std::shared_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
#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'; }
Possible output:
waiting... timeout 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) |