<type_traits>

类模板
<type_traits>

std::conditional

template <bool Cond, class T, class F> struct conditional;
Conditional type
Obtains eitherTorF, depending on whetherCond的 C++ 等效文件是trueorfalse.

如果已知Condis true, member typeconditional::typeis defined as an alias ofT.
如果已知Condis false, member typeconditional::typeis defined as an alias ofF.

模板参数

Cond
A compile-time constant of typebool.
T,F
Types.

成员类型

成员类型定义
类型如果已知Condis trueT
如果已知Condis falseF

示例

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// conditional example
#include <iostream>
#include <type_traits>

int main() {

  short int i = 1;    // code does not compile if type of i is not integral

  typedef std::conditional<true,int,float>::type A;                      // int
  typedef std::conditional<false,int,float>::type B;                     // float
  typedef std::conditional<std::is_integral<A>::value,long,int>::type C; // long
  typedef std::conditional<std::is_integral<B>::value,long,int>::type D; // int

  std::cout << std::boolalpha;
  std::cout << "typedefs of int:" << std::endl;
  std::cout << "A: " << std::is_same<int,A>::value << std::endl;
  std::cout << "B: " << std::is_same<int,B>::value << std::endl;
  std::cout << "C: " << std::is_same<int,C>::value << std::endl;
  std::cout << "D: " << std::is_same<int,D>::value << std::endl;

  return 0;
}

输出
typedefs of int:
A: true
B: false
C: false
D: true


另见