std::future<T>::wait_until

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.

Parameters

timeout_time

maximum time point to block until

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

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).

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.

Example

#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!

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)