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std::numeric_limits::quiet_NaN

static T quiet_NaN();
 (until C++11) 
static constexpr T quiet_NaN();
 (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​
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

Exceptions

(none) (until C++11)
noexcept specification:
noexcept
 (since C++11)

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).

#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

See also

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

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http://en.cppreference.com/w/cpp/types/numeric_limits/quiet_NaN