std::promise (3) - Linux Manuals

std::promise: std::promise

NAME

std::promise - std::promise

Synopsis

Defined in header <future>
template< class R > class promise; (1) (since C++11)
template< class R > class promise<R&>; (2) (since C++11)
template<> class promise<void>; (3) (since C++11)

1) base template
2) non-void specialization, used to communicate objects between threads
3) void specialization, used to communicate stateless events
The class template std::promise provides a facility to store a value or an exception that is later acquired asynchronously via a std::future object created by the std::promise object. Note that the std::promise object is meant to be used only once.
Each promise is associated with a shared state, which contains some state information and a result which may be not yet evaluated, evaluated to a value (possibly void) or evaluated to an exception. A promise may do three things with the shared state:

* make ready: the promise stores the result or the exception in the shared state. Marks the state ready and unblocks any thread waiting on a future associated with the shared state.
* release: the promise gives up its reference to the shared state. If this was the last such reference, the shared state is destroyed. Unless this was a shared state created by std::async which is not yet ready, this operation does not block.
* abandon: the promise stores the exception of type std::future_error with error code std::future_errc::broken_promise, makes the shared state ready, and then releases it.

The promise is the "push" end of the promise-future communication channel: the operation that stores a value in the shared state synchronizes-with (as defined in std::memory_order) the successful return from any function that is waiting on the shared state (such as std::future::get). Concurrent access to the same shared state may conflict otherwise: for example multiple callers of std::shared_future::get must either all be read-only or provide external synchronization.

Member functions

                             constructs the promise object
constructor (public member function)
                             destructs the promise object
destructor (public member function)
                             assigns the shared state
operator= (public member function)
                             swaps two promise objects
swap (public member function)

Getting the result

                             returns a future associated with the promised result
get_future (public member function)

Setting the result

                             sets the result to specific value
set_value (public member function)
                             sets the result to specific value while delivering the notification only at thread exit
set_value_at_thread_exit (public member function)
                             sets the result to indicate an exception
set_exception (public member function)
                             sets the result to indicate an exception while delivering the notification only at thread exit
set_exception_at_thread_exit (public member function)

Non-member functions

std::swap(std::promise) specializes the std::swap algorithm
                        (function template)
(C++11)

Helper classes

std::uses_allocator<std::promise> specializes the std::uses_allocator type trait
                                  (class template specialization)
(C++11)

Example

This example shows how promise<int> can be used as signals between threads.
// Run this code

  #include <vector>
  #include <thread>
  #include <future>
  #include <numeric>
  #include <iostream>
  #include <chrono>

  void accumulate(std::vector<int>::iterator first,
                  std::vector<int>::iterator last,
                  std::promise<int> accumulate_promise)
  {
      int sum = std::accumulate(first, last, 0);
      accumulate_promise.set_value(sum); // Notify future
  }

  void do_work(std::promise<void> barrier)
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
      barrier.set_value();
  }

  int main()
  {
      // Demonstrate using promise<int> to transmit a result between threads.
      std::vector<int> numbers = { 1, 2, 3, 4, 5, 6 };
      std::promise<int> accumulate_promise;
      std::future<int> accumulate_future = accumulate_promise.get_future();
      std::thread work_thread(accumulate, numbers.begin(), numbers.end(),
                              std::move(accumulate_promise));
      accumulate_future.wait(); // wait for result
      std::cout << "result=" << accumulate_future.get() << '\n';
      work_thread.join(); // wait for thread completion

      // Demonstrate using promise<void> to signal state between threads.
      std::promise<void> barrier;
      std::future<void> barrier_future = barrier.get_future();
      std::thread new_work_thread(do_work, std::move(barrier));
      barrier_future.wait();
      new_work_thread.join();
  }

Output:

  result=21