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std:: conditional

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conditional
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Compile-time rational arithmetic
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定义于头文件 <type_traits>
template < bool B, class T, class F >
struct conditional ;
(C++11 起)

提供成员类型定义 type ，当编译时 B 为 true 时其定义为 T ，当 B 为 false 时其定义为 F 。

如果程序为 std::conditional 添加特化，则行为是未定义的。

目录

成员类型

成员类型 定义
type 当 B == true 时为 T ，当 B == false 时为 F

辅助类型

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

可能的实现 

template<bool B, class T, class F>
struct conditional { using type = T; };
template<class T, class F>
struct conditional<false, T, F> { using type = F; };

示例

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

可能的输出： 

int
double
double

参见

(C++11)
有条件地 移除 函数重载或模板特化，使其不参与重载决议
(类模板)