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< template<class...> class Op, class... Args > (library fundamentals TS
constexpr inline bool is_detected_v = is_detected<Op, v3)
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 Expected, template<class...> class Op,
class... Args>
(library fundamentals TS
constexpr inline bool is_detected_exact_v = v3)

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;
template <class To, template<class...> class Op, class...
Args>
(library fundamentals TS
constexpr inline bool is_detected_convertible_v = v3)

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...> {
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() {}