Question

It seems that unless you call std::async a std::future will never be set to any other state than future_status::deferred unless you call get or wait on the future. wait_for & wait_until will continue to not block and return future_status::deferred even if the task has already run and stored the result.

Here’s an example:

#include <future>

void main()
{
    auto func = []() { return 5; };
    auto asyncFuture = std::async(std::launch::async, func);
    auto status = asyncFuture.wait_for(std::chrono::seconds(0));   // timeout (1)

    auto deferredFuture = std::async(std::launch::deferred, func);
    status = deferredFuture.wait_for(std::chrono::seconds(0));     // deferred (2)

    std::packaged_task<int()> task(func);
    auto packagedTaskFuture = task.get_future();
    status = packagedTaskFuture.wait_for(std::chrono::seconds(0)); // deferred (2)

    task();

    status = packagedTaskFuture.wait_for(std::chrono::seconds(0)); // deferred (2)
    packagedTaskFuture.wait();
    status = packagedTaskFuture.wait_for(std::chrono::seconds(0)); // ready (0)
}

I don’t have the current C++11 standard, but the draft standard in 30.6.9 says that when a packaged_task is run it should store the result in the future’s shared state. It’s not very clear whether that includes setting the expected wait_until/wait_for behavior or not.

There were previously issues with VS11 behavior in this area with respect to async calls: http://social.msdn.microsoft.com/Forums/hu/parallelcppnative/thread/4394f2c1-0404-40df-869b-f4fc36fc035c

Additionally it seems like other compilers have problems in this area: C++ 11 future_status::deferred not working

Anyone that may know the standard better: Is this expected behavior or is there an issue with the implemenation in VS11?

Updates: I somehow missed the report for this: http://connect.microsoft.com/VisualStudio/feedback/details/761829/c-11-unexpected-behavior-for-std-future-wait-for-and-std-packaged-task

Answer 1

This is an issue with VS2012. It’s not the only issue either — their implementation of the C++11 thread library has several bugs. I wrote about a few on my blog.