
  using diff_t = typename T::difference_type;

  template <class Ptr>
  using difference_type = std::experimental::detected_or_t<std::ptrdiff_t, diff_t, Ptr>;

  struct Woof { using difference_type = int; };
  struct Bark {};

  static_assert(std::is_same<difference_type<Woof>, int>::value,
                "Woof's difference_type should be int!");
  static_assert(std::is_same<difference_type<Bark>, std::ptrdiff_t>::value,
                "Bark's difference_type should be ptrdiff_t!");

  int main() {}