std::future<T>::wait_until (3) - Linux Manuals

std::future<T>::wait_until: std::future<T>::wait_until

NAME

std::future<T>::wait_until - std::future<T>::wait_until

Synopsis

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

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

// 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!