std::promise<R>::get_future (3) - Linux Man Pages
std::future<T> get_future(); (since C++11)
Returns a future object associated with the same shared state as *this.
An exception is thrown if *this has no shared state or get_future has already been called. To get multiple "pop" ends of the promise-future communication channel, use std::future::share.
Calls to this function do not introduce data races with calls to set_value, set_exception, set_value_at_thread_exit, or set_exception_at_thread_exit (but they need not synchronize with each other).
A future referring to the shared state of *this
std::future_error on the following conditions:
* *this has no shared state. The error category is set to no_state.
* get_future() has already been called on a promise with the same shared state as *this. The error category is set to future_already_retrieved.