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std::conditional

STD：：有条件

Defined in header ​​
template< bool B, class T, class F > struct conditional;​(since C++11)

提供成员类型type，它被定义为T如果B是true在编译时，或F如果B是false...

成员类型

Member typeDefinition
typeT if B == true, F if B == false

帮助者类型

template< bool B, class T, class F > using conditional_t = typename conditional::type;​(since C++14)

可能的实施

模板<bool B，类T，类F>结构条件{ty胡枝子T型；}；模板<类T，类F>结构条件<false，T，F>{ty胡枝子F型；}；

*。

例

二次

#include <iostream>
#include <type_traits>
#include <typeinfo>
 
int main() 
{
    typedef std::conditional<true, int, double>::type Type1;
    typedef std::conditional<false, int, double>::type Type2;
    typedef std::conditional<sizeof(int) >= sizeof(double), int, double>::type Type3;
 
    std::cout << typeid(Type1).name() << '\n';
    std::cout << typeid(Type2).name() << '\n';
    std::cout << typeid(Type3).name() << '\n';
}

二次

产出：

二次

i
d
d

二次

另见

enable_if (C++11)hides a function overload or template specialization based on compile-time boolean (class template)

 © cppreference.com

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

http：//en.cppreference.com/w/cpp/type/条件

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