std::shared_future<T>:: wait_for

Waits for the result to become available. Blocks until specified timeout_duration has elapsed or the result becomes available, whichever comes first. The return value identifies the state of the result.

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

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() is false before the call to this function.

Parameters

Return value

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 std::future_errc::no_state .

Example

#include <chrono>
#include <future>
#include <iostream>
#include <thread>
using namespace std::chrono_literals;
 
int main()
{
    std::shared_future<int> future = std::async(std::launch::async, []()
    {
        std::this_thread::sleep_for(3s);
        return 8;
    });
 
    std::cout << "waiting...\n";
    std::future_status status;
 
    do
    {
        switch (status = future.wait_for(1s); status)
        {
            case std::future_status::deferred:
                std::cout << "deferred\n";
                break;
            case std::future_status::timeout:
                std::cout << "timeout\n";
                break;
            case std::future_status::ready:
                std::cout << "ready!\n";
                break;
        }
    }
    while (status != std::future_status::ready);
 
    std::cout << "result is " << future.get() << '\n';
}

waiting...
timeout
timeout
timeout
ready!
result is 8

See also