Why can't I use shared_from_this with a callbacks interface object?

Question

I'm having trouble with a bad weak_ptr exception when I try to pass a shared_from_this() from a derived class that inherits from the callback interface into a third class which takes the callback base class shared_ptr in the constructor. Why doesn't this work? See simplified test case below:

#include <boost/tr1/memory.hpp>

class MyCallbacks : public std::tr1::enable_shared_from_this<MyCallbacks>
{
public:
  virtual ~MyCallbacks() {}
  virtual void callback1() = 0;
};

class Worker
{
public:
  Worker(std::tr1::shared_ptr<MyCallbacks> cb) : callbacks(cb) {}

  void do_work() { if (callbacks) callbacks->callback1(); }

private:
  std::tr1::shared_ptr<MyCallbacks> callbacks;

};

class Boss : public MyCallbacks
{
public:
  Boss() : worker(shared_from_this()) {}

  void go()
  {
    std::cerr << "Calling worker->do_work" << std::endl;
    worker.do_work();
  }

  void callback1()
  {
    std::cerr << "callback1" << std::endl;
  }

private:
  Worker worker;
};

int main (int argc, char const *argv[])
{
  std::tr1::shared_ptr<Boss> boss(new Boss);
  boss->go();
  return 0;
}

Answer 1

shared_from_this() cannot be called in the constructor because at least one shared_ptr must exist first see the Requires statement at the bottom of this page