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

Question

I am trying to make the following continuation work - but 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 the "done:", so the future fires, but it'll then just "hang" on f.get() .. I am 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: The following code change will make the example work - but 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: The following, adding a launch policy launch::deferred, will also work:
foo.start().then(boost::launch::deferred, [](boost::future<int> f) {
std::cout << "done:" << std::endl;
std::cout << f.get() << std::endl;
});
and this also:
boost::future<int> start() {
boost::future<int> f = p.get_future();
f.set_deferred();
return f;
}

Answer 1

The problem is, your composed future is not kept around. In fact, it is a temporary and it gets destructed as soon as the 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 it prints
done:
23
See it Live On Coliru
If you move the f2.get() before the foo.finish() it will dead lock again.