std::variant_alternative_t
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) |
提供编译时索引访问可能的cv限定变体的类型，并结合变体%28的cv-资格(如果有%29)和备选版本的cv-资格。
正式的，
2%29TransformationTrait成员类型的要求ftype等于索引的替代类型。I
3%29TransformationTrait成员类型的要求ftype他们的名字，分别，std::add_const_t<std::variant_alternative<I,T>>，，，std::add_volatile_t<std::variant_alternative<I,T>>，和std::add_cv_t<std::variant_alternative<I,T>>
成员类型
Member type | Definition |
|---|---|
type | the type of Ith alternative of the variant, where I must be in [0, sizeof...(Types)), otherwise the behavior is undefined |
辅助模板别名
template <size_t I, class T> using variant_alternative_t = typename variant_alternative<I, T>::type; | | (since C++17) |
|---|
例
另见
variant_sizevariant_size_v (C++17) | obtains the size of the variant's list of alternatives at compile time (class template) (variable template) |
|---|---|
tuple_element | obtains the type of the specified element (class template specialization) |
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