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C++ Structures (struct) — Definition, Initialization & Best Practices


Learn how to define and use struct in C++: syntax, initialization, member access, struct vs class, passing by value/reference, and common best practices for safe, efficient structs.

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🔷 What is a Structure in C++?

A structure (struct) in C++ is a user-defined data type that allows you to store different types of data under one name.

👉 It is useful when you want to group related data together.

Example (Real Life):

A Student has:

  • Roll number (int)
  • Name (string)
  • Marks (float)

All these can be grouped using a structure.

struct in cpp

🔷 Declaring a Structure

Syntax:

struct StructureName {
    dataType member1;
    dataType member2;
};

Example:

struct Student {
    int rollNo;
    string name;
    float marks;
};

✔ This only defines the structure, no memory is allocated yet.

🔷 Defining Structure Variables

After declaring a structure, you can create variables of that structure.

Example:

Student s1;

Now s1 can store student data.

🔷 Accessing Structure Members

Use the dot (.) operator to access members.

Example:

#include <iostream>
using namespace std;

struct Student {
    int rollNo;
    string name;
    float marks;
};

int main() {
    Student s1;

    s1.rollNo = 101;
    s1.name = "Ali";
    s1.marks = 85.5;

    cout << s1.rollNo << endl;
    cout << s1.name << endl;
    cout << s1.marks << endl;

    return 0;
}

🔷 Initializing Structure Variables

Method 1: Initialization at declaration

Student s1 = {101, "Ahmed", 90.5};

Method 2: Member-wise initialization

Student s1;
s1.rollNo = 102;
s1.name = "Sara";
s1.marks = 88.0;

🔷 Assigning One Structure Variable to Another

In C++, you can directly assign one structure to another of the same type.

Example:

Student s1 = {101, "Ali", 85};
Student s2;

s2 = s1;   // Copy all values

cout << s2.name;   // Output: Ali

✔ All members are copied automatically.

🔷 Array as Members of Structure

A structure member can also be an array.

Example:

struct Student {
    int rollNo;
    char name[20];
    int marks[3];
};

Example Program:

#include <iostream>
using namespace std;

struct Student {
    int rollNo;
    char name[20];
    int marks[3];
};

int main() {
    Student s1;

    s1.rollNo = 101;
    strcpy(s1.name, "Ali");

    s1.marks[0] = 80;
    s1.marks[1] = 75;
    s1.marks[2] = 90;

    cout << "Roll No: " << s1.rollNo << endl;
    cout << "Name: " << s1.name << endl;

    cout << "Marks: ";
    for(int i = 0; i < 3; i++) {
        cout << s1.marks[i] << " ";
    }

    return 0;
}

🔷 Structure with Array of Structures (Common Use)

Student students[2] = {
    {101, "Ali", 85},
    {102, "Sara", 90}
};

🔷 Why Use Structures?

✔ Group related data ✔ Improve code readability ✔ Used in real-world data (students, employees, products) ✔ Foundation for classes in OOP

🔷 Key Points

  • struct groups different data types
  • Access members using dot (.)
  • Structures can be copied directly
  • Structures can contain arrays
  • Structures make programs organized and meaningful

🔷 Example: Structure for Book Information

📘 Problem Statement

Store and display information of a book using structure.

🔷 Step 1: Declare the Structure

struct Book {
    int bookId;
    string title;
    float price;
};

🔷 Step 2: Complete Program Example

#include <iostream>
using namespace std;

struct Book {
    int bookId;
    string title;
    float price;
};

int main() {
    // Defining structure variable
    Book b1;

    // Assigning values
    b1.bookId = 1;
    b1.title = "C++ Basics";
    b1.price = 550.75;

    // Displaying values
    cout << "Book ID: " << b1.bookId << endl;
    cout << "Title: " << b1.title << endl;
    cout << "Price: " << b1.price << endl;

    return 0;
}

🔷 Output

Book ID: 1
Title: C++ Basics
Price: 550.75

🔷 Example with Initialization at Declaration

Book b2 = {2, "Programming Fundamentals", 620.50};

cout << b2.bookId << endl;
cout << b2.title << endl;
cout << b2.price << endl;

🔷 Example: Assigning One Structure to Another

Book b3;
b3 = b2;   // Copy all data

cout << b3.title;   // Output: Programming Fundamentals

🔷 Example: Structure for Employee Information

📌 Problem Statement

Store and display employee details using a structure.

🔷 Step 1: Declare the Structure

struct Employee {
    int empId;
    string name;
    float salary;
};

🔷 Step 2: Complete Program

#include <iostream>
using namespace std;

struct Employee {
    int empId;
    string name;
    float salary;
};

int main() {
    // Defining structure variable
    Employee e1;

    // Assigning values
    e1.empId = 1001;
    e1.name = "Ahmed";
    e1.salary = 45000.50;

    // Displaying values
    cout << "Employee ID: " << e1.empId << endl;
    cout << "Employee Name: " << e1.name << endl;
    cout << "Employee Salary: " << e1.salary << endl;

    return 0;
}

🔷 Output

Employee ID: 1001
Employee Name: Ahmed
Employee Salary: 45000.5

🔷 Example with Initialization at Declaration

Employee e2 = {1002, "Sara", 52000};

🔷 Example: Copying One Structure Variable to Another

Employee e3;
e3 = e1;   // Copy data

cout << e3.name;   // Output: Ahmed

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