Lesson 203 +10 XP

Object Relationships

Real programs join many classes. C++ describes how objects relate with a small vocabulary: composition, aggregation, association, dependency.

Composition - part of me

A composition means the whole creates and destroys its parts. An Engine inside a Car dies with the car.

struct Engine { int hp; };

struct Car {
    Engine engine;   // owned by value, both live and die together
    Car() : engine{120} {}
};

Aggregation - I hold someone else's

Aggregation means the whole holds references to independent object. A Team points at Player instances that exist on their own.

struct Player { std::string name; };

struct Team {
    std::string teamName;
    std::vector<Player*> members;   // borrowed, not owned
};

Association - we know each other

Association links classes without any ownership. A Doctor and a Patient know each other, but neither owns the other.

Dependency - I use it

The loosest tie: a class uses another only in a function signature.

void printTeam(const Team& team);   // depends on Team

No ownership, and no member whose lifetime is stored.

Loosen the coupling

Prefer the loosest relationship that works. Favor arguments over member; favor interfaces over type names. The less a class knows, the easier it is to test.

TL;DR

Composition: the whole owns the parts and shares their lifetime.
Aggregation: borrowed parts that outlive the whole.
Association: classes that relate without any ownership.
Dependency: one class only uses the other in a parameter.
Loose coupling and clear interfaces make classes reusable.

Object Relationships