BKSP Assistant Programmer Exam: 03.12.2022প্রশ্ন ও সমাধান

Direct answer: A class is a user-defined blueprint/template that groups data members (attributes) and member functions (behavior); an object is a concrete instance of that class, occupying actual memory with its own copy of the data members.

#include <iostream>
using namespace std;

class Box {                       // class = blueprint
public:
    double length, breadth, height;
    double volume() {
        return length * breadth * height;
    }
};

int main() {
    Box b1;                       // b1 = object (instance of Box)
    b1.length = 5; b1.breadth = 3; b1.height = 2;
    cout << "Volume: " << b1.volume() << endl;   // Volume: 30
    return 0;
}

classDiagram
    class Box {
        +double length
        +double breadth
        +double height
        +volume() double
    }

Key distinction:

• The class Box itself occupies no memory for data until an object is created.
• b1 is an object — it has real memory holding actual length/breadth/height values, and can call volume() on those specific values.
• Multiple objects (b1, b2, ...) can be created from the same class, each with independent data.

Exam takeaway: Class = definition/type (no memory for its own data); Object = instance of that class (has real memory and state) — "a class is to an object what a blueprint is to a house."