std::numeric_limits<T>::signaling_NaN

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std::numeric_limits
静态常量
(C++11)
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(C++11)
numeric_limits::signaling_NaN
辅助类型
 
static T signaling_NaN() throw();
 (C++11 前)
static constexpr T signaling_NaN() noexcept;
 (C++11 起)

返回特殊值“发信的非数”，以浮点类型 T 表示。仅若 std::numeric_limits<T>::has_signaling_NaN == true 才有意义。在最常见的浮点数二进制表示 IEEE 754 中，任何所有指数位均为 1 且至少有一位尾数位为 1 的值表示 NaN 。何种尾数位的值表示安静或发信的 NaN ，及符号位是否有意义，是实现定义的。

返回值

T std::numeric_limits<T>::signaling_NaN()
/* non-specialized */ T()
bool false
char ​0​
signed char ​0​
unsigned char ​0​
wchar_t ​0​
char8_t (C++20) ​0​
char16_t (C++11) ​0​
char32_t (C++11) ​0​
short ​0​
unsigned short ​0​
int ​0​
unsigned int ​0​
long ​0​
unsigned long ​0​
long long (C++11) ​0​
unsigned long long (C++11) ​0​
float 实现定义（可为 FLT_SNAN ）
double 实现定义（可为 DBL_SNAN ）
long double 实现定义（可为 LDBL_SNAN ）

注解

NaN 决不与自身比较相等。 IEEE-754 不要求复制 NaN 保留其位表示（符号与载荷），尽管大多数实现都会保留。

当发信 NaN 被用于算术表达式的参数时，可能引发适当的浮点异常，而且 NaN 被“安静化”，即表达式返回安静的 NaN 。

示例

演示用发信的 NaN 引发浮点异常

运行此代码
#include <iostream>
#include <limits>
#include <cfenv>
#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';
}

输出： 

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

参阅

[静态]
 鉴别能表示特殊值“发信的非数”（ NaN ）的浮点类型
(公开静态成员常量)
[静态]
 返回给定浮点类型的安静 NaN 值
(公开静态成员函数)
(C++11)
 检查给定的数是否 NaN
(函数)
来自“https://zh.cppreference.com/mwiki/index.php?title=cpp/types/numeric_limits/signaling_NaN&oldid=73366”