• Class: a user-defined blueprint/template defining data members (attributes) and member functions (behavior)
• Object: a concrete instance of a class, occupying its own memory for data members

Note: struct defaults to public access; class defaults to private — a classic MCQ trap.

class Rectangle
{
private:
    double length, width;
public:
    Rectangle(double l, double w) : length(l), width(w) {}
    double area() { return length * width; }
};

int main()
{
    Rectangle r(5, 3);   // r is an OBJECT -- an instance of the Rectangle CLASS
    cout << r.area();    // 15
}

Class vs Object — the blueprint vs the house

A class is a template; it occupies no memory for data on its own (memory is only reserved when an object is created). An object is a real instance built from that template, occupying its own dedicated memory.

class Rectangle           // BLUEPRINT -- defines what a rectangle looks like, no memory for actual data yet
{
private:
    double length, width;
public:
    Rectangle(double l, double w) : length(l), width(w) {}
    double area() { return length * width; }
};

int main()
{
    Rectangle r1(5, 3);   // OBJECT #1 -- its own length/width in memory
    Rectangle r2(10, 2);  // OBJECT #2 -- separate length/width, doesn't affect r1

    cout << r1.area();    // 15
    cout << r2.area();    // 20 -- independent of r1
}

Multiple objects, one class

Each object gets its own copy of instance data members but shares the same member function code (there's only one area() function in memory, used by every object).

How memory works, briefly

When Rectangle r1(5, 3); runs:

1. Memory is allocated for r1's length and width
2. The constructor initializes them to 5 and 3
3. r1.area() reads r1's own length/width — not some shared or global value

Important MCQs

1. A class, by itself (before any object is created), occupies memory for:
   • a) All its data members b) No data — memory is allocated only when an object is instantiated c) Only static members d) Only private members
2. In C++, the default access specifier for members of a class (not struct) is:
   • a) public b) protected c) private d) none/undefined
3. struct in C++ defaults its members to:
   • a) private b) protected c) public d) static
4. If Rectangle r1(5,3) and Rectangle r2(10,2) are both created, changing r1's data:
   • a) Also changes r2 b) Has no effect on r2 — each object has its own copy of instance data c) Causes a compile error d) Deletes r2