Structures and Unions in C++

Structures and unions allow grouping variables of different data types under a single name. They are the foundation of custom data types in C++ and an important stepping stone toward understanding classes and objects.

Structures — struct

A structure bundles multiple variables (called members) of different types into one unit. Think of it as a record card — a student record can have a name (string), age (int), and marks (float) all in one place.

Defining a Structure:

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

Creating a Structure Variable:

Student s1;
s1.name  = "Arun";
s1.age   = 19;
s1.marks = 88.5;

Full Example:

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

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

int main() {
    Student s1;
    s1.name  = "Priya";
    s1.age   = 20;
    s1.marks = 92.5;

    cout << "Name:  " << s1.name  << endl;
    cout << "Age:   " << s1.age   << endl;
    cout << "Marks: " << s1.marks << endl;
    return 0;
}

Output:

Name:  Priya
Age:   20
Marks: 92.5

Initializing a Struct at Declaration

Student s2 = {"Rohan", 21, 78.0};

Array of Structures

Student batch[3] = {
    {"Alice", 19, 85.0},
    {"Bob",   20, 78.5},
    {"Carol", 19, 91.0}
};

for (int i = 0; i < 3; i++) {
    cout << batch[i].name << " - " << batch[i].marks << endl;
}

Output:

Alice - 85
Bob - 78.5
Carol - 91

Passing Structures to Functions

void printStudent(Student s) {
    cout << s.name << " (Age: " << s.age << ")" << endl;
}

int main() {
    Student s = {"Kiran", 22, 80.0};
    printStudent(s);
    return 0;
}

Output:

Kiran (Age: 22)

Nested Structures

A structure can contain another structure as a member:

struct Address {
    string city;
    string state;
};

struct Person {
    string name;
    int age;
    Address addr;    // nested structure
};

int main() {
    Person p;
    p.name      = "Meera";
    p.age       = 25;
    p.addr.city  = "Pune";
    p.addr.state = "Maharashtra";

    cout << p.name << " lives in " << p.addr.city << endl;
    return 0;
}

Output:

Meera lives in Pune

Unions

A union is similar to a structure, but all members share the same memory location. Only one member can hold a value at a time. The size of a union equals the size of its largest member.

union Data {
    int i;
    float f;
    char c;
};

#include <iostream>
using namespace std;

union Data {
    int i;
    float f;
};

int main() {
    Data d;
    d.i = 10;
    cout << "int:   " << d.i << endl;

    d.f = 3.14;
    cout << "float: " << d.f << endl;
    cout << "int now (corrupted): " << d.i << endl;  // i is now overwritten
    return 0;
}

Output:

int:   10
float: 3.14
int now (corrupted): 1078523331

Struct vs Union

Key Takeaways

• A struct groups multiple variables of different types into one unit.
• Members of a struct are accessed using the dot operator ..
• Structures can be nested, placed in arrays, and passed to functions.
• A union allows all members to share the same memory — only one member is valid at a time.
• Structures are the foundation of OOP concepts like classes in C++.