static T signaling_NaN() throw(); | (until C++11) | |

static constexpr T signaling_NaN() noexcept; | (since C++11) |

Returns the special value "signaling not-a-number", as represented by the floating-point type `T`

. Only meaningful if `std::numeric_limits<T>::has_signaling_NaN == true`

. In IEEE 754, the most common binary representation of floating-point numbers, any value with all bits of the exponent set and at least one bit of the fraction set represents a NaN. It is implementation-defined which values of the fraction represent quiet or signaling NaNs, and whether the sign bit is meaningful.

`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` | `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` |

`float` | implementation-defined |

`double` | implementation-defined |

`long double` | implementation-defined |

A NaN never compares equal to itself. Copying a NaN is not required, by IEEE-754, to preserve its bit representation (sign and payload), though most implementation do.

When a signaling NaN is used as an argument to an arithmetic expression, the appropriate floating-point exception may be raised and the NaN is "quieted", that is, the expression returns a quiet NaN.

Demonstrates the use of a signaling NaN to raise a floating-point exception.

#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'; }

Output:

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

[static] | identifies floating-point types that can represent the special value "signaling not-a-number" (NaN) (public static member constant) |

[static] | returns a quiet NaN value of the given floating-point type (public static member function) |

(C++11) | checks if the given number is NaN (function) |

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