std::numeric_limits::signaling_NaN(3)

NAME
std::numeric_limits::signaling_NaN - std::numeric_limits::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 (since C++20) 0
   char16_t (since C++11) 0
   char32_t (since C++11) 0
   short 0
   unsigned short 0
   int 0
   unsigned int 0
   long 0
   unsigned long 0
   long long (since C++11) 0
   unsigned long long (since C++11) 0
   float implementation-defined (may be FLT_SNAN)
   double implementation-defined (may be DBL_SNAN)
   long double implementation-defined (may be LDBL_SNAN)

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 <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';
 }

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

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