std::packaged_task::valid
From cppreference.com
                    
                                        
                    < cpp | thread | packaged task
                    
                                                            
                    | bool valid() const; | (since C++11) | |
Checks whether *this has a shared state.
Parameters
(none)
Return value
true if *this has a shared state, false otherwise.
Exceptions
noexcept specification:  
noexcept