std::future
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std::future
Member functions
Getting the result
State
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
[edit] Member functions
transfers the shared state from *this to a shared_future and returns it
(public member function) [edit]
(public member function) [edit]
Getting the result
State
waits for the result, returns if it is not available for the specified timeout duration
(public member function) [edit]
(public member function) [edit]
waits for the result, returns if it is not available until specified time point has been reached
(public member function) [edit]
(public member function) [edit]
[edit] Examples
Run this code
#include <future> #include <iostream> #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
[edit] Example with exceptions
Run this code
#include <future> #include <iostream> #include <thread> 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
[edit] See also
(C++11)
(function template) [edit]
(C++11)
(class template) [edit]