std::void_t
| Defined in header <type_traits>
|
||
| template< class... > using void_t = void; |
(since C++17) | |
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
See also
| (C++11) |
hides a function overload or template specialization based on compile-time boolean (class template) |