std::experimental:: is_detected, std::experimental:: detected_t, std::experimental:: detected_or

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

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

#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() {}