Class Diagram

Use Mermaid classDiagram to show classes, interfaces, inheritance, composition, and associations in object-oriented designs.

Basic template

classDiagram
    class User
    class Account
    User --> Account

Core syntax

• Start with classDiagram
• Define a class with class Name
• Add members inside braces
• Use + public, - private, # protected
• Show relationships with:
  • --|> inheritance
  • ..|> interface realization
  • --> association
  • *-- composition
  • o-- aggregation

Common example

classDiagram
    class User {
        +string name
        +login()
    }

    class Report {
        +run()
    }

    class ExportService {
        +export(report)
    }

    class CsvExporter {
        +write(data)
    }

    User --> Report : runs
    Report --> ExportService : uses
    ExportService *-- CsvExporter : owns

Good for

• Domain models
• Code design notes
• Refactoring discussions

Watch for

• Mermaid class diagrams are great for structure, not detailed UML perfection
• Keep method lists short unless the note is specifically about API design