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