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std::future

From cppreference.com
< cpp‎ | thread
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:

  • 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

constructs the future object
(public member function) [edit]
destructs the future object
(public member function) [edit]
moves the future object
(public member function) [edit]
transfers the shared state from *this to a shared_future and returns it
(public member function) [edit]
Getting the result
returns the result
(public member function) [edit]
State
checks if the future has a shared state
(public member function) [edit]
waits for the result to become available
(public member function) [edit]
waits for the result, returns if it is not available for the specified timeout duration
(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]

[edit] Example

#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); }, 
                 std::move(p) ).detach();
 
    std::cout << "Waiting..." << std::flush;
    f1.wait();
    f2.wait();
    f3.wait();
    std::cout << "Done!\nResults are: "
              << f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n';
}

Output:

Waiting...Done!
Results are: 7 8 9

[edit] 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) [edit]
waits for a value (possibly referenced by other futures) that is set asynchronously
(class template) [edit]