Utilities library
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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.


[edit] 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 (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 NAN)
double implementation-defined
long double implementation-defined

[edit] Notes

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

[edit] Example

Several ways to generate a NaN (the output string is compiler-specific):

#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

[edit] See also

identifies floating-point types that can represent the special value "quiet not-a-number" (NaN)
(public static member constant) [edit]
returns a signaling NaN value of the given floating-point type
(public static member function) [edit]
not-a-number (NaN)
(function) [edit]
checks if the given number is NaN
(function) [edit]