Relationship Diagrams

Concept Definition

Relationship diagrams show how classes connect. The most important relationships for this course are inheritance, composition, aggregation, association, and dependency. Each relationship has a different meaning, so the arrow should match the real design.

A useful analogy is a university system. A department owns classrooms, a course uses a teacher, a student knows an advisor, and a report temporarily uses a printer. The diagram should make these differences visible before code hides them.

Why It Matters

Wrong relationships create wrong code. If you model Car as inheriting from Engine, the design says a car is an engine, which is false. If you model strong ownership as a weak association, cleanup and responsibility become unclear.

Relationship diagrams force you to answer two important questions: is this an is-a relationship, and who owns whom?

Syntax Block

class Derived : public Base {};      // inheritance
class Owner { Part part; };          // composition
class User { Service& service; };    // association or dependency

Explained Code

Example: Relationships in C++

#include <string> // std::string

class Shape {
public:
    virtual ~Shape() = default;       // base for inheritance
};

class Circle : public Shape {};       // Circle is a Shape

class Book {
private:
    std::string title;                // simple owned data
};

class Library {
private:
    Book book;                        // Library strongly owns Book
};

classDiagram
    Shape <|-- Circle
    Library *-- Book
    Course o-- Teacher
    Doctor --> Patient
    OrderService ..> Logger

Key Points / Rules

• Use inheritance only for true is-a relationships.
• Use composition when one object strongly owns another.
• Use aggregation for weaker whole-part relationships.
• Use association when objects know each other but ownership is not central.
• Use dependency when one class temporarily uses another.

Common Mistakes

1. Using inheritance for reuse only. Reuse without substitutability often belongs to composition.
2. Confusing aggregation and composition. Ask whether the part's lifetime depends on the whole.
3. Drawing arrows without explaining ownership. A diagram should communicate responsibility.
4. Making every relationship bidirectional. Two-way links increase coupling.

Quick Check

1. Which relationship fits Library owning Book objects?
2. Why is Car inheriting from Engine a bad design?

Viva Answer

Relationship diagrams show how classes are connected. The main design choices are inheritance for is-a, composition for strong has-a, aggregation for weak has-a, association for knowing, and dependency for temporary use.