#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;
}