std::variant_alternative, std::variant_alternative_t

 

C++

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Technical Specifications

 

Utilities library

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
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strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
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pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
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swap
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std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Non-member functions

visit

holds_alternative

get

get_if

operator==operator!=operator<operator<=operator>operator>=

swap

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash

Helper objects

variant_npos

 

Defined in header <variant>
template <std::size_t I, class T> struct variant_alternative; /* undefined */	(1)	(since C++17)
template <size_t I, class... Types> struct variant_alternative<I, variant<Types...>>;	(2)	(since C++17)
template <size_t I, class T> class variant_alternative<I, const T>; template <size_t I, class T> class variant_alternative<I, volatile T>; template <size_t I, class T> class variant_alternative<I, const volatile T>;	(3)	(since C++17)

Provides compile-time indexed access to the types of the alternatives of the possibly cv-qualified variant, combining cv-qualifications of the variant (if any) with the cv-qualifications of the alternative.

Formally,

2) meets the TransformationTrait requirements with a member typedef type equal to the type of the alternative with index I

3) meets the TransformationTrait requirements with a member typedef type that names, respectively, std::add_const_t<std::variant_alternative<I,T>>, std::add_volatile_t<std::variant_alternative<I,T>>, and std::add_cv_t<std::variant_alternative<I,T>>

Member types

Member type	Definition
type	the type of Ith alternative of the variant, where I must be in [0, sizeof...(Types)), otherwise the program is ill-formed.

Helper template alias

template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type;		(since C++17)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2974	C++17	out-of-bounds index resulted in undefined behavior	made ill-formed

See also

variant_sizevariant_size_v (C++17)	obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
tuple_element	obtains the type of the specified element (class template specialization)