#include <iostream>
#include <string>

class Memento
{
	std::string oldString;
public:
	Memento(std::string & str, const std::string newString) : oldString(str)
	{
		str = newString;
	}
	void undo(std::string & str)
	{
		str = oldString;
	}
};

void hello_world(Memento & memento)
{
	std::string message;
	memento.undo(message);
	std::cout << message << std::endl;
}

int main()
{
	std::string hw("Hello world!");
	Memento mem(hw, "Goodbye, cruel world!");
	hello_world(mem);
	return 0;
}