std::numeric_limits::quiet_NaN(3)

NAME
std::numeric_limits::quiet_NaN - std::numeric_limits::quiet_NaN

Synopsis
   static T quiet_NaN() throw(); (until C++11)
   static constexpr T quiet_NaN() noexcept; (since C++11)
   Returns the special value "quiet not-a-number", as represented by the floating-point
   type T. Only meaningful if std::numeric_limits<T>::has_quiet_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>::quiet_NaN()
   /* non-specialized */ T()
   bool false
   char 0
   signed char 0
   unsigned char 0
   wchar_t 0
   char8_t (C++20) 0
   char16_t (C++11) 0
   char32_t (C++11) 0
   short 0
   unsigned short 0
   int 0
   unsigned int 0
   long 0
   unsigned long 0
   long long (C++11) 0
   unsigned long long (C++11) 0
   float implementation-defined (may be NAN)
   double implementation-defined
   long double implementation-defined

Notes
   A NaN never compares equal to itself. Copying a NaN may not preserve its bit
   representation.

Example
   Several ways to generate a NaN (the output string is compiler-specific) // Run this code
 #include <iostream>
 #include <limits>
 #include <cmath>
 int main()
 {
     std::cout << std::numeric_limits<double>::quiet_NaN() << ' ' // nan
               << std::numeric_limits<double>::signaling_NaN() << ' ' // nan
               << std::acos(2) << ' ' // nan
               << std::tgamma(-1) << ' ' // nan
               << std::log(-1) << ' ' // nan
               << std::sqrt(-1) << ' ' // -nan
               << 0 / 0.0 << '\n'; // -nan
     std::cout << "NaN == NaN? " << std::boolalpha
               << ( std::numeric_limits<double>::quiet_NaN() ==
                    std::numeric_limits<double>::quiet_NaN() ) << '\n';
 }

Possible output:
 nan nan nan nan nan -nan -nan
 NaN == NaN? false

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