include <iostream>
  
  using namespace std;
  
  // forward references
  class Fish;
  class Bear;
  class Dinosaur;
  
  class Animal
  {
  public:
    virtual bool eats(const Animal& inPrey) const = 0;
  
    virtual bool eatenBy(const Bear& inBear) const = 0;
    virtual bool eatenBy(const Fish& inFish) const = 0;
    virtual bool eatenBy(const Dinosaur& inDinosaur) const = 0;
  };
  
  class Bear : public Animal
  {
  public:
    virtual bool eats(const Animal& inPrey) const;
  
    virtual bool eatenBy(const Bear& inBear) const;
    virtual bool eatenBy(const Fish& inFish) const;
    virtual bool eatenBy(const Dinosaur& inDinosaur) const;
  };
  
  class Fish : public Animal
  {
  public:
    virtual bool eats(const Animal& inPrey) const;
  
    virtual bool eatenBy(const Bear& inBear) const;
    virtual bool eatenBy(const Fish& inFish) const;
    virtual bool eatenBy(const Dinosaur& inDinosaur) const;
  };
  
  class Dinosaur : public Animal
  {
  public:
    virtual bool eats(const Animal& inPrey) const;
  
    virtual bool eatenBy(const Bear& inBear) const;
    virtual bool eatenBy(const Fish& inFish) const;
    virtual bool eatenBy(const Dinosaur& inDinosaur) const;
  };
  
  bool Bear::eats(const Animal& inPrey) const 
  {
    return inPrey.eatenBy(*this);
  }
  
  bool Bear::eatenBy(const Bear& inBear) const 
  { 
    return false; 
  }
  
  bool Bear::eatenBy(const Fish& inFish) const 
  { 
    return false; 
  }
  
  bool Bear::eatenBy(const Dinosaur& inDinosaur) const 
  { 
    return true;
  }
  
  bool Fish::eats(const Animal& inPrey) const 
  {
    return inPrey.eatenBy(*this);
  }
  
  bool Fish::eatenBy(const Bear& inBear) const 
  { 
    return true; 
  }
  
  bool Fish::eatenBy(const Fish& inFish) const 
  { 
    return true; 
  }
  
  bool Fish::eatenBy(const Dinosaur& inDinosaur) const 
  { 
    return true;
  }
  
  bool Dinosaur::eats(const Animal& inPrey) const 
  {
    return inPrey.eatenBy(*this);
  }
  
  bool Dinosaur::eatenBy(const Bear& inBear) const 
  { 
    return false; 
  }
  
  bool Dinosaur::eatenBy(const Fish& inFish) const 
  { 
    return false; 
  }
  
  bool Dinosaur::eatenBy(const Dinosaur& inDinosaur) const 
  { 
    return true;
  }
  
  int main(int argc, char** argv)
  {
    Fish myFish;
    Bear myBear;
    Dinosaur myDino;
  
    cout << "Fish eats bear? " << myFish.eats(myBear) << endl;
    cout << "Fish eats dino? " << myFish.eats(myDino) << endl;
    cout << "Dino eats bear? " << myDino.eats(myBear) << endl;
    cout << "Bear eats fish? " << myBear.eats(myFish) << endl;
  }