std::void_t (3) - Linux Manuals

std::void_t: std::void_t

NAME

std::void_t - std::void_t

Synopsis

Defined in header <type_traits>
template< class... > (since C++17)
using void_t = void;

Utility metafunction that maps a sequence of any types to the type void

Notes

This metafunction is used in template metaprogramming to detect ill-formed types in SFINAE context:

  // primary template handles types that have no nested ::type member:
  template< class, class = std::void_t<> >
  struct has_type_member : std::false_type { };

  // specialization recognizes types that do have a nested ::type member:
  template< class T >
  struct has_type_member<T, std::void_t<typename T::type>> : std::true_type { };

It can also be used to detect validity of an expression:

  // primary template handles types that do not support pre-increment:
  template< class, class = std::void_t<> >
  struct has_pre_increment_member : std::false_type { };
  // specialization recognizes types that do support pre-increment:
  template< class T >
  struct has_pre_increment_member<T,
             std::void_t<decltype( ++std::declval<T&>() )>
         > : std::true_type { };

Until CWG_1558 (a C++14 defect), unused parameters in alias_templates were not guaranteed to ensure SFINAE and could be ignored, so earlier compilers require a more complex definition of void_t, such as

  template<typename... Ts> struct make_void { typedef void type;};
  template<typename... Ts> using void_t = typename make_void<Ts...>::type;

Examples

// Run this code

  #include <iostream>
  #include <type_traits>
  #include <vector>
  #include <map>

  class A {};

  template <typename T, typename = void>
  struct is_iterable : std::false_type {};
  template <typename T>
  struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
                                    decltype(std::declval<T>().end())>>
      : std::true_type {};

  // An iterator trait which value_type is always the value_type of the
  // iterated container, even with back_insert_iterator which value_type is void

  template <typename T, typename = void>
  struct iterator_trait
  : std::iterator_traits<T> {};
  template <typename T>
  struct iterator_trait<T, std::void_t<typename T::container_type>>
  : std::iterator_traits<typename T::container_type::iterator> {};

  int main()
  {
      std::cout << std::boolalpha;
      std::cout << is_iterable<std::vector<double>>::value << '\n';
      std::cout << is_iterable<std::map<int, double>>::value << '\n';
      std::cout << is_iterable<double>::value << '\n';
      std::cout << is_iterable<A>::value << '\n';

      std::vector<int> v;

      std::cout << std::is_same<iterator_trait<decltype(std::back_inserter(v))>::value_type
      , iterator_trait<decltype(v.cbegin())>::value_type >::value << '\n';
  }

Output:

  true
  true
  false
  false
  true