I have two classes, say A and B. A has a member variable of B and B also has a member variable of type A. A actually instantiates B and passes a reference of self to B's constructor. B then stores the reference of A in its member variable. How do I show this kind of aggregation in a UML diagram?

Also, I would like to know the difference between uni-directional & bi-directional aggregation & composition. Examples would be highly useful.

Thanks,
Suresh