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std::variant_size_v

性病：变体[医]大小，STD：：变体[医]大小[医]v型

Defined in header ​​
template <class T> struct variant_size; /* undefined */(1)(since C++17)
template <class... Types> struct variant_size<std::variant<Types...>> : std::integral_constant<std::size_t, sizeof...(Types)> { };(2)(since C++17)
template <class T> class variant_size<const T>; template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>;(3)(since C++17)

以编译时常量表达式的形式提供对可能的cv限定变量中的替代方案的访问。

正式的，

2%29UnaryTypeTrait具有基本特征的需求std::integral_constant<std::size_t, sizeof...(Types)>

3%29UnaryTypeTrait具有基本特征的需求std::integral_constant<std::size_t, variant_size<T>>

辅助变量模板

template inline constexpr std::size_t variant_size_v = std::variant_size::value;​(since C++17)

继承自STD：积分[医]常量

成员常数

value staticsizeof...(Types) (public static member constant)

成员函数

operator std::size_tconverts the object to std::size_t, returns value (public member function)
operator() (C++14)returns value (public member function)

成员类型

TypeDefinition
value_typestd::size_t
typestd::integral_constant<std::size_t, value>

注记

所有的专门化std::variant_size满足感UnaryTypeTrait带着基本特征std::integral_constant<std::size_t, N>对一些人来说N...

例

另见

variant_alternativevariant_alternative_t (C++17)obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
tuple_sizeobtains the size of tuple at compile time (class template specialization)

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在CreativeCommonsAttribution下授权-ShareAlike未移植许可v3.0。

http：//en.cppreference.com/w/cpp/实用程序/变体/变体[医]大小

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