std::variant_alternative, std::variant_alternative_t

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)

Example

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

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

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