std::system_error::code
From cppreference.com
                    
                                        
                    < cpp | error | system error
                    
                                                            
                    |   const std::error_code& code() const noexcept;  | 
(since C++11) | |
Returns the stored error code.
Parameters
(none)
Return value
The stored error code.
See also
|    [virtual]  | 
   returns an explanatory string  (virtual public member function)  |