Unified Modeling Language Reference Manual, The, Second Edition

shared aggregation

An aggregation that does not represent ownership.

Semantics

One end of a binary association can be marked as being an aggregation, meaning that an instance of the class connected to the end is considered to be composed of parts that are instances of the class at the other end. An aggregation can be composite or shared. A composite aggregation conveys the sense of ownership. An object may be part of at most one composite aggregation. A shared aggregation does not have this restriction. An object can belong to many shared aggregations.

There are no precise semantics to shared aggregation except for the restriction that chains of aggregation links may not form a cycle. Modelers use shared aggregation in various ways to represent ...