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std::numeric_limits::signaling_NaN

STD：：数字[医]限制：：信令[医]南

static T signaling_NaN(​(until C++11)
static constexpr T signaling_NaN(​(since C++11)

返回特殊值“信令”。无号“，由浮点类型表示。T.只有在以下情况下才有意义std::numeric_limits<T>::has_signaling_NaN==true在ieee 754中，浮点数最常见的二进制表示形式，指数集的所有位数和分数集的至少一位的任何值都代表NaN。它是实现-定义的分数的值代表安静或信令的nans，以及符号位是否有意义。

返回值

Tstd::numeric_limits::signaling_NaN()
/* non-specialized */T()
boolfalse
char​0​
signed char​0​
unsigned char​0​
wchar_t​0​
char16_t​0​
char32_t​0​
short​0​
unsigned short​0​
int​0​
unsigned int​0​
long​0​
unsigned long​0​
long long​0​
unsigned long long​0​
floatimplementation-defined
doubleimplementation-defined
long doubleimplementation-defined

例外

(none)(until C++11)
noexcept specification: noexcept(since C++11)

注记

NaN从来都比不上它自己。复制NaN并不是ieee-754所要求的，以保留其位表示形式%28符号和有效载荷%29，尽管大多数实现都是这样做的。

当信号NaN被用作算术表达式的参数时，可能会引发适当的浮点异常，而NaN被“静默”，也就是说，该表达式返回一个安静的NaN。

例

演示如何使用信令NAN引发浮点异常。

二次

#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';
}

二次

产出：

二次

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

二次

另见

has_signaling_NaN staticidentifies floating-point types that can represent the special value "signaling not-a-number" (NaN) (public static member constant)
quiet_NaN staticreturns a quiet NaN value of the given floating-point type (public static member function)
isnan (C++11)checks if the given number is NaN (function)

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