The alias templatedetected_or is an alias for an unspecified class type with two public member typedefsvalue_t andtype, which are defined as follows:

• If thetemplate-idOp<Args...> denotes a valid type, thenvalue_t is an alias forstd::true_type, andtype is an alias forOp<Args...>;
• Otherwise,value_t is an alias forstd::false_type andtype is an alias forDefault.

The alias templateis_detected is equivalent totypename detected_or<std::experimental::nonesuch, Op, Args...>::value_t. It is an alias forstd::true_type if thetemplate-idOp<Args...> denotes a valid type; otherwise it is an alias forstd::false_type.

The alias templatedetected_t is equivalent totypename detected_or<std::experimental::nonesuch, Op, Args...>::type. It is an alias forOp<Args...> if thattemplate-id denotes a valid type; otherwise it is an alias for the classstd::experimental::nonesuch.

[edit]Additional utilities

The alias templateis_detected_exact checks whetherdetected_t<Op, Args...> isExpected.

The alias templateis_detected_convertible checks whetherdetected_t<Op, Args...> is convertible toTo.

[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 <cstddef>#include <experimental/type_traits> 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(){}

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