On Thu, Jun 27, 2013 at 9:01 PM, Vicente J. Botet Escriba < 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.
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