#include <iostream>
#include <string>

class animal
{
	public:
		virtual std::string say() const = 0;
};

class cat:
	public animal
{
	public:
		virtual std::string say() const
		{
			return "meow";
		}
};

class dog:
	public animal
{
	public:
		virtual std::string say() const
		{
			return "bark";
		}
};

class animal_container
{
	public:
		void check(const animal &a) const
		{
			std::cout << a.say() << std::endl;
		}
};

int main()
{
	animal_container a;
	cat c;
	dog d;
	a.check(c);
	a.check(d);
	a.check(d);
	a.check(c);
	return 0;
}