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

std::numeric_limits<T>::quiet_NaN: std::numeric_limits<T>::quiet_NaN

NAME

std::numeric_limits<T>::quiet_NaN - std::numeric_limits<T>::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 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 NAN or another implementation-defined 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() << ' '
                << std::numeric_limits<double>::signaling_NaN() << ' '
                << std::acos(2) << ' '
                << std::tgamma(-1) << ' '
                << std::log(-1) << ' '
                << std::sqrt(-1) << ' '
                << 0 / 0.0 << '\n';

      std::cout << "NaN == NaN? " << std::boolalpha
                << ( std::numeric_limits<double>::quiet_NaN()
                     == std::numeric_limits<double>::quiet_NaN() ) << '\n';
  }

Output:

  nan nan nan nan nan -nan -nan
  NaN == NaN? false