std::isunordered
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <cmath> | ||
| bool isunordered( float x, float y ); | (since C++11) | |
| bool isunordered( double x, double y ); | (since C++11) | |
| bool isunordered( long double x, long double y ); | (since C++11) | |
Determines if the floating point numbers x and y are unordered, that is, one or both are NaN and thus cannot be meaningfully compared with each other.
[edit] Parameters
| x | - | floating point value | 
| y | - | floating point value | 
[edit] Return value
true if either x or y is NaN, false otherwise
[edit] See also
| (C++11) | categorizes the given floating point value (function) | 
| (C++11) | checks if the given number is NaN (function) | 
| C documentation for isunordered | |


