# std::numeric_limits<T>::signaling_NaN

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< cpp‎ | types‎ | numeric limits

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 | ​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 | implementation-defined
double | implementation-defined
long double | implementation-defined

### 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 <iostream>
    #include <limits>
    #include <cfenv>
    #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)

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