std::future

cppreference std::future

The class template std::future provides a mechanism to access the result of asynchronous operations:

1 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.

2 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.

3 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: 进行了非常好的抽象

std::future VS std::shared_future

Note that std::future references shared state that is not shared with any other asynchronous return objects (as opposed to std::shared_future).

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 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();
}
// g++ --std=c++11 test.cpp -lpthread

NOTE: 上述代码在"g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28)"中，编译报错如下:

test.cpp: In lambda function:
test.cpp:18:26: error: ‘class std::promise<int>’ has no member named ‘set_value_at_thread_exit’
     std::thread( [&p]{ p.set_value_at_thread_exit(9); }).detach();

Example with exceptions

#include <thread>
#include <iostream>
#include <future>

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();
}
// g++ --std=c++11 test.cpp -lpthread