std::variant_alternative, std::variant_alternative_t
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std::variant
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variant_alternative
Defined in header
<variant>
template <std::size_t I, class T>
struct variant_alternative; /* undefined */
(1)
(since C++17)
struct variant_alternative; /* undefined */
template <std::size_t I, class... Types>
struct variant_alternative<I, variant<Types...>>;
(2)
(since C++17)
struct variant_alternative<I, variant<Types...>>;
template <std::size_t I, class T> class variant_alternative<I, const T>;
(3)
(since C++17)
template <std::size_t I, class T>
(3)
(since C++17) class variant_alternative<I, volatile T>;
template <std::size_t I, class T>
(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 I3) 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>>[edit] 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.
[edit] Helper template alias
template <size_t I, class T>
using variant_alternative_t = typename variant_alternative<I, T>::type;
(since C++17)
using variant_alternative_t = typename variant_alternative<I, T>::type;
[edit] 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.
[edit] 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 |
[edit] See also
(C++17)
variant's list of alternatives at compile time(class template) (variable template)[edit]