< cpp‎ | thread
Concurrency support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and Barriers
Safe Reclamation
Hazard Pointers

Atomic types
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
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.


[edit] Member functions

constructs the promise object
(public member function) [edit]
destructs the promise object
(public member function) [edit]
assigns the shared state
(public member function) [edit]
swaps two promise objects
(public member function) [edit]
Getting the result
returns a future associated with the promised result
(public member function) [edit]
Setting the result
sets the result to specific value
(public member function) [edit]
sets the result to specific value while delivering the notification only at thread exit
(public member function) [edit]
sets the result to indicate an exception
(public member function) [edit]
sets the result to indicate an exception while delivering the notification only at thread exit
(public member function) [edit]

[edit] Non-member functions

specializes the std::swap algorithm
(function template) [edit]

[edit] Helper classes

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

[edit] Example

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

#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)
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(),
    // 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));