Composition vs aggregation vs association summary

Relationships between objects

There are many different kinds of relationships two objects may have in real-life, and we use specific “relation type” words to describe these relationships. For example: a square “is-a” shape. A car “has-a” steering wheel. A computer programmer “uses-a” keyboard. A flower “depends-on” a bee for pollination. A student is a “member-of” a class. And your brain exists as “part-of” you (at least, we can reasonably assume so if you’ve gotten this far).

All of these relation types have useful analogies in C++.

In this chapter, we’ll explore the nuances of the relation types “part-of”, “has-a”, “uses-a”, “depends-on”, and “member-of”, and show how they can be useful in the context of C++ classes. We’ll also explore a couple of related topics that don’t fit nicely anywhere else.

 

 

Composition vs aggregation vs association summary

Here’s a summary table to help you remember the difference between composition, aggregation, and association:

Property	Composition	Aggregation	Association
Relationship type	Whole/part	Whole/part	Otherwise unrelated
Members can belong to multiple classes	No	Yes	Yes
Members existence managed by class	Yes	No	No
Directionality	Unidirectional	Unidirectional	Unidirectional or bidirectional
Relationship verb	Part-of	Has-a	Uses-a