NAN
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <cmath> | ||
| #define NAN /*implementation defined*/ | (since C++11) | |
The macro NAN expands to constant expression of type float which evaluates to a quiet not-a-number (QNaN) value. If the implementation does not support QNaNs, this macro constant is not defined. 
[edit] See also
| (C++11)(C++11)(C++11) | not-a-number (NaN) (function) | 
| (C++11) | checks if the given number is NaN (function) | 
| [static] | identifies floating-point types that can represent the special value "quiet not-a-number" (NaN) (public static member constant of std::numeric_limits) | 
| [static] | identifies floating-point types that can represent the special value "signaling not-a-number" (NaN) (public static member constant of std::numeric_limits) | 
| [static] | returns a quiet NaN value of the given floating-point type (public static member function of std::numeric_limits) | 
| [static] | returns a signaling NaN value of the given floating-point type (public static member function of std::numeric_limits) | 
| C documentation for NAN | |


