NAME¶

std::future::wait_until - std::future::wait_until

Synopsis¶

template< class Clock, class Duration >
std::future_status wait_until( const (since C++11)
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.

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)

Parameters¶

timeout_time - maximum time point to block until

Return value¶

Constant Explanation
The shared state contains a deferred function using lazy
future_status::deferred 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

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 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.

Example¶

// Run this code

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

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!

See also¶

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