main.cpp

class Undoable
{
public:
	Undoable() {}
	void setString(std::string str)
	{	
		if (str.find("🍺") != std::string::npos)
		{
			std::cout << "No beer tonight! String not set.\n";
			return;
		}
		oldString = currentString;
		currentString = str;
	}
	std::string getString() const
	{
		return currentString;
	}
	void undo()
	{
		setString(oldString);
	}
private:
	std::string currentString;
	std::string oldString;
};

void test_undoclass()
{
	Undoable u;
	u.setString("bar");
	u.setString("foo");
	std::cout << u.getString() << "\n";
	u.undo();
	std::cout << u.getString() << "\n";
	u.undo();
	std::cout << u.getString() << "\n";
	u.setString("Getting some 🍺 tonight!");
}