Pattern for future conversion

问题

currently we are using asynchronous values very heavily. Assume that I have a function which does something like this:

int do_something(const boost::posix_time::time_duration& sleep_time)
{
    BOOST_MESSAGE("Sleeping a bit");
    boost::this_thread::sleep(sleep_time);
    BOOST_MESSAGE("Finished taking a nap");

    return 42;
}

At some point in code we create a task which creates a future to such an int value which will be set by a packaged_task - like this (worker_queue is a boost::asio::io_service in this example):

boost::unique_future<int> createAsynchronousValue(const boost::posix_time::seconds& sleep)
{
    boost::shared_ptr< boost::packaged_task<int> > task(
        new boost::packaged_task<int>(boost::bind(do_something, sleep)));
    boost::unique_future<int> ret = task->get_future();

    // Trigger execution
    working_queue.post(boost::bind(&boost::packaged_task<int>::operator (), task));

    return boost::move(ret);
}

At another point in code I want to wrap this function to return some higher level object which should also be a future. I need a conversion function which takes the first value and transforms it to another value (in our actual code we have some layering and doing asynchronous RPC which returns futures to responses - these responses should be converted to futures to real objects, PODs or even void future to be able to wait on it or catch exceptions). So this is the conversion function in this example:

float converter(boost::shared_future<int> value)
{
    BOOST_MESSAGE("Converting value " << value.get());
    return 1.0f * value.get();
}

Then I thought of creating a lazy future as described in the Boost docs to do this conversion only if wanted:

void invoke_lazy_task(boost::packaged_task<float>& task)
{
    try
    {
        task();
    }
    catch(boost::task_already_started&)
    {}
}

And then I have a function (might be a higher level API) to create a wrapped future:

boost::unique_future<float> createWrappedFuture(const boost::posix_time::seconds& sleep)
{
    boost::shared_future<int> int_future(createAsynchronousValue(sleep));
    BOOST_MESSAGE("Creating converter task");
    boost::packaged_task<float> wrapper(boost::bind(converter, int_future));

    BOOST_MESSAGE("Setting wait callback");
    wrapper.set_wait_callback(invoke_lazy_task);

    BOOST_MESSAGE("Creating future to converter task");
    boost::unique_future<float> future = wrapper.get_future();

    BOOST_MESSAGE("Returning the future");
    return boost::move(future);
}

At the end I want to be able to use it like this:

{    
    boost::unique_future<float> future = createWrappedFuture(boost::posix_time::seconds(1));
    BOOST_MESSAGE("Waiting for the future");
    future.wait();
    BOOST_CHECK_EQUAL(future.get(), 42.0f);
}

But here I end up getting an exception about a broken promise. The reason seems to be pretty clear for me because the packaged_task which does the conversion goes out of scope.

So my questing is: How do I deal with such situations. How can I prevent the task from being destroyed? Is there a pattern for this?

Bests,

Ronny

回答1:

You need to manage the lifetime of task object properly.

The most correct way is to return boost::packaged_task<float> instead of boost::unique_future<float> from createWrappedFuture(). The caller will be responsible to get future object and to prolongate task lifetime until future value is ready.

Or you can place task object into some 'pending' queue (global or class member) the similar way you did in createAsynchronousValue. But in this case you will need to explcitly manage task lifetime and remove it from queue after completion. So don't think this solution has advantages against returning task object itself.

来源：https://stackoverflow.com/questions/13066667/pattern-for-future-conversion

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boost-thread