NAME¶

std::variant_alternative,std::variant_alternative_t - std::variant_alternative,std::variant_alternative_t

Synopsis¶

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

class variant_alternative<I, volatile T>; (3) (since C++17)
template <std::size_t I, class T> (deprecated in C++20)

class variant_alternative<I, const volatile T>;

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, (since C++17)
T>::type;

Example¶

// Run this code

#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 made ill-formed
behavior

See also¶

variant_size obtains the size of the variant's list of
variant_size_v alternatives at compile time
(C++17) (class template) (variable template)
std::tuple_element<std::tuple> obtains the type of the specified element
(C++11) (class template specialization)