std::numeric_limits<T>::signaling_NaN (3) - Linux Manuals

std::numeric_limits<T>::signaling_NaN: std::numeric_limits<T>::signaling_NaN

NAME

std::numeric_limits<T>::signaling_NaN - std::numeric_limits<T>::signaling_NaN

Synopsis

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.

Return value

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

Notes

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.

Example

Demonstrates the use of a signaling NaN to raise a floating-point exception
// Run this code

  #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