Struct: Creating Your Own Data Types

You already know how to use arrays and vectors to store lots of data. But there is a big problem: arrays can only store one data type. What if you want to store a student’s name (string), age (int), and grade (double) together as a single unit?

Well, this is where struct comes to the rescue!

Analogy: Student Registration Form

Imagine you are asked to fill out a school registration form. The form has:

Name  : _______________
Age   : ___
Class : ___
Grade : ___

This form groups several different pieces of data into one complete sheet. You don’t write the name on one piece of paper, the age on another, and the grade on yet another. Everything is one package.

In C++, this form is a struct! A struct is how we create a custom data type that can hold multiple fields at once.

Other analogies:

• ID card = a struct containing name, ID number, address, date of birth
• Business card = a struct containing name, title, phone number, email
• Receipt = a struct containing item name, price, quantity

The Problem Without Struct

Suppose you want to store data for 3 students: their names, ages, and grades. Without struct, you would need separate arrays:

#include <iostream>
#include <string>

int main() {
    // Data scattered across separate arrays... messy!
    std::string names[3] = {"Budi", "Ani", "Citra"};
    int ages[3] = {15, 16, 15};
    double grades[3] = {85.5, 92.0, 78.3};

    // To display student 0's data
    std::cout << "Name : " << names[0] << std::endl;
    std::cout << "Age  : " << ages[0] << std::endl;
    std::cout << "Grade: " << grades[0] << std::endl;

    return 0;
}

Output:

Name : Budi
Age  : 15
Grade: 85.5

It looks fine, but there is a big problem:

• The data is not linked together. You have to remember that names[0], ages[0], and grades[0] belong to the same person.
• If you want to sort by grade, you have to move all three arrays at once. Very error-prone!
• Imagine if the data had 10 fields… would you need 10 separate arrays?

The Solution: Struct!

With struct, all related data is combined into one:

#include <iostream>
#include <string>

struct Student {
    std::string name;
    int age;
    double grade;
};

int main() {
    Student s1;
    s1.name = "Budi";
    s1.age = 15;
    s1.grade = 85.5;

    std::cout << "Name : " << s1.name << std::endl;
    std::cout << "Age  : " << s1.age << std::endl;
    std::cout << "Grade: " << s1.grade << std::endl;

    return 0;
}

Output:

Name : Budi
Age  : 15
Grade: 85.5

Now Budi’s data is one package in the variable s1. Clean and impossible to mix up!

Struct Syntax

Here is how to declare a struct:

struct StructName {
    data_type field1;
    data_type field2;
    data_type field3;
    // ... you can add as many as you want
};  // <-- DON'T FORGET the semicolon here!

Full example:

struct Student {
    std::string name;
    int age;
    double grade;
};

Creating Struct Variables and Accessing Fields

After a struct is defined, you can create variables of that type:

#include <iostream>
#include <string>

struct Product {
    std::string name;
    int price;
    int stock;
};

int main() {
    // Create a struct variable
    Product item1;

    // Fill in fields with the dot operator (.)
    item1.name = "2B Pencil";
    item1.price = 3000;
    item1.stock = 50;

    // Read fields
    std::cout << "Product: " << item1.name << std::endl;
    std::cout << "Price  : Rp " << item1.price << std::endl;
    std::cout << "Stock  : " << item1.stock << " units" << std::endl;

    // Modify a field
    item1.stock = item1.stock - 1;
    std::cout << "Stock after selling 1: " << item1.stock << " units" << std::endl;

    return 0;
}

Output:

Product: 2B Pencil
Price  : Rp 3000
Stock  : 50 units
Stock after selling 1: 49 units

Direct Initialization

Besides filling in fields one by one, you can initialize them all at once when creating the variable:

#include <iostream>
#include <string>

struct Book {
    std::string title;
    std::string author;
    int year;
    double price;
};

int main() {
    // Direct initialization — order matches the fields in the struct
    Book book1 = {"Laskar Pelangi", "Andrea Hirata", 2005, 79000.0};

    // Modern C++: designated initializers (C++20)
    // Book book2 = {.title = "Bumi", .author = "Tere Liye", .year = 2014, .price = 89000.0};

    // Initialization with empty braces = all fields get default values
    Book book3 = {};  // name = "", year = 0, price = 0.0

    std::cout << "Title : " << book1.title << std::endl;
    std::cout << "Author: " << book1.author << std::endl;
    std::cout << "Year  : " << book1.year << std::endl;
    std::cout << "Price : Rp " << book1.price << std::endl;

    return 0;
}

Output:

Title : Laskar Pelangi
Author: Andrea Hirata
Year  : 2005
Price : Rp 79000

Array of Struct

You can create an array whose elements are structs! This is like having a stack of student data forms.

#include <iostream>
#include <string>

struct Student {
    std::string name;
    int age;
    double grade;
};

int main() {
    // Array of struct
    Student classroom[3] = {
        {"Budi", 15, 85.5},
        {"Ani", 16, 92.0},
        {"Citra", 15, 78.3}
    };

    std::cout << "=== Student Data ===" << std::endl;
    for (int i = 0; i < 3; i++) {
        std::cout << "Student " << (i + 1) << ": "
                  << classroom[i].name << " (age "
                  << classroom[i].age << "), grade: "
                  << classroom[i].grade << std::endl;
    }

    return 0;
}

Output:

=== Student Data ===
Student 1: Budi (age 15), grade: 85.5
Student 2: Ani (age 16), grade: 92.0
Student 3: Citra (age 15), grade: 78.3

Vector of Struct (Dynamic Size)

If you don’t know how many entries will be added, use std::vector so the size can grow:

#include <iostream>
#include <string>
#include <vector>

struct Employee {
    std::string name;
    std::string position;
    int salary;
};

int main() {
    std::vector<Employee> team;

    // Add data using push_back
    team.push_back({"Andi", "Programmer", 8000000});
    team.push_back({"Budi", "Designer", 7500000});
    team.push_back({"Citra", "Manager", 12000000});

    std::cout << "=== Team Data ===" << std::endl;
    for (int i = 0; i < team.size(); i++) {
        std::cout << team[i].name << " - "
                  << team[i].position << " - Rp "
                  << team[i].salary << std::endl;
    }

    std::cout << std::endl;
    std::cout << "Number of team members: " << team.size() << std::endl;

    return 0;
}

Output:

=== Team Data ===
Andi - Programmer - Rp 8000000
Budi - Designer - Rp 7500000
Citra - Manager - Rp 12000000

Number of team members: 3

Type Alias with using

Sometimes struct names can get long. You can create an alias with the using keyword:

#include <iostream>
#include <string>
#include <vector>

struct StudentGradeData {
    std::string name;
    double midtermGrade;
    double finalGrade;
    double assignmentGrade;
};

// Create an alias for a shorter name
using Grade = StudentGradeData;

int main() {
    // Now you can use 'Grade' instead of 'StudentGradeData'
    Grade n1 = {"Budi", 80.0, 85.0, 90.0};

    double average = (n1.midtermGrade + n1.finalGrade + n1.assignmentGrade) / 3.0;

    std::cout << "Name      : " << n1.name << std::endl;
    std::cout << "Midterm   : " << n1.midtermGrade << std::endl;
    std::cout << "Final     : " << n1.finalGrade << std::endl;
    std::cout << "Assignment: " << n1.assignmentGrade << std::endl;
    std::cout << "Average   : " << average << std::endl;

    return 0;
}

Output:

Name      : Budi
Midterm   : 80
Final     : 85
Assignment: 90
Average   : 85

Practical Example: Library Data

Let’s build a more complete example — a mini library book data system:

#include <iostream>
#include <string>
#include <vector>

struct Book {
    std::string title;
    std::string author;
    int year;
    bool available;
};

int main() {
    std::vector<Book> library = {
        {"Laskar Pelangi", "Andrea Hirata", 2005, true},
        {"Bumi", "Tere Liye", 2014, false},
        {"Filosofi Teras", "Henry Manampiring", 2018, true},
        {"Pulang", "Tere Liye", 2015, true},
        {"Negeri 5 Menara", "Ahmad Fuadi", 2009, false}
    };

    std::cout << "=== LIBRARY CATALOG ===" << std::endl;
    std::cout << std::endl;

    for (int i = 0; i < library.size(); i++) {
        std::cout << (i + 1) << ". " << library[i].title << std::endl;
        std::cout << "   Author : " << library[i].author << std::endl;
        std::cout << "   Year   : " << library[i].year << std::endl;
        std::cout << "   Status : "
                  << (library[i].available ? "Available" : "Borrowed")
                  << std::endl;
        std::cout << std::endl;
    }

    // Count available books
    int availableCount = 0;
    for (int i = 0; i < library.size(); i++) {
        if (library[i].available) {
            availableCount++;
        }
    }

    std::cout << "Total books: " << library.size() << std::endl;
    std::cout << "Books available: " << availableCount << std::endl;

    return 0;
}

Output:

=== LIBRARY CATALOG ===

1. Laskar Pelangi
   Author : Andrea Hirata
   Year   : 2005
   Status : Available

2. Bumi
   Author : Tere Liye
   Year   : 2014
   Status : Borrowed

3. Filosofi Teras
   Author : Henry Manampiring
   Year   : 2018
   Status : Available

4. Pulang
   Author : Tere Liye
   Year   : 2015
   Status : Available

5. Negeri 5 Menara
   Author : Ahmad Fuadi
   Year   : 2009
   Status : Borrowed

Total books: 5
Books available: 3

Common Mistakes

1. Forgetting the semicolon after the struct

// WRONG — missing ; at the end!
struct Student {
    std::string name;
    int age;
}  // <-- Error! Must have ;

// CORRECT
struct Student {
    std::string name;
    int age;
};  // <-- Semicolon is required!

2. Initialization order does not match

struct Student {
    std::string name;
    int age;
    double grade;
};

// WRONG — reversed order, age and name are swapped
// Student s = {15, "Budi", 85.5};  // Error!

// CORRECT — order matches: name, age, grade
Student s = {"Budi", 15, 85.5};

3. Accessing fields without the dot operator

Student s1 = {"Budi", 15, 85.5};

// WRONG
// std::cout << s1 << std::endl;  // Structs can't be printed directly with cout!

// CORRECT — access each field individually
std::cout << s1.name << std::endl;
std::cout << s1.age << std::endl;

4. Defining a struct inside main

// NOT IDEAL — a struct inside main can only be used within main
int main() {
    struct Student {
        std::string name;
    };
}

// BETTER — define the struct outside main (global)
struct Student {
    std::string name;
};

int main() {
    Student s;
}

Exercises

Exercise 1: Create a struct Contact that stores a name, phone number, and email. Create 3 Contact variables and display their data.

Example output:

=== Contact List ===
1. Andi  | 081234567890 | andi@email.com
2. Budi  | 089876543210 | budi@email.com
3. Citra | 082111222333 | citra@email.com

Exercise 2: Create a struct Product with fields for name, price, and stock. Insert 5 products into a std::vector<Product>, then display all products and calculate the total inventory value (price x stock for each product, then summed up).

Exercise 3: Build a simple student registration program. The Student struct contains name, class, and average grade. Use a while loop to ask for user input (type “done” to stop), store entries in a std::vector<Student>, then display all data and find the student with the highest grade.

Summary

Struct is an important foundation toward Object-Oriented Programming that you will learn later. In the next lesson, we will learn how to combine structs with functions to create more well-structured programs!