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

Subject: Re: [Boost-users] future<>.then and promises
From: Tobias Furuholm (furuhlm_at_[hidden])
Date: 2013-06-27 16:53:34

On Thu, Jun 27, 2013 at 9:01 PM, Vicente J. Botet Escriba <
vicente.botet_at_[hidden]> 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.

>
>
>> 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...

> 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?

The examples are complete except for includes and that they are put in a
main function. So e.g. the first example looks like this when complete

#include <boost/thread.hpp>

int main()
{
   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);
}

The command line follows:

clang++ -m32 --std=c++11 -DBOOST_THREAD_VERSION=4
-I/usr/local/boost_1_54_0_beta1 -I/usr/include/x86_64-linux-gnu/c++/4.7/32
main.cpp  -pthread -o test -L/usr/local/boost_1_54_0_beta1/stage/lib
-lboost_system -lboost_thread

The -m32 is because I build for a 32-bit target on a 64 bit machine. Boost
is also compiled with 32-bit.

Regards,
Tobias