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std::future - cppreference.com

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).

#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

#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

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