Fast C++ Delegate: Boost.Function 'drop-in' replacement and multicast

// FD.Delegate library examples

// Copyright Douglas Gregor 2001-2003. Use, modification and
// distribution is subject to the Boost Software License, Version
// 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

// For more information, see http://www.boost.org

// Copyright (C) 2007 JaeWook Choi
// , modified from Boost.Signals button_click.cpp

#include <boost/bind.hpp>
#include <boost/signals/signal1.hpp>
#include <fd/delegate/delegate1.hpp>
#include <iostream>

struct print_string : public boost::signals::trackable {
  typedef void result_type;

  void print(const std::string& s) const { std::cout << s << std::endl; }
};

struct my_button {
  typedef boost::signal1<void, const std::string&> click_signal_type;
  typedef click_signal_type::slot_type click_slot_type;

  boost::signals::connection on_click_connect(const click_slot_type& s)
    { return on_click.connect(s); }

  my_button(const std::string& l) : label(l) {}

  virtual ~my_button() {}

  void click();

protected:
  virtual void clicked() { on_click(label); }

private:
  std::string label;
  click_signal_type on_click;
};

void my_button::click()
{
  clicked();
}

struct my_button2 {
  typedef fd::delegate1<void, std::string const &> click_signal_type;
  typedef click_signal_type click_slot_type;

  fd::multicast::token on_click_connect(const click_slot_type& s)
  { return on_click.add(s); }

  my_button2(const std::string& l) : label(l) {}

  virtual ~my_button2() {}

  void click();

protected:
  virtual void clicked() { on_click(label); }

private:
  std::string label;
  click_signal_type on_click;
};

void my_button2::click()
{
  clicked();
}

int main()
{
  my_button* b = new my_button("OK!");
  print_string* ps = new print_string();
  b->on_click_connect(boost::bind(&print_string::print, ps, _1));

  b->click(); // prints OK!

  delete ps;

  b->click(); // prints nothing

  // --------------------------------------------------------------------------------

  my_button2* b2 = new my_button2("OK,too!");
  print_string* ps2 = new print_string();
  b2->on_click_connect(boost::bind(&print_string::print, ps2, _1));

  b2->click(); // prints OK,too!

  delete ps2;

  // FD.Delegate does not have trackable feature.
  b2->click(); // Access Violation! but it might prints OK,too! by luck.

  return 0;
}