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std::future<T>::wait_until

From cppreference.com
< cpp‎ | thread‎ | future
 
 
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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.

If the future is the result of a call to async that used lazy evaluation, this function returns immediately without waiting.

The behavior is undefined if valid() is false before the call to this function, or Clock does not meet the Clock requirements. The programs is ill-formed if std::chrono::is_clock_v<Clock> is false. (since C++20)

Contents

[edit] Parameters

timeout_time - maximum time point to block until

[edit] Return value

Constant Explanation
future_status::deferred The shared state contains a deferred function using lazy evaluation, so the result will be computed only when explicitly requested
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 std::future_error with an error condition of future_errc::no_state.

The standard recommends that the clock tied to timeout_time be used to measure time; that clock 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 adjustments to the system clock, but not to 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

#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 takes 1 second to complete
    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 takes 5 seconds to complete
    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

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