Defined in header <future> | ||
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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:
std::async, std::packaged_task, or std::promise) can provide a std::future object to the creator of that asynchronous operation. std::future. These methods may block if the asynchronous operation has not yet provided a value. 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).
| constructs the future object (public member function) |
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| destructs the future object (public member function) |
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| moves the future object (public member function) |
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transfers the shared state from *this to a shared_future and returns it (public member function) |
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Getting the result |
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| returns the result (public member function) |
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State |
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| checks if the future has a shared state (public member function) |
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| waits for the result to become available (public member function) |
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| waits for the result, returns if it is not available for the specified timeout duration (public member function) |
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| waits for the result, returns if it is not available until specified time point has been reached (public member function) |
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#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
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(C++11) | runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template) |
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(C++11) | waits for a value (possibly referenced by other futures) that is set asynchronously (class template) |
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