Std::future::wait_for (3) Linux Manual Page

std::future<T>::wait_for – std::future<T>::wait_for

Synopsis

template <class Rep, class Period>
(since C++ 11)

    std::future_status wait_for(const std::chrono::duration<Rep, Period> &timeout_duration) const;

Waits for the result to become available. Blocks until specified timeout_duration has elapsed or the result becomes available, whichever comes first. Returns value identifies the state of the result.
This function may block for longer than timeout_duration due to scheduling or resource contention delays.
The standard recommends that a steady clock is used to measure the duration. If an implementation uses a system clock instead, the wait time may also be sensitive to clock adjustments.
The behavior is undefined if valid()== false before the call to this function.

Parameters

timeout_duration – maximum duration to block for

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.

Example

// Run this code

#include <iostream>

#include <future>

#include <thread>

#include <chrono>
int main()

{
    std::future<int> future = std::async(std::launch::async, []() {
        std::this_thread::sleep_for(std::chrono::seconds(3));
        return 8;
    });
    std::cout << "waiting...\n";
    std::future_status status;
    do {
        status = future.wait_for(std::chrono::seconds(1));
        if (status == std::future_status::deferred) {
            std::cout << "deferred\n";
        } else if (status == std::future_status::timeout) {
            std::cout << "timeout\n";
        } else if (status == std::future_status::ready) {
            std::cout << "ready!\n";
        }
    } while (status != std::future_status::ready);
    std::cout << "result is " << future.get() << '\n';
}

Possible output:

  waiting…
  timeout
  timeout
  ready!
  result is 8

std::future::wait_for (3) Linux Manual Page