std::variant_alternative,std::variant_alternative_t(3)

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)