Question

I would like to implement a method like this:

boost::unique_future<int> foo()
{
    return defer([=] // call this lambda on boost::unique_future<int>::get();
    {
         return prime(10);
    });
}

I know of boost::promise and boost::packaged_task and set_wait_callback, however since the returned future takes a reference to either this will not work?

I know there is std::async and std::launch::sync, but how can emulate it using boost and vs2010?

std::unique_future<int> foo()
{
    return std::async(std::launch::sync, [=] 
    {
         return prime(10);
    });
}

Answer 1

You can use set_wait_callback, you just have to dynamically allocate the promise, and delete it in the callback after you set the value:

#include <boost/thread.hpp>
#include <iostream>

void callback(boost::promise<int>&p) {
    std::cout<<"callback"<<std::endl;
    p.set_value(42);
    delete &p;
}

boost::unique_future<int> f()
{
    boost::promise<int> *p=new boost::promise<int>;
    p->set_wait_callback(callback);
    return p->get_future();
}

int main()
{
    boost::unique_future<int> uf(f());
    std::cout<<"main()"<<std::endl;
    int val=uf.get();
    std::cout<<"val="<<val<<std::endl;
}