function
<cmath> <ctgmath>

nan

double nan (const char* tagp);
Generate quiet NaN
Returns a quiet NaN (Not-A-Number) value of type double.

The NaN values are used to identify undefined or non-representable values for floating-point elements, such as the square root of negative numbers or the result of 0/0.

The argument can be used by library implementations to distinguish different NaN values in a implementation-specific manner.

Similarly, nanf and nanl return NaN values of type float and long double, respectively.

Parameters

tagp
An implementation-specific C-string.
If this is an empty string (""), the function returns a generic NaN value (the same as returned by passing "NAN" to strtod).

Return Value

A quiet NaN value.

See also

isnan
Is Not-A-Number (macro/function )
nextafter
Next representable value (function )
nexttoward
Next representable value toward precise value (function )
NAN
Not-A-Number (constant )

C++