Aggregation

The next idea is aggregation, which is designated by the HAS-A relationship. This is when a single class contains a collection of instances of other classes that are obtained from somewhere else in your program. These are considered to have a weak HAS-A relationship as they can exist outside of the confines of the class.

In this case, I created a new class called CombatEncounter which can have an unlimited number of enemies that can be added to it. However when using aggregation, those enemies will exist before the CombatEncounter starts; and when it finishes, they will also still exist. Through code it would look something like this:

class CombatEncounter 
{ 
  public: 
    void AddEnemy(Enemy* pEnemy); 
  private: 
    std::list<Enemy*> enemies; 
 
};

Inside of UML, it would look like this: