std::void_t

 

C++

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Type support

Basic types
Fixed width integer types (C++11)
Fixed width floating-point types (C++23)
ptrdiff_t size_t max_align_t (C++11)     byte (C++17)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast

 

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 { };

See also

enable_if (C++11)

conditionally removes a function overload or template specialization from overload resolution (class template)