std::variant_alternative, std::variant_alternative_t
Defined in header <variant>
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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>; |
(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,
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<I,T>>, std::add_volatile_t<std::variant_alternative<I,T>>, and std::add_cv_t<std::variant_alternative<I,T>>Member types
Member type | Definition |
type | the type of I th alternative of the variant, where I must be in [0, sizeof...(Types)) , otherwise the program is ill-formed.
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Helper template alias
template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; |
(since C++17) | |
Example
This section is incomplete Reason: no example |
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 2974 | C++17 | out-of-bounds index resulted in undefined behavior | made ill-formed |
See also
(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) |