std::numeric_limits<T>:: signaling_NaN

返回由浮点类型 T 表示的特定值“信令 非数值”。仅当 std:: numeric_limits < T > :: has_signaling_NaN == true 时具有意义。在IEEE 754（最常用的浮点数二进制表示标准）中，任何指数位全为1且小数位至少有一位为1的值均表示NaN。小数位的哪些值表示静默NaN或信令NaN，以及符号位是否具有意义，均由实现定义。

返回值

注释

NaN 永远不会与自身相等。根据 IEEE-754 标准，复制 NaN 不要求保留其位表示（符号和 有效载荷 ），但大多数实现会保留。

当信号NaN被用作算术表达式的参数时，可能会引发相应的浮点异常，同时该NaN会被"静默化"，即表达式将返回一个静默NaN。

示例

#include <cfenv>
#include <iostream>
#include <limits>
#pragma STDC_FENV_ACCESS on
void show_fe_exceptions()
{
    int n = std::fetestexcept(FE_ALL_EXCEPT);
    if (n & FE_INVALID)
        std::cout << "FE_INVALID is raised\n";
    else if (n == 0)
        std::cout << "no exceptions are raised\n";
    std::feclearexcept(FE_ALL_EXCEPT);
}
int main()
{
    double snan = std::numeric_limits<double>::signaling_NaN();
    std::cout << "After sNaN was obtained, ";
    show_fe_exceptions();
    double qnan = snan * 2.0;
    std::cout << "After sNaN was multiplied by 2, ";
    show_fe_exceptions();
    double qnan2 = qnan * 2.0;
    std::cout << "After the quieted NaN was multiplied by 2, ";
    show_fe_exceptions();
    std::cout << "The result is " << qnan2 << '\n';
}

After sNaN was obtained, no exceptions are raised
After sNaN was multiplied by 2, FE_INVALID is raised
After the quieted NaN was multiplied by 2, no exceptions are raised
The result is nan

参见