Difference Between Structure and Class in C++

In C++, both structures (struct) and classes (class) are user-defined data types used to group related data and functions into a single unit. They support almost the same features, including member functions, constructors, inheritance, and polymorphism. The primary differences between them lie in their default access control and inheritance behavior.

• Both can contain data members and member functions.
• Both support constructors, inheritance, and virtual functions.
• The main differences are the default member access and default inheritance mode.

Structure

A structure is a user-defined data type that groups related variables under a single name. In C++, structures can also contain member functions, constructors, and other class-like features.

• Declared using the struct keyword.
• Members are public by default.
• Supports functions, constructors, and inheritance.
• Commonly used for storing and organizing data.

#include <iostream>
using namespace std;

struct Student{
    
    int id;
    string name;
};

int main(){

    Student s;

    s.id = 101;
    s.name = "Rahul";

    cout << s.id << " " << s.name;

    return 0;
}

101 Rahul

Explanation

• Student is a structure containing two data members.
• Members are public by default.
• Therefore, s.id and s.name can be accessed directly outside the structure.

Class

A class is a user-defined data type that encapsulates data and functions into a single unit. It is one of the fundamental building blocks of Object-Oriented Programming (OOP).

• Declared using the class keyword.
• Members are private by default.
• Supports encapsulation, inheritance, and polymorphism.
• Widely used for implementing object-oriented programs.

#include <iostream>
using namespace std;

class Student{
    
private:
    int id;

public:
    void setId(int i) {
        id = i;
    }

    void display() {
        cout << id;
    }
};

int main(){

    Student s;

    s.setId(101);
    s.display();

    return 0;
}

101

Explanation

• id is a private data member.
• It cannot be accessed directly outside the class.
• Public member functions are used to access and modify the data.

Structure Members are Public by Default

#include <iostream>

using namespace std;

struct Test {
    // x is public
    int x;
};

int main()
{
    Test t;
    t.x = 20;

    // works fine because x is public
    cout << t.x;
}

20

Explanation

• The member x is declared inside a structure without an explicit access specifier.
• Since structure members are public by default, x can be accessed directly outside the structure.
• Therefore, assigning and printing t.x works successfully.

Key Differences Between Structure and Class

Although structures and classes are functionally very similar, they differ in a few default behaviors.