std::experimental::is_detected,std::experimental::detected_t, (3) Linux Manual Page

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()
{
}

See also