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Table of contents
1.
Introduction
2.
Overview of Interfaces and concrete classes
3.
Implementation in C++
4.
Brief Summary of SOLID Principles
5.
Benefits of SOLID Principles
6.
Frequently Asked Questions
6.1.
Why are SOLID principles important?
6.2.
Are there any downsides to using SOLID principles?
6.3.
Can SOLID principles be applied to any programming language?
7.
Conclusion
Last Updated: Mar 27, 2024
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What are SOLID Principles in Low Level System Design?

Author Suraj Pandey
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Introduction

As software development projects grow in size and complexity, managing code becomes more challenging. Codebases become harder to maintain, understand, and change. This is where SOLID principles come in - a set of design principles for writing maintainable, scalable, and extensible software. 

These principles were introduced by Robert C. Martin, a renowned software engineer, and author, and have become a cornerstone of modern software development.

Solid Principles in System Design

SOLID is an acronym that stands for five individual principles: Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. These principles provide a framework for creating high-quality software that is resilient to changes and promotes modular, testable, and efficient code. 

By adhering to SOLID principles, developers can produce easier code to maintain, update, and extend, resulting in more robust and reliable software applications.

This blog post aims to delve into the significance and advantages of SOLID principles, which serve as fundamental design principles for creating maintainable, scalable, and extensible software solutions. Additionally, we will concisely review interfaces and concrete classes, two essential elements of SOLID principles. 

By the end of this post, you will have gained a comprehensive understanding of SOLID principles and be equipped with the knowledge to develop cleaner, more efficient, and more maintainable code.

Overview of Interfaces and concrete classes

Interfaces and concrete classes are essential concepts in object-oriented programming; let's understand them through real-world examples.

Imagine you want to buy a car and go to a car dealership. The dealership has several car models on display, each with specific features and capabilities. In this scenario, the car models can be considered concrete classes, as they are specific types of cars with unique features and capabilities.

Now, let's consider the process of test-driving cars. Before you buy a car, you want to test drive it to see how it performs on the road. However, you don't need to know how the car's engine works or how the transmission is designed to test drive it. You just need to know how to use the car's pedals, steering wheel, and other basic controls. In programming, an interface defines a set of functions that a class must implement, and in this case, the set of functions you need to know to test drive a car can be considered an interface.

To summarize, concrete classes can be considered specific types of objects with unique features and capabilities. In contrast, an interface can be thought of as a set of instructions that define how to use an object's basic controls. By separating these concerns, programming becomes more modular and easier to understand and maintain.

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Implementation in C++

Let's start by defining an interface for the basic controls of a car. In this case, we'll define an interface called ICarControls:

class ICarControls {
public:
  virtual void accelerate() = 0;
  virtual void brake() = 0;
  virtual void steer() = 0;
};

This interface defines three functions: accelerate(), brake(), and steer(). Any class that implements this interface must provide implementations for these functions.

Next, let's define a concrete class for a specific car model. In this example, we'll create a class called SportsCar:

class SportsCar : public ICarControls {
public:
  void accelerate() override {
    // code to make the car accelerate
  }

  void brake() override {
    // code to make the car brake
  }

  void steer() override {
    // code to steer the car
  }

  // additional functions and variables specific to the SportsCar class
};

This class implements the ICarControls interface by providing implementations for the accelerate(), brake(), and steer() functions. It also has additional functions and variables specific to the SportsCar class.

Finally, let's create a function to test drive the car. In this case, we'll create a function called testDrive() that takes an ICarControls object as a parameter:

void testDrive(ICarControls& car) {
  // code to test drive the car using the basic controls
  car.accelerate();
  car.steer();
  car.brake();
}

This function takes an object that implements the ICarControls interface as a parameter and uses the accelerate(), steer(), and brake() functions to test drive the car.

Interfaces and concrete classes are important concepts in SOLID principles, as they provide a way to separate concerns and promote modularity, flexibility, and maintainability in object-oriented programming.

Brief Summary of SOLID Principles

The SOLID principles are a set of design principles that are used to guide the development of object-oriented software. They are designed to make the software more modular, maintainable, and flexible over time.

Here's a brief summary of each principle:

  1. Single Responsibility Principle (SRP): A class should have only one reason to change. This means that a class should only have one responsibility or job to do and not be responsible for multiple unrelated tasks.
    To learn more about the Single Responsibility Principle and how it can be applied, please refer to the following article.
     
  2. Open/Closed Principle (OCP): A class should be open for extension but closed for modification. This means that a class should be designed to allow new functionality to be added without modifying the existing code. This can be achieved through the use of abstraction and interfaces.
    To learn more about the Open/Closed Principle and how it can be applied, please refer to the following article.
     
  3. Liskov Substitution Principle (LSP): Subtypes must be substitutable for their base types. This means that a derived class should be able to be used in place of its base class without causing any problems or unexpected behavior.
    To learn more about the Liskov Substitution Principle and how it can be applied, please refer to the following article.
     
  4. Interface Segregation Principle (ISP): Clients should not be forced to depend on methods they do not use. This means that interfaces should be designed to be specific to clients' needs rather than trying to include all possible methods.
    To learn more about the Interface Segregation Principle and how it can be applied, please refer to the following article.
     
  5. Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules. Instead, both should depend on abstractions. This means that dependencies should be inverted so that high-level modules depend on interfaces or abstractions rather than low-level modules.
    To learn more about the Dependency Inversion Principle and how it can be applied, please refer to the following article.


By following these principles, the software can be made more modular, flexible, and easier to maintain over time.

Benefits of SOLID Principles

Here are some of the key benefits of adhering to SOLID principles:

  1. Improved code maintainability: SOLID principles promote modularity, which makes code easier to understand, update, and debug. By designing code with SOLID principles in mind, developers can ensure that changes to one part of the system do not adversely affect other parts of the system.
  2. Enhanced scalability: SOLID principles emphasize loose coupling between software components, allowing for easier application scaling as the user base grows. By designing modular and easy-to-modify code, developers can add new functionality to the system without compromising its overall structure.
  3. Better code quality: SOLID principles promote the use of best practices, such as dependency injection, single responsibility, and open/closed principles. This results in more robust and reliable code that is easier to test and maintain.
  4. Increased development efficiency: SOLID principles can help reduce development time by making it easier to isolate and fix bugs and add new features to the system. By designing code that is easy to modify and extend, developers can save time and effort in the long run.


Overall, adhering to SOLID principles can help developers create more reliable, efficient, and maintainable software, which can ultimately result in better user experiences and improved business outcomes.

Frequently Asked Questions

Why are SOLID principles important?

SOLID principles help developers create flexible, maintainable, and scalable code. They also help reduce code complexity, increase code reusability, and make code easier to test and debug.

Are there any downsides to using SOLID principles?

While SOLID principles can be beneficial in many ways, there are some potential downsides. Following these principles may require additional effort and time during the development process, which could increase project costs. Additionally, strict adherence to SOLID principles may only sometimes be practical or necessary, especially for small or simple projects.

Can SOLID principles be applied to any programming language?

While SOLID principles are often associated with object-oriented programming (OOP) languages like Java and C++, many of these principles can be applied to other types of languages, including functional programming languages like Haskell and JavaScript.

Conclusion

In conclusion, we have discussed the importance of SOLID principles in software development. We introduced the concept of interfaces and concrete classes and how they form the basis for SOLID principles. We then looked at implementing SOLID principles in C++, a popular programming language used in the industry.

Furthermore, we briefly summarized the five SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. These principles help us create software that is easy to maintain, test, and extend. They provide a solid foundation for building modular, flexible, and scalable software systems.

Lastly, we discussed the benefits of SOLID principles, including improved code quality, reduced technical debt, increased reusability, and easier maintenance. Adhering to these principles makes it easier for developers to collaborate and work on large projects.

You can also consider our System Design Course to give your career an edge over others.

We would love to hear your thoughts and feedback in the comments below. Good luck with your system design journey!

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