Re: [Boost-users] future<>.then and promises

28 Jun 2013


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On Thu, Jun 27, 2013 at 9:01 PM, Vicente J. Botet Escriba 
mailto:vicente.botet@wanadoo.fr> wrote:
Le 27/06/13 14:26, Tobias Furuholm a écrit :
I am trying out future<>.then and I am seeing some results
        that confuses me. I have tried to illustrate this with a few
        examples. The first example is using a promise together with
        future<>.then. It generates no output:
boost::promise<int> promise;
          boost::future<int> fut = promise.get_future();
          promise.set_value(123);
fut.then([](boost::future<int>& f) {
             std::cout << "The value is: " << f.get() << std::endl;
             return f.get();
          });
boost::chrono::milliseconds duration(1000);
          boost::this_thread::sleep_for(duration);
Why is fut.then not triggered? (assigning fut.then to another
        future makes no difference)
I guess you need to call fut.then() before the value is set. I
    will need to check if this works when the future is not created
    with async().
It makes no difference.
Sorry I was wrong.
There is an issue with the current implementation. If the parent future 
has not been created with async, you need to add the launch policy on
Le 27/06/13 22:53, Tobias Furuholm a écrit :
the then call. Please, could you add a ticket to the Trac system? I will 
try to fix it soon.
...
By adding a launch policy I get the output "The value is: 123"
        which is the output I would expect from all my examples:
boost::promise<int> promise;
          boost::future<int> fut = promise.get_future();
promise.set_value(123);
fut.then(boost::launch::async, [](boost::future<int>& f) {
             std::cout << "The value is: " << f.get() << std::endl;
             return f.get();
          });
boost::chrono::milliseconds duration(1000);
          boost::this_thread::sleep_for(duration);
Could you tell me the version of BOOST_THREAD_VERSION are you using?
I use  BOOST_THREAD_VERSION=4
To make this more real, let's move promise.set_value to after
        .then
boost::promise<int> promise;
          boost::future<int> fut = promise.get_future();
fut.then(boost::launch::async, [](boost::future<int>& f) {
             std::cout << "The value is: " << f.get() << std::endl;
             return f.get();
          });
promise.set_value(123);
boost::chrono::milliseconds duration(1000);
          boost::this_thread::sleep_for(duration);
This program outputs nothing and never terminates. But when I
        assign the result of fut.then to another future I get the
        expected output once again:
boost::promise<int> promise;
          boost::future<int> fut = promise.get_future();
*boost::future<int> fut2* = fut.then(boost::launch::async,
        [](boost::future<int>& f) {
             std::cout << "The value is: " << f.get() << std::endl;
             return f.get();
          });
promise.set_value(123);
boost::chrono::milliseconds duration(1000);
          boost::this_thread::sleep_for(duration);
Where is the assignment?
I made the assignment bold to make it more visible...
When you use a policy launch::async the resulting future will block on 
the destructor. When there is no assignment the temporary is destroyed 
before the promise is set.
...
Again I will need to check this.
Finally, as I bonus I add an example where I use async to
        generate a future instead.
boost::future<int> fut = boost::async([]() { return 123; });
fut.then([](boost::future<int>& f) {
             std::cout << "The value is: " << f.get() << std::endl;
             return f.get();
          });
boost::chrono::milliseconds duration(1000);
          boost::this_thread::sleep_for(duration);
future::get() can be called only once depending on the value of
    BOOST_THREAD_VERSION
This generates the output "The value is: " (without 123). Once
        again, by assigning the result of fut.then to another future I
        get the expected output.
As the fut shared state is set asynchronously the temporary ends by been 
set, so the trace is done.

Thanks,
Vicente