Listing 6

template<typename delegate, typename ptr_obj, typename mem_fun>
class delegate_impl : public callable<typename delegate::dret,
  typename delegate::dparams>
{
public:
  typedef delegate_impl<delegate, ptr_obj, mem_fun> self_type;
  typedef typename delegate::dret ret;
  typedef typename delegate::dparams params;
  typedef typename delegate::p1 p1;
  // repeat for parameters 2 and higher...
  delegate_impl(ptr_obj o, mem_fun mf) 
    : _done(false), _fwd(0), 
    _ptr_obj(o), _mem_fun(mf) {}
  void operator()()
  { _fwd = bind(_mem_fun, _ptr_obj); }
  void operator()(p1 par1) 
  { _fwd = bind(_mem_fun, _ptr_obj, par1); }
  // repeat operator() for 2 and more parameters...
  virtual ~delegate_impl() {}
  bool invoke() 
  {
    _result = _fwd();
    mutex::scoped_lock lk(_mx);
    _done = true;
    _completed.notify_all();
    if(0 != _ac)
      _ac->accept_result(_result);
    return true;
  }
  bool is_completed() const { return _done; }
  void wait_completion()
  {
    mutex::scoped_lock lk(_mx);
    if(_done)
      return;
    _completed.wait(lk);
  }
  virtual self_type* make_clone() const
  { return new self_type(_ptr_obj, _mem_fun); }
  ret result() const { return _result; }
private:
  bool _done;
  function0<ret> _fwd;
  ret _result;
  mutex _mx;
  condition _completed;
  ptr_obj _ptr_obj;
  mem_fun _mem_fun;
};