# std::shared_future

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< cpp‎ | thread

Defined in header `<future>` |  |
template< class T > class shared_future; | (1) | (since C++11)
template< class T > class shared_future<T&>; | (2) | (since C++11)
template<> class shared_future<void>; | (3) | (since C++11)

The class template `std::shared_future` provides a mechanism to access the result of asynchronous operations, similar to std::future, except that multiple threads are allowed to wait for the same shared state. Unlike std::future, which is only moveable (so only one instance can refer to any particular asynchronous result), `std::shared_future` is copyable and multiple shared future objects may refer to the same shared state.

Access to the same shared state from multiple threads is safe if each thread does it through its own copy of a `shared_future` object.

### Member functions

(constructor) | constructs the future object (public member function)
(destructor) | destructs the future object (public member function)
operator= | assigns the contents (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)
wait_for | waits for the result, returns if it is not available for the specified timeout duration (public member function)
wait_until | waits for the result, returns if it is not available until specified time point has been reached (public member function)

### Example

A `shared_future` may be used to signal multiple threads simultaneously, similar to std::condition_variable::notify_all()

Run this code

    #include <iostream>
    #include <future>
    #include <chrono>

    int main()
    {
        std::promise<void> ready_promise, t1_ready_promise, t2_ready_promise;
        std::shared_future<void> ready_future(ready_promise.get_future());

        std::chrono::time_point<std::chrono::high_resolution_clock> start;

        auto fun1 = [&, ready_future]() -> std::chrono::duration<double, std::milli>
        {
            t1_ready_promise.set_value();
            ready_future.wait(); // waits for the signal from main()
            return std::chrono::high_resolution_clock::now() - start;
        };

        auto fun2 = [&, ready_future]() -> std::chrono::duration<double, std::milli>
        {
            t2_ready_promise.set_value();
            ready_future.wait(); // waits for the signal from main()
            return std::chrono::high_resolution_clock::now() - start;
        };

        auto result1 = std::async(std::launch::async, fun1);
        auto result2 = std::async(std::launch::async, fun2);

        // wait for the threads to become ready
        t1_ready_promise.get_future().wait();
        t2_ready_promise.get_future().wait();

        // the threads are ready, start the clock
        start = std::chrono::high_resolution_clock::now();

        // signal the threads to go
        ready_promise.set_value();

        std::cout << "Thread 1 received the signal "
                  << result1.get().count() << " ms after start\n"
                  << "Thread 2 received the signal "
                  << result2.get().count() << " ms after start\n";
    }

Possible output:

    Thread 1 received the signal 0.072 ms after start
    Thread 2 received the signal 0.041 ms after start

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