Structs (Records)

1. What is a Struct?

So far, if we wanted to store a student's information (for example, their name, their age, and their grade), we needed three separate variables:

string name = "Frieren";
int age = 1250;
double grade = 10.0;

This works for a single person, but it becomes chaotic if we have to manage many students and pass this data between functions.

A struct (or record) allows us to create our own "data type" by grouping different variables inside it, even if they are of different types.

The ID Card or Medical Record Analogy: Imagine a struct as a medical record, an ID card, or a phone contact. It is a single entity (the contact) that contains different sections or fields inside (name, phone number, email).

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2. How to define a Struct

The definition of a struct must always be placed at the very top of the file, before the functions and main(), so that the entire program knows about it.

#include <iostream>
using namespace std;

// We define our new data type
struct Student {
    string name;
    int age;
    double grade;
}; // IMPORTANT: DO NOT forget THIS semicolon! It is mandatory.

int main() {
    // ...
}

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3. How to declare and access a Struct

Once we have created the "blueprint" (the struct), we can create variables of this new type inside main() or in functions.

To access the different fields inside the struct, we use a dot (.).

int main() {
    // We create a variable of type 'Student' named 'student1'
    Student student1;

    // We fill in the data by accessing the fields with a dot
    student1.name = "Megumin";
    student1.age = 14;
    student1.grade = 9.2;

    // We can read the data the exact same way
    cout << "The student " << student1.name << " has a " << student1.grade << endl;
}

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4. Reading Structs from the terminal

We can also use cin to directly fill in the fields of a struct. We just have to remember to do it field by field:

int main() {
    Student student2;

    cout << "Enter the student's name, age and grade:" << endl;
    cin >> student2.name >> student2.age >> student2.grade;

    cout << "Data saved successfully!" << endl;
}

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5. Structs and Functions (Very Important)

structs can contain a lot of information (imagine a struct that stores all the data of a video game or a student's entire academic record).

If we pass a struct to a function by value, C++ will make an exact copy of all its variables every time we call the function, wasting memory unnecessarily and making the program run extremely slowly. Therefore, we apply a golden rule: we always pass them by reference.

1. Normal pass by reference (To modify)

If we want the function to modify any data within the struct, we use the &.

// This function adds one point to the student's grade
void applyBonus(Student& s) {
    s.grade = s.grade + 1.0;
    if (s.grade > 10.0) {
        s.grade = 10.0;
    }
}

2. Constant pass by reference (Read-only)

If we only want to read the data to print it or do calculations, but we do not want to modify it, we use const and &. This is the most efficient and secure way, as we avoid making copies while preventing the function from mistakenly modifying the original data.

// This function only reads the data and prints it
void printStudent(const Student& s) {
    cout << "--- STUDENT INFO ---" << endl;
    cout << "Name: " << s.name << endl;
    cout << "Age: " << s.age << endl;
    cout << "Grade: " << s.grade << endl;
}

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6. Final Exercise

Below you can see a complete example of how to use structs alongside functions by applying pass by reference and limit control.

final.cpp

/*
 * Final Exercise
 * Example: Adventurer's Status Screen With Structs and Functions (With Limits).
 * Made by: aso
 */

#include <iostream>
using namespace std;

// We define the struct at the very top so the functions can recognize it
struct Adventurer {
    string name;
    int level;
    double health;
};

// pre: true
// post: reads the data from the terminal and stores it in the struct,
// ensuring the values make sense.
void readAdventurer(Adventurer& a) {
    cout << "Enter the adventurer's name, level (1-100) and health (max 9999):" << endl;
    cout << "Example: Frieren 99 1500.5" << endl;

    cin >> a.name >> a.level >> a.health;

    // Error handling: we repeat the read process until the data is valid
    while (a.level < 1 || a.level > 100 || a.health <= 0 || a.health > 9999.0) {
        cout << "Error: Invalid stats for " << a.name << ". Level must be 1-100 and Health 1-9999." << endl;
        cout << "Please enter the data again:" << endl;
        cin >> a.name >> a.level >> a.health;
    }
}

// pre: true
// post: prints the adventurer's current status
void printAdventurer(const Adventurer& a) {
    cout << "--- ADVENTURER STATUS ---" << endl;
    cout << "Name:    " << a.name << endl;
    cout << "Level:   " << a.level << endl;
    cout << "Health: " << a.health << " HP" << endl;
}

// pre: true
// post: increases the adventurer's level by 1 and boosts their health, respecting the limits
void levelUp(Adventurer& a) {
    if (a.level >= 100) {
        cout << ">> " << a.name << " is already at maximum level! <<" << endl;
    } else {
        a.level++; // Level up by 1
        a.health = a.health + 50.0; // Give 50 extra health points

        // We make sure not to exceed the maximum health due to the bonus
        if (a.health > 9999.0) {
            a.health = 9999.0;
        }

        cout << ">> " << a.name << " leveled up to level " << a.level << "! <<" << endl;
    }
}

int main() {
    Adventurer hero; // We declare our variable of the new struct type

    // 1. Fill in the data (Modification) with the limit filter active
    readAdventurer(hero);
    cout << endl;

    // 2. Print the status screen (Read-only)
    printAdventurer(hero);
    cout << endl;

    // 3. Level up the character (Modification)
    levelUp(hero);
    cout << endl;

    // 4. Print again to check the changes (Read-only)
    printAdventurer(hero);
}