Getting return value from a boost::threaded member function?

Question

I have a worker class like the one below:

class Worker{
public:
  int Do(){
    int ret = 100;
    // do stuff
    return ret;
  }
}

It's intended to be executed with boost::thread and boost::bind, like:

Worker worker;
boost::function<int()> th_func = boost::bind(&Worker::Do, &worker);
boost::thread th(th_func);
th.join();

My question is, how do I get the return value of Worker::Do?

Thanks in advance.

Answer 1

I don't think you can get the return value.

Instead, you can store the value as a member of Worker:

class Worker{
public:
  void Do(){
    int ret = 100;
    // do stuff
    m_ReturnValue = ret;
  }
  int m_ReturnValue;
}

And use it like so:

Worker worker;
boost::function<void()> th_func = boost::bind(&Worker::Do, &worker);
boost::thread th(th_func);
th.join();
//do something with worker.m_ReturnValue

Answer 2

Another option is to use promises/futures.

class Worker{
public:
  void Do( boost::promise<int> & p){
    int ret = 100;
    // do stuff
    p.set_value(ret);
  }
};
//Later...
boost::promise<int> p;
boost::thread t( boost::bind(&Worker::Do, &worker, boost::ref(p));
int retval = p.get_future().get(); //This will block until the promise is set.

And if you can use c++0x, then using std::async will package up all of the above and just do:

std::future<int> f = std::async( std::bind(&Worker::Do, &worker) );
int retval = f.get(); //Will block until do returns an int.

Answer 3

In addition, you also have some redundant calls to boost::bind() and boost::function(). You can instead do the following:

class Worker{
    public:
       void operator(){
          int ret = 100;
          // do stuff
          m_ReturnValue = ret;
       }
    int m_ReturnValue;
}

Worker worker;
boost::thread th(worker());//or boost::thread th(boost::ref(worker));

You can do this because Thread's constructor is a convenience wrapper around an internal bind() call. Thread Constructor with arguments

Answer 4

class Worker{
public:
  int Do(){
  int ret = 100;
  // do stuff
  return ret;
  }
}

Worker worker;
boost::packaged_task<int> ptask(boost::bind(&Worker::Do, &worker));
boost::unique_future<int> future_int = ptask.get_future();
boost::thread th(boost::move(ptask));
th.join();
if (future_int.is_ready())
   int return_value = future_int.get();

You can take a look at the "boost::future" concept, ref this link

Answer 5

Another option is using the Boost.Lambda library. Then you can write the code as follows without changing the Worker class:

Worker worker;
int ret;
boost::thread th( boost::lambda::var( ret ) = worker.Do() );
th.join();

This is useful in particular when you cannot change the function to call. Like this, the return value is wrapped in a local variable ret.