std::future

template< class T > class future;

template< class T > class future<T&>;

template<>          class future<void>;

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::future object 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

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';
}

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

See also