class Entity : public Engine
{
    protected:
        //…
    public:
        //…
        virtual void Physics()
        {
            //Default physics.//
        }
        virtual void Other(){}//Could be used for AI.
};
struct DefaultAI
{
    void AIDefault()
    {
        //Default AI behavior.//
    }
};
class FirstAI: public Entity, public DefaultAI
{
    //…
    public:
        //…
        void Other()
        {
            //The AI most of the AI’s would have.
            Default();
        }
};
class SecondAI: public Entity
{
   //…
public:
   //…
   void Other()
   {
      //New AI behavior.//
   }
   void Physics()
   {
      //New physics.//
   }
};
//Could be in the singleton.//
static void Shared( Entity *ent )
{
   ent->Physics();
   ent->Other();
}
class Single
{
    std::vector objects;
    public:
        void Instantiate( std::string what )
        {
            //…
        }
        void Update()
        {
             //Somewhere in the engines cycle.//
             Shared( objects[i] );
        }
};