std::future_error::code
From cppreference.com
                    
                                        
                    < cpp | thread | future 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
|   returns the explanatory string specific to the error code  (public member function)  |