std::future<T>::wait_for

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. 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

timeout_duration

-

maximum duration to block for

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

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

Run this code

#include <chrono>
#include <future>
#include <iostream>
#include <thread>
using namespace std::chrono_literals;
 
int main()
{
    std::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';
}

Possible output:

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

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Member functions
future::future
future::~future
future::operator=
future::share
Getting the result
future::get
State
future::valid
future::wait
future::wait_for
future::wait_until

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