std::shared_future::wait_for

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std::shared_future
 
template< class Rep, class Period >
std::future_status wait_for( const std::chrono::duration<Rep,Period>& timeout_duration ) const;
 (since C++11)

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::shared_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

See also

 waits for the result to become available
(public member function)
waits for the result, returns if it is not available until specified time point has been reached
(public member function)
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