bool valid() const;
Checks if the future refers to a shared state. This is the case only for futures returned by std::promise::get_future(), std::packaged_task::get_future() or std::async() until the first time get()or share() is called.
true if *this refers to a shared state, otherwise false.
|This section is incomplete|
Reason: no example
| waits for the result to become available |
(public member function)