#include <string>
#include <iostream>

class HelloWorld
{
public:
	virtual ~HelloWorld() { }
	void output()
	{
		write_string("Hello world!");
		write_endl();
	};
	virtual void write_string(const std::string &)=0;
	virtual void write_endl()=0;
};

class HelloWorldImpl : public HelloWorld
{
public:
	void write_string(const std::string & str)
	{
		std::cout << str;
	}
	void write_endl() 
	{ 
		std::cout << std::endl; 
	}
};

void hello_world(HelloWorld & hw)
{
	hw.output();
}

int main()
{
	HelloWorldImpl hw;
	hello_world(hw);
	return 0;
}