std::promise<R>::set_exception (3) - Linux Man Pages
void set_exception( std::exception_ptr p ); (since C++11)
Atomically stores the exception pointer p into the shared state and makes the state ready.
The operation behaves as though set_value, set_exception, set_value_at_thread_exit, and set_exception_at_thread_exit acquire a single mutex associated with the promise object while updating the promise object.
An exception is thrown if there is no shared state or the shared state already stores a value or exception.
Calls to this function do not introduce data races with calls to get_future (but they need not synchronize with each other).
p - exception pointer to store. The behavior is undefined if p is null.
std::future_error on the following conditions:
* *this has no shared state. The error category is set to no_state.
* The shared state already stores a value or exception. The error category is set to promise_already_satisfied.
// Run this code