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std::variant_alternative, std::variant_alternative_t - cppreference.com

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Defined in header <variant>

template <std::size_t I, class T> struct variant_alternative; /* undefined */

 (1) (since C++17)

template <std::size_t I, class... Types> struct variant_alternative<I, variant<Types...>>;

 (2) (since C++17)

template <std::size_t I, class T> class variant_alternative<I, const T>;

 (3) (since C++17)

template <std::size_t I, class T> class variant_alternative<I, volatile T>; template <std::size_t I, class T> class variant_alternative<I, const volatile T>;

 (3) (since C++17)
(deprecated in C++20)

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_t<I,T>>, std::add_volatile_t<std::variant_alternative_t<I,T>>, and std::add_cv_t<std::variant_alternative_t<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

#include <variant>
#include <iostream>

using my_variant = std::variant<int, float>;
static_assert(std::is_same_v
    <int,   std::variant_alternative_t<0, my_variant>>);
static_assert(std::is_same_v
    <float, std::variant_alternative_t<1, my_variant>>);
// cv-qualification on the variant type propagates to the extracted alternative type.
static_assert(std::is_same_v
    <const int, std::variant_alternative_t<0, const my_variant>>);

int main()
{
    std::cout << "All static assertions passed.\n";
}

Output: 

All static assertions passed.

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