Boost:Signals

struct HelloWorld 
{
  void operator()() const 
  { 
    std::cout << "Hello, World!" << std::endl;
  } 
};

// ...

// Signal with no arguments and a void return value
boost::signal<void ()> sig;

// Connect a HelloWorld slot
HelloWorld hello;
sig.connect(hello);

// Call all of the slots
sig();