Structures & Unions

1. Understanding Structures

A structure is a user-defined data type that groups related data items of different types under a single name. Structures allow you to create complex data types that represent real-world entities. For example, a Student structure can contain roll number (int), name (string), and marks (float) - all related to a single student entity.

Structures are particularly useful when you need to pass multiple related values to functions, return multiple values from functions, or organize data in a logical way. They provide better code organization and readability compared to using separate variables for each piece of related data.

Basic Structure

#include <iostream>
#include <string>
using namespace std;

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

int main() {
    Student s1;
    s1.rollNo = 101;
    s1.name = "John";
    s1.marks = 85.5;
    
    cout << "Roll No: " << s1.rollNo << endl;
    cout << "Name: " << s1.name << endl;
    cout << "Marks: " << s1.marks << endl;
    
    return 0;
}

Initialization

#include <iostream>
#include <string>
using namespace std;

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

int main() {
    // Initialization at declaration
    Student s1 = {101, "John", 85.5};
    
    // Designated initialization (C++20)
    Student s2 = {.rollNo = 102, .name = "Alice", .marks = 90.0};
    
    return 0;
}

2. Structure with Functions

#include <iostream>
#include <string>
using namespace std;

struct Rectangle {
    int length;
    int width;
    
    // Member function
    int area() {
        return length * width;
    }
    
    void setDimensions(int l, int w) {
        length = l;
        width = w;
    }
};

int main() {
    Rectangle r;
    r.setDimensions(5, 3);
    cout << "Area: " << r.area() << endl;
    
    return 0;
}

3. Array of Structures

#include <iostream>
#include <string>
using namespace std;

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

int main() {
    Student students[3];
    
    // Input
    for (int i = 0; i < 3; i++) {
        cout << "Enter roll no, name, marks: ";
        cin >> students[i].rollNo;
        cin >> students[i].name;
        cin >> students[i].marks;
    }
    
    // Display
    for (int i = 0; i < 3; i++) {
        cout << students[i].rollNo << " " 
             << students[i].name << " " 
             << students[i].marks << endl;
    }
    
    return 0;
}

4. Nested Structures

#include <iostream>
#include <string>
using namespace std;

struct Date {
    int day;
    int month;
    int year;
};

struct Employee {
    int id;
    string name;
    Date birthDate;  // Nested structure
    float salary;
};

int main() {
    Employee emp;
    emp.id = 101;
    emp.name = "John";
    emp.birthDate.day = 15;
    emp.birthDate.month = 5;
    emp.birthDate.year = 1990;
    emp.salary = 50000;
    
    cout << "Employee: " << emp.name << endl;
    cout << "DOB: " << emp.birthDate.day << "/" 
         << emp.birthDate.month << "/" 
         << emp.birthDate.year << endl;
    
    return 0;
}

5. Structure Pointers

#include <iostream>
#include <string>
using namespace std;

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

int main() {
    Student s = {101, "John", 85.5};
    Student *ptr = &s;
    
    // Accessing using pointer
    cout << (*ptr).rollNo << endl;
    cout << ptr->name << endl;  // Arrow operator
    cout << ptr->marks << endl;
    
    // Dynamic allocation
    Student *s2 = new Student;
    s2->rollNo = 102;
    s2->name = "Alice";
    s2->marks = 90.0;
    
    delete s2;
    
    return 0;
}

6. typedef with Structures

#include <iostream>
#include <string>
using namespace std;

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

typedef Student Stu;  // Create alias

int main() {
    Stu s1;  // Same as Student s1;
    s1.rollNo = 101;
    s1.name = "John";
    s1.marks = 85.5;
    
    return 0;
}

7. Unions

A union is similar to a structure, but all members share the same memory location. Only one member can be accessed at a time:

#include <iostream>
using namespace std;

union Data {
    int intValue;
    float floatValue;
    char charValue;
};

int main() {
    Data d;
    
    d.intValue = 10;
    cout << "Integer: " << d.intValue << endl;
    
    d.floatValue = 3.14;
    cout << "Float: " << d.floatValue << endl;
    // intValue is now overwritten
    
    d.charValue = 'A';
    cout << "Char: " << d.charValue << endl;
    // Both intValue and floatValue are overwritten
    
    return 0;
}

8. Structure vs Union

Summary

Frequently Asked Questions

Conclusion

Structures and unions are essential tools in C++ for organizing and managing related data. Structures allow you to group different data types together, creating meaningful abstractions that model real-world entities. Unions provide memory-efficient alternatives when only one member needs to be active at a time.

Understanding when to use structures versus unions, how to work with nested structures, and how to efficiently pass structures to functions are important skills. Structures form the foundation for classes in C++, and many concepts you learn with structures directly apply to object-oriented programming.

As you progress in C++, you'll find that structures are used extensively in system programming, data structures, and when interfacing with C code. Mastery of structures and unions enables you to write more organized, maintainable, and efficient C++ programs.

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