Defined in header <variant> | ||
|---|---|---|
template <std::size_t I, class T> struct variant_alternative; /* undefined */ | (1) | (since C++17) |
template <size_t I, class... Types> struct variant_alternative<I, variant<Types...>>; | (2) | (since C++17) |
template <size_t I, class T> class variant_alternative<I, const T>; template <size_t I, class T> class variant_alternative<I, volatile T>; template <size_t I, class T> class variant_alternative<I, const volatile T>; | (3) | (since C++17) |
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,
type equal to the type of the alternative with index I
type that names, respectively, std::add_const_t<std::variant_alternative<I,T>>, std::add_volatile_t<std::variant_alternative<I,T>>, and std::add_cv_t<std::variant_alternative<I,T>>
| 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. |
template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; | (since C++17) |
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 |
|
(C++17) | obtains the size of the variant's list of alternatives at compile time (class template) (variable template) |
| obtains the type of the specified element (class template specialization) |
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