# std::future<T>::wait_for

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< cpp‎ | thread‎ | 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. 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 <iostream>
    #include <future>
    #include <thread>
    #include <chrono>
    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

### See also

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)

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