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std::numeric_limits<T>::signaling_NaN

来自 cppreference.cn
< cpp‎ | 类型‎ | 数值限制
 
 
 
 
 
static T signaling_NaN() throw();
(C++11 前)
static constexpr T signaling_NaN() noexcept;
(C++11 起)

返回浮点类型 T 表示的特殊值“发出信号的非数字 (NaN)”。仅当 std::numeric_limits<T>::has_signaling_NaN == true 时有意义。在 IEEE 754 中,最常见的浮点数二进制表示方式中,所有指数位都置位且至少有一个小数位也置位的值表示一个 NaN。小数位哪些值表示安静 NaN 或发出信号的 NaN,以及符号位是否有意义,这些都是实现定义的。

目录

[编辑] 返回值

T std::numeric_limits<T>::signaling_NaN()
/* 未特化 */ T()
bool false
char 0
signed char 0
unsigned char 0
wchar_t 0
char8_t (C++20起) 0
char16_t (C++11起) 0
char32_t (C++11起) 0
short 0
unsigned short 0
int 0
unsigned int 0
long 0
unsigned long 0
long long (C++11起) 0
unsigned long long (C++11起) 0
float 实现定义(可能是 FLT_SNAN
double 实现定义(可能是 DBL_SNAN
long double 实现定义(可能是 LDBL_SNAN

[编辑] 注意

一个 NaN 永远不会与自身相等。根据 IEEE-754,复制一个 NaN 不需要保留其位表示(符号和有效载荷),尽管大多数实现会这样做。

当发出信号的 NaN 作为算术表达式的参数使用时,可能会引发适当的浮点异常,并且该 NaN 会被“安静化”,即表达式返回一个安静 NaN。

[编辑] 示例

演示使用发出信号的 NaN 引发浮点异常

#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';
}

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

[编辑] 另请参阅

确定可以表示特殊值“发信的非数”(signaling not-a-number,NaN)的浮点类型
(公共静态成员常量) [编辑]
[静态]
返回给定浮点类型的安静 NaN 值
(公共静态成员函数) [编辑]
(C++11)
检查给定数字是否为 NaN
(函数) [编辑]