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# std::future::wait_until

From cppreference.com
< cpp‎ | thread‎ | future

[edit template]

template< class Clock, class Duration > std::future_status wait_until( const std::chrono::time_point<Clock,Duration>& timeout_time ) const; |  | (since C++11)

`wait_until` waits for a result to become available. It blocks until specified `timeout_time` has been reached or the result becomes available, whichever comes first. The return value indicates why `wait_until` returned.

The behavior is undefined if valid()== false before the call to this function.

### [edit] Parameters

timeout_time | - | maximum time point to block until

### [edit] Return value

Constant | Explanation
`future_status::deferred` | The function to calculate the result has not been started yet
`future_status::ready` | The result is ready
`future_status::timeout` | The timeout has expired

### [edit] Exceptions

Any exception thrown by clock, time_point, or duration during the execution (clocks, time points, and durations provided by the standard library never throw)

### [edit] Notes

The implementations are encouraged to detect the case when valid == false before the call and throw a future_error with an error condition of future_errc::no_state.

The clock tied to `timeout_time` is used, which is not required to be a monotonic clock.There are no guarantees regarding the behavior of this function if the clock is adjusted discontinuously, but the existing implementations convert `timeout_time` from `Clock` to std::chrono::system_clock and delegate to POSIX pthread_cond_timedwait so that the wait honors ajustments to the system clock, but not to the the user-provided `Clock`. In any case, the function also may wait for longer than until after `timeout_time` has been reached due to scheduling or resource contention delays.

### [edit] Example

Run this code

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

    int main()
    {
        std::chrono::system_clock::time_point two_seconds_passed
            = std::chrono::system_clock::now() + std::chrono::seconds(2);

        // Make a future that that takes 1 second to completed
        std::promise<int> p1;
        std::future<int> f_completes = p1.get_future();
        std::thread([](std::promise<int> p1)
                    {
                        std::this_thread::sleep_for(std::chrono::seconds(1));
                        p1.set_value_at_thread_exit(9);
                    },
                    std::move(p1)
        ).detach();

        // Make a future that that takes 5 seconds to completed
        std::promise<int> p2;
        std::future<int> f_times_out = p2.get_future();
        std::thread([](std::promise<int> p2)
                    {
                        std::this_thread::sleep_for(std::chrono::seconds(5));
                        p2.set_value_at_thread_exit(8);
                    },
                    std::move(p2)
        ).detach();

        std::cout << "Waiting for 2 seconds..." << std::endl;

        if(std::future_status::ready == f_completes.wait_until(two_seconds_passed))
            { std::cout << "f_completes: " << f_completes.get() << "\n"; }
        else
            { std::cout << "f_completes did not complete!\n"; }

        if(std::future_status::ready == f_times_out.wait_until(two_seconds_passed))
            { std::cout << "f_times_out: " << f_times_out.get() << "\n"; }
        else
            { std::cout << "f_times_out did not complete!\n"; }

        std::cout << "Done!\n";
    }

Possible output:

    Waiting for 2 seconds...
    f_completes: 9
    f_times_out did not complete!
    Done!

### [edit] See also

wait | waits for the result to become available (public member function) [edit]
wait_for | waits for the result, returns if it is not available for the specified timeout duration (public member function) [edit]

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