std::error_code::category
From cppreference.com
                    
                                        
                    < cpp | error | error code
                    
                                                            
                    | const std::error_category& category() const; | (since C++11) | |
Returns the error category of the error value.
Parameters
(none)
Return value
The error category of the error value.
Exceptions
noexcept specification:  
noexcept
  See also
| obtains the value of the error_code (public member function) |