Properties: Attributes & Association (review)

• Another notation for property (alternative to attribute)
• Use attributes for properties that are data types, e.g., primitive types or whatever is considered a data type in your design
• Use associations for class types, especially if the methods are of interest at the design level

Association: One class has an element of another class

UML Class Relationships

Composition

• Target belongs only to the source
• Source (e.g., University) controls the lifetime of the target (e.g., College)

Composition Code

• Code: fields/data members of
  • direct type
  • containers
  • pointers/references to objects created (and destroyed) internally (discussed later)

Aggregation

• part of relationship
• Special kind of association with roles and multiplicities often used
• Targets may be shared, e.g., a member can belong to many different clubs
• The source (e.g., Club) doesn't control the lifetime of the target (e.g., Member)
• Code: Fields (data members) are pointers/references to this type, and the objects are created externally

Aggregation: C++ References

• Simpler syntax than pointers
• Sure (?) that the object exists
• Can only be changed in a member initialization list
• Cannot be directly part of a container (workaround: std::reference_wrapper)
• References to local objects whose lifetime may disappear

Aggregation: C++ Pointers

• Much more flexible, can be changed at any time
• More complex syntax
• May be nullptr, so have to check before use
• More difficult to manage lifetime, Who deletes?

Aggregation: std::shared_ptr<>

• Similar to std::unique_ptr<>, but allows multiple clients
• As the last client is deleted, then the pointer is deleted
• Easier to manage lifetime, as the pointer is shared
• Suggest as the first choice

Composition Can Occur Even With:

Composition with References

• References to other fields
• Still composition since the class Club controls the lifetime of the object being referenced, e.g., emptyPolicy and fullPolicy

Composition with Pointers

• Pointers to objects created and destroyed by the class
• Typically allocated in the constructor (or later on) and deleted in the destructor
• The class Club controls the lifetime of policy
• This is still composition

Composition with Smart Pointers

• When the Club destructor is called, the std::unique_ptr<> handles the delete of the allocated memory from the objects
• This is still composition

Composition vs. Aggregation

• Prefer Composition to Aggregation as the object's lifetime is clearer
• Aggregation has particular roles in more complex relationships, especially with polymorphic data members
• Two are often confused, and terms used interchangeably