std::future
From cppreference.com
| Defined in header <future>
|
||
| template< class T > class future; |
(1) | (since C++11) |
| template< class T > class future<T&>; |
(2) | (since C++11) |
| template<> class future<void>; |
(3) | (since C++11) |
The class template std::future provides a mechanism to access the result of asynchronous operations:
- An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a
std::futureobject to the creator of that asynchronous operation.
- The creator of the asynchronous operation can then use a variety of methods to query, wait for, or extract a value from the
std::future. These methods may block if the asynchronous operation has not yet provided a value.
- When the asynchronous operation is ready to send a result to the creator, it can do so by modifying shared state (e.g. std::promise::set_value) that is linked to the creator's
std::future.
Note that std::future references shared state that is not shared with any other asynchronous return objects (as opposed to std::shared_future).
Member functions
| constructs the future object (public member function) | |
| destructs the future object (public member function) | |
| moves the future object (public member function) | |
transfers the shared state from *this to a shared_future and returns it (public member function) | |
Getting the result | |
| returns the result (public member function) | |
State | |
| checks if the future has a shared state (public member function) | |
| waits for the result to become available (public member function) | |
| waits for the result, returns if it is not available for the specified timeout duration (public member function) | |
| waits for the result, returns if it is not available until specified time point has been reached (public member function) | |
Examples
Run this code
#include <iostream> #include <future> #include <thread> int main() { // future from a packaged_task std::packaged_task<int()> task([]{ return 7; }); // wrap the function std::future<int> f1 = task.get_future(); // get a future std::thread t(std::move(task)); // launch on a thread // future from an async() std::future<int> f2 = std::async(std::launch::async, []{ return 8; }); // future from a promise std::promise<int> p; std::future<int> f3 = p.get_future(); std::thread( [&p]{ p.set_value_at_thread_exit(9); }).detach(); std::cout << "Waiting..." << std::flush; f1.wait(); f2.wait(); f3.wait(); std::cout << "Done!\nResults are: " << f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n'; t.join(); }
Output:
Waiting...Done! Results are: 7 8 9
Example with exceptions
Run this code
#include <thread> #include <iostream> #include <future> int main() { std::promise<int> p; std::future<int> f = p.get_future(); std::thread t([&p]{ try { // code that may throw throw std::runtime_error("Example"); } catch(...) { try { // store anything thrown in the promise p.set_exception(std::current_exception()); } catch(...) {} // set_exception() may throw too } }); try { std::cout << f.get(); } catch(const std::exception& e) { std::cout << "Exception from the thread: " << e.what() << '\n'; } t.join(); }
Output:
Exception from the thread: Example
See also
| (C++11) |
runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template) |
| (C++11) |
waits for a value (possibly referenced by other futures) that is set asynchronously (class template) |