[boost] [threads] Launch deferred for async

Subject: [boost] [threads] Launch deferred for async
From: Stephan Menzel (stephan.menzel_at_[hidden])
Date: 2018-11-26 13:30:13

Hello all,

I am using boost::async() to create futures from tasks in order to wrap
some behavior that may or may not be sync. This behavior is implemented
behind an interface and can be small and fast operations or longer running
and blocking things. All return a future as a result type, so I can hide
the ugly truth.

For the smaller stuff I'd like to use launch::deferred to avoid threading
altogether and just execute the task when the user get()s the future.

    boost::future<bool> ret = boost::async(boost::launch::deferred, [this]
() -> bool {
        // do some small cpu only impl
        return result;
    });

But when i get the result:

    const bool result = ret.get();

...the application crashes. Debugging this I came to this in
boost/thread/future.hpp:9373

    } else if (underlying_cast<int>(policy) & int(launch::deferred)) {
      std::terminate();
      //BOOST_THREAD_FUTURE<R> ret;
      //return ::boost::move(ret);
      //          return
boost::detail::make_future_deferred_shared_state<Rp>(
      //              BF(
      //                  thread_detail::decay_copy(boost::forward<F>(f))
      //              )
      //          );
    } else {

To me this sounds like the application will always std::terminate() when
launch::deferred is chosen. Why is that?

When I use launch::any it seems to work but why is there a parameter that
always yields unwanted results. What is the reason behind it? Can I get
launch::deferred as it is described (not threading, just executing the task
sync when get() is called) somehow?

Platform is x64 Windows 10, MSVC14.1.

Cheers,
Stephan