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Design Pattern by Gamma et al said

Consider the distinction between object aggregation and acquaintance and how differently they manifest themselves at compile- and run-times.

Aggregation implies that one object owns or is responsible for another object. Generally we speak of an object having or being part of another object. Aggregation implies that an aggregate object and its owner have identical lifetimes.

Acquaintance implies that an object merely knows of another object. Sometimes acquaintance is called "association" or the "using" relationship. Acquainted objects may request operations of each other, but they aren't responsible for each other. Acquaintance is a weaker relationship than aggregation and suggests much looser coupling between objects.

Another useful thing to show is which classes instantiate which others. We call this the "creates" relationship. The arrow points to the class that's instantiated. In Figure c,

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Can more than one objects aggregate the same object? If object A aggregate object B, can another object say C also aggregate B?

Can more than one objects acquaint the same object? If object A acquaint object B, can another object say C also acquaint B?

Can more than one objects instantiate the same object?

Tim
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1 Answers1

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To my understanding and the common definitions I know, Composition is an association where the lifetime of the associated objects is tied to the lifetime of the owner. Aggregation describes an association where the lifetime of the aggregated objects is independent from the lifetime of the owner. Acquaintance is the same as association.

According to this definition an object can be aggregated by multiple owners in an aggregate association. If the association is of type composition then sharing is by definition not intended.

Association or acquaintance are more generic description of a relationship between objects. So it depends on the details of the relationship if a single object can have multiple owners. But when being that generic, it is generally possible.

Wikipedia Object composition and aggregation

But I think there exists another definition which was introduced by Martin Fowler. It's the one you were quoting. To me this is the less known definition since the one that I rendered is defined in the UML rules which are widely spread and have become a de facto standard. According to M. Fowler's definition aggregation is equal to composition whereas acquaintance matches the UML definition of aggregation.

So the answer to your questions is, when looking through the eyes of M. Fowler: the rules for composition apply when talking about Fowler's aggregation which means multiple owners can't aggregate the same objects and in terms of acquaintance the rules of the in the UML defined aggregation association apply meaning that multiple owners can acquaint the same objects independently (from their lifetime).0

And of course you can create instances of an object as often as you want or as memory is available and where you want. The object or the class is only the definition whereas the instance (as a result of the instantiation) is the actual allocated memory area to store data of the object (e.g. fields). The class definition describes the layout of this allocated memory area.

BionicCode
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  • Thanks. How about "Another useful thing to show is which classes instantiate which others. We call this the 'creates' relationship."? Can more than one objects instantiate the same object? – Tim Jun 06 '19 at 16:30
  • So the answer is yes, when object A and object B and object C are each creating an instance of object X. But this instances of object X are never the same when looking at their memory locations. – BionicCode Jun 06 '19 at 16:43
  • Thanks. I'd appreciate if you could also consider https://stackoverflow.com/questions/56479898/how-is-instantiation-different-from-aggregation-and-from-acquaintance and https://stackoverflow.com/questions/56478884/how-is-instantiation-different-from-aggregation-and-from-acquaintance – Tim Jun 06 '19 at 16:45
  • Instantiation creates an object instance while aggregation describes the relationship between objects (that are already created or instantiated) – BionicCode Jun 06 '19 at 16:50
  • "When object A instantiates/aggrgate/acquaint object B, must object A have a field member referencing object B?" YES – BionicCode Jun 06 '19 at 16:52
  • When object A instantiates object B, must object A have a field member referencing object B, or object A's method creates and returns object B? (Could you reply the two questions at their links) – Tim Jun 06 '19 at 17:11
  • Thanks for replying in the other links. I'd appreciate if you could also consider https://stackoverflow.com/questions/56482491/what-is-the-creates-instantiation-relationship-like-between-two-classes and https://stackoverflow.com/questions/56483736/does-factory-method-pattern-follow-template-pattern-and-abstract-factory-patter – Tim Jun 06 '19 at 23:00
  • I already explained the 'creates instance' relationship in the other question. I even provide code examples. – BionicCode Jun 06 '19 at 23:13
  • The last paragraph also anwsers the question regarding the fields. [My Answer](https://stackoverflow.com/questions/56478884/when-object-a-instantiates-aggrgate-acquaint-object-b-must-object-a-have-a-fiel/56485674#56485674) – BionicCode Jun 06 '19 at 23:21