When embarking on Low-Level Design (LLD), structural diagrams serve as your system's blueprint. They define the static architecture of your software—mapping out exactly what components exist and how they are wired together before any execution logic is written. At the absolute center of structural modeling sits the Class Diagram.

Key ideas:

  • A Class Diagram provides a static view of an application, detailing classes, interfaces, attributes, and methods.

  • It translates abstract object-oriented concepts into a clear structural map.

  • It forms the direct bridge between conceptual design and your physical codebase setup.

The Anatomy of a Class Block

In a UML Class Diagram, a class is represented by a simple rectangle divided vertically into three distinct compartments. Each compartment has a dedicated responsibility for describing the object.

1. The Top Compartment (Class Name)

This contains the name of the class, centered and bolded (e.g., Book or Customer). If the name is italicized, it indicates an Abstract Class. If it is enclosed in guillemets (e.g., <<interface>>It represents a pure behavioral contract or Interface.

2. The Middle Compartment (Attributes / Properties)

This section lists the state variables or fields that objects of this class will hold. They follow a specific notation format: visibility name : type = default_value

3. The Bottom Compartment (Methods / Operations)

This section houses the behaviors or functions that the class can execute. They follow a matching notation format: visibility name(parameter_list) : return_type

Visibility Modifiers (Access Specifiers)

To enforce encapsulation, Class Diagrams explicitly denote which parts of the system can view or modify an object's internal properties. This is achieved using four standard prefix symbols:

  • - (Private): Accessible only within the defining class itself. This is the default choice for almost all class attributes.

  • + (Public): Globally accessible by any other class or external subsystem module. This is typically reserved for business execution methods.

  • # (Protected): Accessible within the defining class and any derived subclasses that inherit from it.

  • ~ (Package/Package-Private)Accessible only by classes residing inside the same folder or namespace module.

Step-by-Step Modeling Example: A Retail Product Class

To see how a conceptual object converts into a structural UML block, let us analyze a standard inventory item for an e-commerce platform:

"An inventory entity named Product has a private alphanumeric SKU string, a private description, a private decimal price, and a default status tracking flag. It allows public access to change the retail price, check stock availability, and calculate bulk discount reductions."

C++ Code Representation of the Class Structure

Here is how that exact visual three-compartment block maps directly into standard C++ header definitions:


Architectural Mapping: Diagram to Code

When translating a class diagram block into code, look for these explicit patterns:

  • Compartment Transitions: The middle section dictates what variables go inside your private access specifier block. The bottom section dictates what function signatures must be declared under your public access specifier block.

  • Types and Returns: The type trailing the colon (: string or : double) maps directly to the data type declaration in C++ (std::string or double).

  • Const Correctness: If a method does not change any internal properties of the class (like isAvailable() : boolean), make sure to add the const keyword to its C++ implementation to protect the object's state.

Common Pitfalls in Class Modeling

  • Blindly Exposing Setters: Do not list public set methods for every attribute in your middle compartment. Exposing raw setters (like setPrice(), setStatus()) breaks encapsulation and allows external classes to bypass validation rules. Expose specific business actions (like updatePrice()) instead.

  • Overloading with Implementation Details: A class diagram is a blueprint, not a raw code dump. Do not clutter the diagram by listing standard constructors, memory destructors, or trivial helper methods. Keep the focus entirely on high-level attributes and essential business capabilities.

  • Ignoring Case and Type Specifications: Leaving the parameter types or return values blank in your operations box creates ambiguity for developers writing the implementation code. Always specify explicit data contracts.

Summary

  • Class Diagrams present the foundational structural view of an object-oriented application.

  • The three-compartment rectangle cleanly separates the entity's identity, data state, and behavioral interfaces.

  • Visibility prefixes(-, +, #) explicitly define access scopes, ensuring clean data encapsulation across your system modules.