std::promise<R>::set_value_at_thread_exit (3) - Linux Manuals

std::promise<R>::set_value_at_thread_exit: std::promise<R>::set_value_at_thread_exit

NAME

std::promise<R>::set_value_at_thread_exit - std::promise<R>::set_value_at_thread_exit

Synopsis

void set_value_at_thread_exit( const R& value ); (1) (member only of generic promise template)
                                                     (since C++11)
void set_value_at_thread_exit( R&& value ); (2) (member only of generic promise template)
                                                     (since C++11)
void set_value_at_thread_exit( R& value ); (3) (member only of promise<R&> template specialization)
                                                     (since C++11)
void set_value_at_thread_exit() (4) (member only of promise<void> template specialization)
                                                     (since C++11)

Stores the value into the shared state without making the state ready immediately. The state is made ready when the current thread exits, after all variables with thread-local storage duration have been destroyed.
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).

Parameters

value - value to store in the shared state

Return value

(none)

Exceptions

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.

Additionally:
1-2) Any exception thrown by the copy constructor of value
3) Any exception thrown by the move constructor of value

Example

// Run this code

  #include <iostream>
  #include <future>
  #include <thread>

  int main()
  {
      using namespace std::chrono_literals;
      std::promise<int> p;
      std::future<int> f = p.get_future();
      std::thread([&p] {
            std::this_thread::sleep_for(1s);
            p.set_value_at_thread_exit(9);
      }).detach();

      std::cout << "Waiting..." << std::flush;
      f.wait();
      std::cout << "Done!\nResult is: " << f.get() << '\n';
  }

Output:

  Waiting...Done!
  Result is: 9