std::numeric_limits<T>::signaling_NaN

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::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.

# Declarations

static T signaling_NaN() throw();

(until C++11)

static constexpr T signaling_NaN() noexcept;

(since C++11)

# 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

#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';
}

# See also