# std::promise

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

### Non-member functions

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

### Helper classes

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

### 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));

        // 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

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