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

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

NAME

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

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