| 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.
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;| (C++11)
| hides a function overload or template specialization based on compile-time boolean (class template) |
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