bool valid() const noexcept;
Checks if the future refers to a shared state.
This is the case only for futures that were not default-constructed or moved from (i.e. returned by std::promise::get_future(), std::packaged_task::get_future() or std::async()) until the first time get() or share() is called.
The behavior is undefined if any member function other than the destructor, the move-assignment operator, or
valid is called on a
future that does not refer to shared state (although implementations are encouraged to throw std::future_error indicating
no_state in this case). It is valid to move from a future object for which
valid() is false.
 Return value
true if *this refers to a shared state, otherwise false.
true true false
 See also
| waits for the result to become available |
(public member function)