#include <iostream>
#include <string>

class Observer
{
public:
	virtual ~Observer() { }
	virtual void on_message(const std::string &)=0;
};

class MessageBoard
{
	Observer & observer;
public:
	MessageBoard(Observer & obs) : observer(obs) { }

	void post(const std::string & str)
	{
		observer.on_message(str);
	};
};

class Writer : public Observer
{
public:
	void on_message(const std::string & str)
	{
		std::cout << str << std::endl;
	}
};

void hello_world(MessageBoard & message_board)
{
	message_board.post("Hello world!");
}

int main()
{
	Writer writer;
	MessageBoard message_board(writer);
	hello_world(message_board);
	return 0;
}