#pragma once
class EntityManagerInterface
{
    public:
    virtual entity_sptr AddEntity(Entity* newentityptr)=0;
    virtual void AddEntity(entity_sptr newentity)=0;
};

typedef EntityManagerInterface* entitymanager_ptr;

class EntityManagerInterface {
public:
    virtual entity_sptr AddEntity(Entity* newentityptr) = 0;
    virtual void AddEntity(entity_sptr newentity) = 0;
};

class EntityManager : public EntityManagerInterface {
    // virtual functions are implemented in this class..
};

Entity e;
EntityManager em;
EntityManagerInterface& emi = em;

emi.AddEntity(&e); // calls EntityManager::AddEntity(Entity *)

#include <iostream>

struct Base {
  virtual void foo(void) const {
    std::cout << "I'm a Base!\n";
  }
  virtual ~Base() {}
};

struct A : Base {
  virtual void foo(void) const {
    std::cout << "I'm an A!\n";
  }
};

struct B : Base {
  virtual void foo(void) const {
    std::cout << "I'm a B!\n";
  }
};

void foo(const Base & base) {
  base.foo();
}

int main(int, char **) {
  A a;
  B b;
  foo(a);
  foo(b);
  return 0;
}