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;

std::void_t

Notes

Examples

See also