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) |