NAME¶

std::numeric_limits::signaling_NaN - std::numeric_limits::signaling_NaN

Synopsis¶

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 (C++20) 0
char16_t (C++11) 0
char32_t (C++11) 0
short 0
unsigned short 0
int 0
unsigned int 0
long 0
unsigned long 0
long long (C++11) 0
unsigned long long (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 <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 identifies floating-point types that can represent the special
[static] value "signaling not-a-number" (NaN)
(public static member constant)
quiet_NaN returns a quiet NaN value of the given floating-point type
[static] (public static member function)
isnan checks if the given number is NaN
(C++11) (function)