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

Subject: Re: [Boost-users] future<>.then and promises
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-06-27 15:01:09

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().
>
> 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?

> 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?
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.
>
> I use boost 1.54 beta 1 with clang 3.2 on Linux and.
>
Please, could you provide complete examples, and the command line?

Best,
Vicente