Arrays Within a Class & Memory Allocation for Objects

1. Arrays as Member Data Within a Class (In Simple Words)

In C++, variables inside a class don’t just have to be single numbers like int rollNo;.

You can also create Arrays inside a class! This is helpful when an object needs to store a collection of related items — like a student’s marks in 5 subjects, or a supermarket shopping cart storing item codes and prices.

#include <iostream>
using namespace std;

const int MAX_ITEMS = 50;

class ShoppingCart {
    private:
        int itemCode[MAX_ITEMS];    // Array to store product IDs
        float itemPrice[MAX_ITEMS]; // Array to store product prices
        int itemCount;              // Keeps track of how many items are added

    public:
        void initialize() { 
            itemCount = 0; 
        }
        
        void addItem(int code, float price) {
            itemCode[itemCount] = code;
            itemPrice[itemCount] = price;
            itemCount++;
        }

        void displayCart() {
            float grandTotal = 0.0;
            cout << "\n--- Supermarket Shopping Cart ---" << endl;
            for (int i = 0; i < itemCount; i++) {
                cout << "Item #" << itemCode[i] << " : Rs. " << itemPrice[i] << endl;
                grandTotal += itemPrice[i];
            }
            cout << "Grand Total: Rs. " << grandTotal << endl;
        }
};

int main() {
    ShoppingCart cart;
    cart.initialize();
    cart.addItem(101, 250.0);
    cart.addItem(102, 120.5);
    cart.addItem(103, 89.0);
    cart.displayCart();

    return 0;
}

2. How Does C++ Allocate Memory for Objects?

A very common myth among students is:
“If I create 1,000 objects of a class, are the functions also copied 1,000 times in RAM?”
NO! C++ is much smarter than that.

┌─────────────────────────────────────────────────────────────┐
│                 SHARED CODE MEMORY SEGMENT                  │
│                                                             │
│   Member Functions (Stored only 1 time in RAM):             │
│   • void Student::getDetails()                              │
│   • void Student::display()                                 │
└──────────────────────────────┬──────────────────────────────┘
                               │ (Shared by all student objects)
        ┌──────────────────────┴──────────────────────┐
        ▼                                             ▼
┌───────────────────────────┐             ┌───────────────────────────┐
│     OBJECT 1 (s1 in RAM)  │             │     OBJECT 2 (s2 in RAM)  │
├───────────────────────────┤             ├───────────────────────────┤
│ rollNo : 101              │             │ rollNo : 102              │
│ marks  : 85.5             │             │ marks  : 92.0             │
│ (Takes separate 8 bytes)  │             │ (Takes separate 8 bytes)  │
└───────────────────────────┘             └───────────────────────────┘

Why is this design so clever?

1. Saves Massive RAM: If your class has 20 functions taking 100 KB of compiled code, creating 10,000 objects won’t waste 1 GB of memory! The 100 KB code is stored only once.
2. How does the shared function know which object called it?
   Whenever you call s1.display(), C++ secretly passes a hidden pointer called the this pointer (&s1). This tells the shared function: “Hey, read the data of s1, not s2!”

DAVV Exam Preparation Tips

Frequently Asked 5-Mark Questions:

1. How is memory allocated for objects and member functions in C++?
   Model Answer:
   • Member functions are allocated only once in the shared Code Segment of RAM.
   • Data members (variables) get separate memory for every object created on the Stack or Heap.
   • Draw the shared code vs. separate objects diagram.
2. Write a C++ class with an array member to calculate the average of 5 subjects.
   Model Answer:
   • Class Student with float marks[5].
   • Function to input marks using a for loop, calculate total sum, and print sum / 5.0.

Frequently Asked 10-Mark Questions:

1. Explain memory allocation for classes and objects in C++. How do multiple objects share member functions without confusing their data? Write a complete inventory/shopping list program.
   Model Answer:
   • Explain memory layout: Code Segment (shared functions) vs. Stack/Heap (separate object variables).
   • Explain the role of the hidden this pointer in distinguishing between calling objects.
   • Write out the complete ShoppingCart class with arrays for item codes and prices, along with add and display functions.