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).
Contents |
Member functions
| constructs the future object (public member function of std::{{{1}}})
| |
| 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) | |
Example
{{example | 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(std::move(task)).detach(); // 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( [](std::promise<int> p){
/*p.set_value_at_thread_exit(9); Dont suppoted in current gcc version*/
p.set_value(9);
}, std::move(p)/*promise(const promise&) = delete; */).detach();
std::cout << "Waiting..." << std::flush;
f1.wait();
f2.wait();
f3.wait();
std::cout << "Done!\nResults are: "
<< f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n';
}
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) |
