std::numeric_limits<T>::signaling_NaN

 

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