std::system_error::what
From cppreference.com
                    
                                        
                    < cpp | error | system error
                    
                                                            
                    | virtual const char* what() const noexcept; | (since C++11) | |
Returns the explanatory string.
Parameters
(none)
Return value
Pointer to a null-terminated string with explanatory information.
See also
| returns error code (public member function) |