bool valid() const;
Checks if the future refers to a shared state.
This is the case only for futures that were not default-constructed or moved from. Unlike std::future,
std::shared_future's shared state is not invalidated when
get() is called.
The behavior is undefined if any member function other than the destructor, the move-assignment operator, or
valid is called on a
shared_future that does not refer to shared state (although implementations are encouraged to throw std::future_error indicating
no_state in this case).
 Return value
true if *this refers to a shared state, otherwise false.
|This section is incomplete|
Reason: no example
 See also
| waits for the result to become available |
(public member function)