Isn't it really a matter of strong vs. weak associations?
If something "is a" then that represents a composition. For instance a triangle is a composition of points. If the Triangle is destroyed, so are it's points.
If something "contains a" that represents and aggregation. Were this the triangle from above, the points would still exist independantly if the triangle is destroyed because it is a stronger binding.
If something "is a" then that represents a composition. For instance a triangle is a composition of points. If the Triangle is destroyed, so are it's points.
If something "contains a" that represents and aggregation. Were this the triangle from above, the points would still exist independantly if the triangle is destroyed because it is a stronger binding.