std::error_condition::category
From cppreference.com
                    
                                        
                    < cpp | error | error condition
                    
                                                            
                    |   const error_category& category() const noexcept;  | 
(since C++11) | |
Returns the stored error category.
Parameters
(none)
Return value
The stored error category.
See also
|    (C++11)  | 
  base class for error categories  (class)  |