std::promise(3)

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

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

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

Non-member functions
   std::swap(std::promise) specializes the std::swap algorithm
   (C++11) (function template)

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

Example
   This example shows how promise<int> can be used as signals between threads. // Run this code
 #include <chrono>
 #include <future>
 #include <iostream>
 #include <numeric>
 #include <thread>
 #include <vector>
 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));
     // future::get() will wait until the future has a valid result and retrieves it.
     // Calling wait() before get() is not needed
     // 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