37th BCS Written Test: 2017প্রশ্ন ও সমাধান

An object in C++ is a concrete instance of a class — it occupies actual memory and holds real values for the class's data members, while the class itself is only a blueprint/template with no memory until instantiated.

#include <iostream>
using namespace std;

class Car {
public:
    string model;
    int speed;

    void display() {
        cout << model << " running at " << speed << " km/h" << endl;
    }
};

int main() {
    Car c1;              // c1 is an object of class Car
    c1.model = "Sedan";
    c1.speed = 120;
    c1.display();

    Car c2;              // a second, independent object
    c2.model = "SUV";
    c2.speed = 100;
    c2.display();

    return 0;
}

Sample output:

Sedan running at 120 km/h
SUV running at 100 km/h

Key points:

• Each object has its own copy of data members (c1.speed is independent of c2.speed) but shares the class's member functions.
• Objects can be created on the stack (Car c1;) or the heap (Car *c3 = new Car();), the latter requiring delete to free memory.
• A class can have any number of objects, each with a distinct identity and state.

Exam takeaway: class = blueprint (no memory); object = instance of that blueprint (occupies memory, has state) — the same relationship as a "recipe" vs "the actual dish cooked from it."