std::numeric_limits<T>::signaling_NaN
来自 cppreference.cn
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
运行此代码
#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) | |
[静态] |
返回给定浮点类型的安静 NaN 值 (公共静态成员函数) |
(C++11) |
检查给定数字是否为 NaN (函数) |