Get the status of a std::future

c++ multithreading c++11 future
David Brown picture David Brown · Jun 5, 2012 · Viewed 33.4k times · Source

Is it possible to check if a std::future has finished or not? As far as I can tell the only way to do it would be to call wait_for with a zero duration and check if the status is ready or not, but is there a better way?

Answer

Jonathan Wakely picture Jonathan Wakely · Jun 6, 2012

You are correct, and apart from calling wait_until with a time in the past (which is equivalent) there is no better way.

You could always write a little wrapper if you want a more convenient syntax:

template<typename R>
  bool is_ready(std::future<R> const& f)
  { return f.wait_for(std::chrono::seconds(0)) == std::future_status::ready; }

N.B. if the function is deferred this will never return true, so it's probably better to check wait_for directly in the case where you might want to run the deferred task synchronously after a certain time has passed or when system load is low.

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