NAME¶

std::future::wait_for - std::future::wait_for

Synopsis¶

template< class Rep, class Period >
std::future_status wait_for( const std::chrono::duration<Rep,Period>& (since C++11)
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. 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
The shared state contains a deferred function using lazy
future_status::deferred 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)
waits for the result, returns if it is not available until specified time
wait_until point has been reached
(public member function)