std::future(3)

NAME
std::future - std::future

Synopsis
   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::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
   constructor constructs the future object
                 (public member function)
   destructor destructs the future object
                 (public member function)
   operator= moves the future object
                 (public member function)
                 transfers the shared state from *this to a shared_future and returns
   share it
                 (public member function)

Getting the result
   get returns the result
                 (public member function)

State
   valid checks if the future has a shared state
                 (public member function)
   wait waits for the result to become available
                 (public member function)
                 waits for the result, returns if it is not available for the specified
   wait_for timeout duration
                 (public member function)
                 waits for the result, returns if it is not available until specified
   wait_until time point has been reached
                 (public member function)

Examples
// Run this code
 #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();
 }

Output:
 Waiting...Done!
 Results are: 7 8 9
    Example with exceptions // Run this code
 #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();
 }

Output:
 Exception from the thread: Example

See also
   async runs a function asynchronously (potentially in a new thread) and
   (C++11) returns a std::future that will hold the result
                 (function template)
   shared_future waits for a value (possibly referenced by other futures) that is set
   (C++11) asynchronously
                 (class template)