🎀 ATM SYSTEM DESIGN (C++ | OOPS | SOLID)

A production-style ATM simulation built in C++ using Object-Oriented Programming and SOLID principles, focused on clean architecture, security, logging, and testability.

This project reflects real-world system design, making it a strong resume, interview, and DRDO-ready project.

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✨ Features

🎀 Secure PIN Authentication

• PIN verification before access
• Card locks after 3 failed attempts

🎀 Core ATM Operations

• Balance Inquiry
• Deposit
• Withdraw

🎀 Audit Trail / Mini Statement

• Persistent transaction logging
• Timestamped entries
• Stored in logs/transactions.log

🎀 Transaction Logger

• Unique transaction IDs
• Date & time stamping
• File-based persistence

🎀 Built-in Unit Tests

• Deposit validation
• Negative withdrawal protection
• PIN lock security test
• Tests runnable directly from the executable

🎀 Clean & Modular Architecture

• Loosely coupled components
• Easy to extend and maintain
• Clear separation of concerns

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✨ OOPS Concepts Used

💗 Encapsulation
Sensitive data like balance and PIN are protected within classes.

💗 Abstraction
Transaction interfaces hide internal implementation details.

💗 Inheritance
Deposit, Withdraw, BalanceInquiry inherit from Transaction.

💗 Polymorphism
ATM dynamically executes different transaction types.

💗 Single Responsibility Principle
Each class handles one responsibility only.

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✨ Project Structure

ATM-System-Design/

│

├── src/

│ └── atm/

│ ├── account/ # Account & balance logic

│ ├── core/ # ATM engine & transaction logger

│ ├── transaction/ # Deposit, Withdraw, Balance Inquiry

│ └── user/ # User & Card authentication

│

├── logs/

│ └── transactions.log # Persistent audit trail

│

├── tests/

│ └── testRunner.cpp # Unit test suite

│

├── src/main.cpp

├── README.md

├── LICENSE

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✨ How to Build & Run (Local)

🔹 Compile

g++ -Isrc -Itests
src/main.cpp
src/atm/account/.cpp
src/atm/core/.cpp
src/atm/transaction/.cpp
src/atm/user/.cpp
tests/testRunner.cpp
-std=c++17 -Wall -o atm_system

🔹 Run ./atm_system

✨ Run in the Browser (No Setup Required)

This project is fully automated for cloud execution. Click below to start:

🎀 Run on Replit (One-Click)
(Best for quick testing)

🎀 Open in GitHub Codespaces
(Best for deep code exploration)

✨ Steps

🎀 Click Create codespace on main

🎀Wait for the environment to load (1–2 minutes)

🎀 In the terminal, run: ./atm_system

🎀 Choose:

1 → Run ATM

2 → Run Tests

💖 No compiler setup 💖 No dependency issues 💖 Fully reproducible environment

✨ Running Tests

When prompted:

1. Run ATM
2. Run Tests

Select 2 to execute all unit tests.

✨ Example Output [PASS] Negative Withdraw [PASS] Deposit Overflow [PASS] PIN Lock

SUMMARY: 3 passed, 0 failed

🎯 Interview-Level Explanation (IMPORTANT)

If asked “How can I run your project?”, answer:

“I’ve containerized the development environment using GitHub Codespaces. Anyone can open the repository, launch a codespace, and run the ATM system using a single make run command — no local dependencies required.”

🔥 This signals system design maturity, not just coding.

✨ Why This Project?

🎀 Practice real-world OOPS & system design 🎀 Learn secure authentication flows 🎀 Implement persistent logging & audit trails 🎀 Demonstrate testing mindset 🎀 Build a strong interview-ready project

✨ Future Enhancements

🎀 Multi-user support 🎀 Database-backed persistence 🎀 Admin mode & analytics 🎀 Encryption for PIN storage 🎀 CI pipeline for automated tests

✨ UML Class Diagram

The UML class diagram below represents the core architecture of the ATM system and demonstrates inheritance, abstraction, polymorphism, and separation of concerns.

📌 atm-uml-class-diagram

✨ Author

Aparna Jha 🎀 Aspiring Software Engineer | System Design & OOPS Enthusiast

🌸 Final Note

This project prioritizes clarity, correctness, extensibility, and security — exactly what interviewers and research organizations like DRDO look for.

Pink-powered. System-designed. Interview-ready. 🎀✨