std::shared_future<T>::valid (3) - Linux Manuals
bool valid() const noexcept; (since C++11)
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 copy-assignment operator, 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). It is valid to move or copy from a shared_future object for which valid() is false
true if *this refers to a shared state, otherwise false.
This section is incomplete
Reason: no example
wait (public member function)