< cpp‎ | thread
Thread support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
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
(public member function)
destructs the promise object
(public member function)
assigns the shared state
(public member function)
swaps two promise objects
(public member function)
Getting the result
returns a future associated with the promised result
(public member function)
Setting the result
sets the result to specific value
(public member function)
sets the result to specific value while delivering the notification only at thread exit
(public member function)
sets the result to indicate an exception
(public member function)
sets the result to indicate an exception while delivering the notification only at thread exit
(public member function)

Non-member functions

specializes the std::swap algorithm
(function template)

Helper classes

specializes the std::uses_allocator type trait
(class template specialization)


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

#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)
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(),
    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));