Get the status of a std::future
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?
Solution 1:
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.
Solution 2:
There's an is_ready member function in the works for std::future. In the meantime, the VC implementation has an _Is_ready() member.
Solution 3:
My first bet would be to call wait_for
with a 0 duration, and check the result code that can be one of future_status::ready
, future_status::deferred
or future_status::timeout
.
valid()
will return true
if *this
refers to a shared state, independently of whether that state is ready or not. See cppreference.