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std::is_union

STD：：是[医]联合

Defined in header ​​
template< class T > struct is_union;​(since C++11)

检查是否T是工会型的。提供成员常量。value，等于true如果T是工会型的。否则，value等于false...

模板参数

T-a type to check

辅助变量模板

template< class T > inline constexpr bool is_union_v = is_union::value;​(since C++17)

继承自STD：积分[医]常量

成员常数

value statictrue if T is a union type , false otherwise (public static member constant)

成员函数

operator boolconverts the object to bool, returns value (public member function)
operator() (C++14)returns value (public member function)

成员类型

TypeDefinition
value_typebool
typestd::integral_constant<bool, value>

例

二次

#include <iostream>
#include <type_traits>
 
struct A {};
 
typedef union {
    int a;
    float b;
} B;
 
struct C {
    B d;
};
 
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_union<A>::value << '\n';
    std::cout << std::is_union<B>::value << '\n';
    std::cout << std::is_union<C>::value << '\n';
    std::cout << std::is_union<int>::value << '\n';
}

二次

产出：

二次

false
true
false
false

二次

另见

is_class (C++11)checks if a type is a non-union class type (class template)

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http：//en.cppreference.com/w/cpp/type/is[医]联合

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