UML Associations (objectmentor)

The black diamond represents composition. It is placed on the Circle class because it is the Circle that is composed of a Point. The arrowhead on the other end of the relationship denotes that the relationship is navigable in only one direction. That is, Point does not know about Circle. In UML relationships are presumed to be bidirectional unless the arrowhead is present to restrict them. Had I omitted the arrowhead, it would have meant that Point knew about Circle. At the code level, this would imply a #include “circle.h” within point.h. For this reason, I tend to use a lot of arrowheads. ... In this case we have represented the composition relationship as a member variable. We could alsohave used a pointer so long as the destructor of Circle deleted the pointer. ... There are other forms of containment that do not have whole / part implications. For example, Each Window refers back to its parent Frame. This is not aggregation since it is not reasonable to consider a parent Frame to be part of a child Window. We use the association relationship to depict this.

http://www.objectmentor.com/resources/articles/umlClassDiagrams.pdf