std::experimental::is_detected, std::experimental::detected_t, std::experimental::detected_or
Defined in header <experimental/type_traits>
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template< template<class...> class Op, class... Args > using is_detected = /* see below */; |
(library fundamentals TS v2) | |
template< template<class...> class Op, class... Args > using detected_t = /* see below */; |
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template< class Default, template<class...> class Op, class... Args > using detected_or = /* see below */; |
(library fundamentals TS v2) | |
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, andtype
is an alias for Op<Args...>; - Otherwise,
value_t
is an alias for std::false_type andtype
is an alias forDefault
.
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 > constexpr bool is_detected_v = is_detected<Op, Args...>::value; |
(library fundamentals TS v2) | |
template< class Default, template<class...> class Op, class... Args > using detected_or_t = typename detected_or<Default, Op, Args...>::type; |
(library fundamentals TS v2) | |
template <class Expected, template<class...> class Op, class... Args> using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>; |
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template <class Expected, template<class...> class Op, class... Args> constexpr bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value; |
(library fundamentals TS v2) | |
template <class To, template<class...> class Op, class... Args> using is_detected_convertible = std::is_convertible<detected_t<Op, Args...>, To>; |
(library fundamentals TS v2) | |
template <class To, template<class...> class Op, class... Args> constexpr bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value; |
(library fundamentals TS v2) | |
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
#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() {}