std::promise
| 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) |
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::asyncwhich 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
(C++11) |
specializes the std::swap algorithm (function template) |
Helper classes
| specializes the std::uses_allocator type trait (class template specialization) |
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)
{
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