using boost.lambda in std::for_each with boost::function

I'm currently learning new features in C++11 and boost, such as lambda and boost::function.
I'm trying to use boost.lambda in std::for_each, with the iterated type being boost::function.
The code looks like this:

void task1(int a)
{
  std::cout << "task1: " << a << std::endl;
}

void task2(const std::string& str)
{
  std::cout << "task2: " << str << std::endl;
}

int main()
{
  std::list<boost::function<void()> > functions;
  functions.push_back(boost::bind(&task1, 5));
  functions.push_back(boost::bind(&task2, "test string"));

  // working
  std::list<boost::function<void()> >::iterator i = functions.begin();
  for (; i != functions.end(); ++i)
  {
    (*i)();
  }
  // also working
  std::for_each(functions.begin(), functions.end(), [](boost::function<void()>& f){f();});

  // trying to use boost::lambda but none compiles.
  std::for_each(functions.begin(), functions.end(), boost::lambda::bind(_1));

  std::for_each(
      functions.begin(),
      functions.end(),
      boost::lambda::bind(&boost::function<void()>::operator(), &_1, _1));

  std::for_each(
      functions.begin(),
      functions.end(),
      boost::lambda::bind(std::mem_fn(&boost::function<void()>::operator(), _1));

  return 0;
}

How can call a boost::function object with boost::lambda? I think I'm supposed to wrap it with boost::lambda::bind(), but I just don't know how. I have read the boost.lambda document, but I didn't find anything useful there.