std::isnan

From Cppreference

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Defined in header <cmath>
int isnan( float arg );		(since C++11)
int isnan( double arg );		(since C++11)
int isnan( long double arg );		(since C++11)

Determines if the given floating point number arg is not-a-number (NaN).

[edit] Parameters

arg

-

floating point value

[edit] Return value

nonzero value if arg is NaN, ​0​ otherwise

[edit] See also

fpclassify (C++11)	categorizes the given floating point value (function)
isfinite (C++11)	checks if the given number has finite value (function)
isinf (C++11)	checks if the given number is infinite (function)
isnormal (C++11)	checks if the given number is normal (function)