In this chapter, we learned about some different kinds of relationships between two objects.
Summary
The process of building complex objects from simpler ones is called object composition. There are two types of object composition: composition, and aggregation.
Composition exists when a member of a class has a part-of relationship with the class. In a composition relationship, the class manages the existence of the members. To qualify as a composition, an object and a part must have the following relationship:
- The part (member) is part of the object (class)
- The part (member) can only belong to one object (class) at a time
- The part (member) has its existence managed by the object (class)
- The part (member) does not know about the existence of the object (class)
Compositions are typically implemented via normal member variables, or by pointers where the class manages all the memory allocation and deallocation. If you can implement a class as a composition, you should implement a class as a composition.
Aggregations exists when a class has a has-a relationship with the member. In an aggregation relationship, the class does not manage the existence of the members. To qualify as an aggregation, an object and its parts must have the following relationship:
- The part (member) is part of the object (class)
- The part (member) can belong to more than one object (class) at a time
- The part (member) does not have its existence managed by the object (class)
- The part (member) does not know about the existence of the object (class)
Aggregations are typically implemented via pointer or reference.
Associations are a looser type of relationship, where the class uses-an otherwise unrelated object. To qualify as an association, an object and an associated object must have the following relationship:
- The associated object (member) is otherwise unrelated to the object (class)
- The associated object (member) can belong to more than one object (class) at a time
- The associated object (member) does not have its existence managed by the object (class)
- The associated object (member) may or may not know about the existence of the object (class)
Associations may be implemented via pointer or reference, or by a more indirect means (such as holding the index or key of the associated object).
In a dependency, one class uses another class to perform a task. The dependent class typically is not a member of the class using it, but rather is temporarily created, used, and then destroyed, or passed into a member function from an external source.
In a container class one class provides a container to hold multiple objects of another type. A value container is a composition that stores copies of the objects it is holding. A reference container is an aggregation that stores pointers or references to objects that live outside the container.
std::initializer_list can be used to implement constructors, assignment operators, and other functions that accept a list initialization parameter. std::initializer_list lives in the <initializer_list> header.
| Property\Type | Composition | Aggregation | Association | Dependency |
|---|---|---|---|---|
| Relationship type | Whole/part | Whole/part | Otherwise unrelated | Otherwise unrelated |
| Members can belong to multiple classes | No | Yes | Yes | Yes |
| Members existence managed by class | Yes | No | No | No |
| Directionality | Unidirectional | Unidirectional | Unidirectional or bidirectional | Unidirectional |
| Relationship verb | Part-of | Has-a | Uses-a | Depends-on |
Quiz time
This chapter is pretty straightforward and a little more abstract than the previous ones, so this quiz will be short and to the point.
- What type of relationship (composition, aggregation, association, or dependency) do the following describe?
1a) An Animal class that contains an animal type (enum) and name (string).
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1b) A text editor class with a save() function that takes a File object as an argument. The save() function writes the contents of the editor to disk.
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1c) An Adventurer class that can carry various kinds of Items, such as swords, wands, potions, or spellbooks. These Items can be dropped and picked up by other Adventurers.
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1d) The Player prays at a Shrine to receive a new power.
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1e) A Computer class that contains a CPU class. The CPU can be removed from the Computer and tested on its own.
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1f) A blacksmith has a specific anvil that he uses to do his work.
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- Select one: If you can design a class using (composition, aggregation, association, or dependency), then you should.