c++ - boost::future and continuations - value set, but future still blocks -

i trying make following continuation work - f.get() blocks. whats wrong?

#include <iostream>  #define boost_thread_provides_future #define boost_thread_provides_future_continuation #include <boost/thread/future.hpp>  struct foo {     boost::future<int> start() {       return p.get_future();    }     void finish() {       p.set_value(23);    }     boost::promise<int> p; };  int main () {     foo foo;     foo.start().then([](boost::future<int> f) {       std::cout << "done:" << std::endl;       std::cout << f.get() << std::endl;    });     foo.finish(); } 

it'll print "done:", future fires, it'll "hang" on f.get() .. lost.

to build:

clang++ -o test8 -std=c++11 -stdlib=libc++ -lboost_thread -lboost_system \   -i/home/oberstet/boost_1_55_0 -l/home/oberstet/boost_1_55_0/stage/lib \   test8.cpp 

update: following code change make example work - why? since f2 isn't used anyway. puzzled again.

   boost::future<void> f2 = foo.start().then([](boost::future<int> f) {       std::cout << "done:" << std::endl;       std::cout << f.get() << std::endl;    }); 

update 2: following, adding launch policy launch::deferred, work:

   foo.start().then(boost::launch::deferred, [](boost::future<int> f) {       std::cout << "done:" << std::endl;       std::cout << f.get() << std::endl;    }); 

and also:

   boost::future<int> start() {       boost::future<int> f = p.get_future();       f.set_deferred();       return f;    } 

the problem is, composed future not kept around. in fact, temporary , gets destructed statement (with .then()) ends.

fix it:

int main () {     foo foo;      auto f1 = foo.start();     auto f2 = f1.then([](boost::future<int> f) {         std::cout << "done:" << std::endl;         std::cout << f.get() << std::endl;     });      foo.finish();     f2.get(); } 

now prints

done: 23 

see live on coliru

if move f2.get() before foo.finish() dead lock again.