Access object in derived class from Vector

Question

I have an Entity baseclass which the classes Player and Enemy Inherit.

class Entity
{
  public:

    virtual void Update(sf::RenderWindow &window) {};
    virtual void Draw(sf::RenderWindow &window) {};

};

Both player and enemy contain a sprite object that looks like this:

class Player : Entity
{
   public:

   sf::Sprite sprite

    void Update(sf::RenderWindow &window);
    void Draw(sf::RenderWindow &window)
}

Player and Enemy are created inside a vector which is set up like this:

class EntityManager
{
   public:
   void CollisionCheck();
   private:
   std::vector<Entity*> entityVector;
}

I'm looking to use a collision detection function of this form:

bool Collision::CircleTest(const sf::Sprite& Object1, const sf::Sprite& Object2)

So I'm trying to do something like this:

void EntityManager::ColCheck()
{
   if (Collision::CircleTest(entityVector[0]->sprite, entityVector[1]->sprite))
      {
         cout << "COLLISION\n";
      }
}

This results in the following compile error:

‘class Entity’ has no member named ‘sprite’

I'm unsure how to create a dummy sprite in Entity so that I can access the player and enemy sprites using the above method. Is this possible?

I'm stumped and would greatly appreciate any help anyone can offer!

Answer 1

If everything in your code that derives from Entity has a sprite object, then you should declare that object in the base class.

Not declaring the object in the base class means that there could be a class inheriting from Entity that does not have a sprite object, which means that ColCheck has no valid basis for assuming that elements of entityVector point to something that has a variable called sprite. Make sense?

Answer 2

You probably shouldn't create a dummy unless having a sprite is something ALL entities have.

What you might want is to use a visitor pattern or possibly one of the many multiple-dispatch implementations. Which and what will end up having to be up to you.

Answer 3

If both Player and Enemy classes contain sprite, why not declare it inside Entity? This should solve your problem.

Answer 4

You could make a member function sprite() that is declared as a pure virtual function inside Entity:

class Entity {
public:
    virtual void Update(sf::RenderWindow &window) {};
    virtual void Draw(sf::RenderWindow &window) {};

    virtual sf::Sprite& sprite() = 0;
};

Then, the Player and Enemy implementations would return the sf::Sprite instance variables each has. However, the other posters bring up a valid point; it might make sense to have the sprite be in the base class if all derived classes are going to have one.