std::variant_alternative, std::variant_alternative_t - cppreference.com (original) (raw)

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,

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

[edit] Member types

[edit] Helper template alias

| template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; | | (since C++17) | | ----------------------------------------------------------------------------------------------------------- | | ------------- |

[edit] Example

#include #include   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.

[edit] See also