std::experimental::is_detected, std::experimental::detected_t, std::experimental::detected_or - cppreference.com (original) (raw)

| Defined in header <experimental/type_traits> | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ---------------------------- | | 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 */; | | (library fundamentals TS v2) | | 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:

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.

[edit] Additional utilities

| template< template<class...> class Op, class... Args > constexpr bool is_detected_v = is_detected<Op, Args...>::value; | | (library fundamentals TS v2) | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ---------------------------- | | template< template<class...> class Op, class... Args > constexpr inline bool is_detected_v = is_detected<Op, Args...>::value; | | (library fundamentals TS v3) | | 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...>>; | | (library fundamentals TS v2) | | 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 Expected, template<class...> class Op, class... Args > constexpr inline bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value; | | (library fundamentals TS v3) | | 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) | | template< class To, template<class...> class Op, class... Args > constexpr inline bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value; | | (library fundamentals TS v3) |

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.

[edit] 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...>;

[edit] Example

#include #include <experimental/type_traits>   template 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 using diff_t = typename T::difference_type;   template 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, int>::value, "Woof's difference_type should be int!"); static_assert(std::is_same<difference_type, std::ptrdiff_t>::value, "Bark's difference_type should be ptrdiff_t!");   int main() {}