What is the main difference between a class and a struct in C++?

Table of Contents

• Introduction
• Difference Between Class and Struct in C++
  • 1. Access Control (Default Visibility)
    • Practical Use:
  • 2. Inheritance Default Access
    • Practical Use:
  • 3. Encapsulation and Design Intent
• Practical Examples
  • Example 1: Using a Class for Encapsulation
  • Example 2: Using a Struct for Simple Data Structures
• Conclusion

Introduction

In C++, both class and struct are used to define custom data types that can encapsulate data and functions. While they are similar in many ways, especially compared to C, the key difference between them lies in how they handle access control. This distinction affects how data and functions within class and struct are used and exposed in C++ programs.

This guide explains the main differences between class and struct in C++ and highlights when you should use one over the other.

Difference Between Class and Struct in C++

1. Access Control (Default Visibility)

The most significant difference between class and struct in C++ is how they handle access control by default.

• Class: In a class, members are private by default. This means that unless specified otherwise, member variables and functions cannot be accessed outside the class.

  Example:

• Struct: In a struct, members are public by default. This means that data and functions can be accessed directly unless explicitly marked as private or protected.

  Example:

Practical Use:

• Use class when you want encapsulation to control access to data, enforcing good design principles.
• Use struct for POD (Plain Old Data) structures where encapsulation is less critical.

2. Inheritance Default Access

Another distinction arises in inheritance. When inheriting from a base class or struct, the default access level differs based on whether you're using a class or a struct.

• Class: In a class, inheritance is private by default. This means that base class members are inherited privately unless explicitly marked as public or protected.

  Example:

• Struct: In a struct, inheritance is public by default. This means that base class members are inherited publicly, allowing access to the derived class members unless marked otherwise.

  Example:

Practical Use:

• Use class for inheritance where you want to limit access to base class members.
• Use struct when you want a simple public inheritance.

3. Encapsulation and Design Intent

• Class: In C++, the class keyword is generally used when implementing object-oriented designs. It emphasizes encapsulation by default, keeping member variables private and exposing only necessary functions (getters/setters, public methods) to interact with an object.
• Struct: The struct keyword is commonly used when creating data structures where direct access to member variables is intended. While you can also add functions to a struct in C++, the use of struct typically suggests a simpler, more transparent design where encapsulation is not a priority.

Practical Examples

Example 1: Using a Class for Encapsulation

In this example, encapsulation is enforced by making width and height private, and only the public methods setDimensions and area are exposed.

Example 2: Using a Struct for Simple Data Structures

Here, the struct is used for a simple data structure, and member variables are accessed directly without encapsulation.

Conclusion

In C++, the main difference between class and struct lies in their default access control and inheritance behavior. In a class, members are private by default, making it ideal for encapsulation in object-oriented design. In contrast, a struct defaults to public members and is commonly used for simpler, data-focused structures. Choosing between class and struct depends on whether you prioritize encapsulation and inheritance or require a straightforward, transparent data container.