std::future<T>::valid (3) - Linux Man Pages
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 (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
true if *this refers to a shared state, otherwise false.
This section is incomplete
Reason: no example
wait (public member function)