std::variant_alternative, std::variant_alternative_t

Defined in header <variant>.

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

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

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";
}

Result

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