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FP_NORMAL, FP_SUBNORMAL, FP_ZERO, FP_INFINITE, FP_NAN

定义于头文件 <cmath>
#define FP_NORMAL    /*implementation defined*/
 (C++11 起)
#define FP_SUBNORMAL /*implementation defined*/
 (C++11 起)
#define FP_ZERO      /*implementation defined*/
 (C++11 起)
#define FP_INFINITE  /*implementation defined*/
 (C++11 起)
#define FP_NAN       /*implementation defined*/
 (C++11 起)

FP_NORMAL 、 FP_SUBNORMAL 、 FP_ZERO 、 FP_INFINITE 、 FP_NAN 宏各代表一个独自的浮点数类别。它们都展开成整数常量表达式。

 
常量 解释
FP_NORMAL 指示值为正规，即不是无穷大、非正规、非数或零
FP_SUBNORMAL 指示值为非正规
FP_ZERO 指示值为正或负零
FP_INFINITE 指示值无法以底层类型表示（正或负无穷大）
FP_NAN 指示值是非数（ NaN ）

示例

运行此代码
#include <iostream>
#include <cmath>
#include <cfloat>
 
const char* show_classification(double x) {
    switch(std::fpclassify(x)) {
        case FP_INFINITE:  return "Inf";
        case FP_NAN:       return "NaN";
        case FP_NORMAL:    return "normal";
        case FP_SUBNORMAL: return "subnormal";
        case FP_ZERO:      return "zero";
        default:           return "unknown";
    }
}
int main()
{
    std::cout << "1.0/0.0 is " << show_classification(1/0.0) << '\n'
              << "0.0/0.0 is " << show_classification(0.0/0.0) << '\n'
              << "DBL_MIN/2 is " << show_classification(DBL_MIN/2) << '\n'
              << "-0.0 is " << show_classification(-0.0) << '\n'
              << "1.0 is " << show_classification(1.0) << '\n';
}

输出： 

1.0/0.0 is Inf
0.0/0.0 is NaN
DBL_MIN/2 is subnormal
-0.0 is zero
1.0 is normal

参阅

(C++11)
 归类给定的浮点值
(函数)