unit 3 opps

 

1️⃣ Relationships in OOP

In Object-Oriented Programming, objects and classes are connected through relationships.

Main types:

• Inheritance (IS-A)

• Association (Uses/Has-A)

• Aggregation (Has-A, weak form)

• Composition (strong Aggregation – optional mention)

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2️⃣ Inheritance

🔹 Definition

Inheritance is a mechanism by which one class acquires the properties and behavior of another class.

👉 It promotes code reusability.

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🔹 Purpose of Inheritance

✅ Code Reuse
✅ Extensibility
✅ Logical hierarchy
✅ Polymorphism support
✅ Reduces redundancy

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🔹 Syntax in C++

class Parent {
    // base class
};

class Child : public Parent {
    // derived class
};

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🔹 “IS-A” Relationship

Inheritance represents an IS-A relationship.

Example:

• Dog IS-A Animal

• Car IS-A Vehicle

So, Dog inherits from Animal.

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🔹 Example in C++

#include <iostream>
using namespace std;

class Animal {
public:
    void eat() {
        cout << "Animal is eating\n";
    }
};

class Dog : public Animal {
public:
    void bark() {
        cout << "Dog is barking\n";
    }
};

int main() {
    Dog d;
    d.eat();   // inherited
    d.bark();  // own function
}

Here:

• Dog IS-A Animal

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🔹 Types of Inheritance in C++

1️⃣ Single Inheritance

One base → one derived

class A {};
class B : public A {};

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2️⃣ Multiple Inheritance

One derived → multiple base

class A {};
class B {};
class C : public A, public B {};

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3️⃣ Multilevel Inheritance

A → B → C

class A {};
class B : public A {};
class C : public B {};

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4️⃣ Hierarchical Inheritance

One base → multiple derived

class A {};
class B : public A {};
class C : public A {};

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5️⃣ Hybrid Inheritance

Combination of multiple types.

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3️⃣ Association

🔹 Definition

Association represents a relationship where one class uses another class.

👉 It represents a USES-A or HAS-A relationship.

Example:

• Student uses Library

• Teacher uses Marker

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🔹 Example in C++

class Engine {
public:
    void start() {
        cout << "Engine started\n";
    }
};

class Car {
private:
    Engine e;   // Car HAS-A Engine (Association)

public:
    void startCar() {
        e.start();
    }
};

Here:
Car uses Engine.

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4️⃣ Aggregation

🔹 Definition

Aggregation is a special type of association representing a weak HAS-A relationship.

👉 The contained object can exist independently.

Example:

• Department has Teachers

• Teacher can exist without Department

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🔹 Example

class Teacher {
public:
    string name;
};

class Department {
public:
    Teacher *t;  // Aggregation
};

If Department is destroyed, Teacher can still exist.

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🔹 Difference: Association vs Aggregation

Feature	Association	Aggregation
Relationship	Uses	Has-A
Ownership	No ownership	Weak ownership
Life Cycle	Independent	Independent

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5️⃣ Abstract Classes

🔹 Definition

A class that contains at least one pure virtual function is called an abstract class.

👉 Cannot create object of abstract class.

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🔹 Syntax

class Shape {
public:
    virtual void area() = 0;   // pure virtual
};

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🔹 Example

class Shape {
public:
    virtual void area() = 0;
};

class Circle : public Shape {
public:
    void area() {
        cout << "Area of Circle\n";
    }
};

int main() {
    Circle c;
    c.area();
}

You cannot do:

Shape s;   // ERROR

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🔹 Purpose of Abstract Class

✅ To achieve abstraction
✅ To define common interface
✅ To support runtime polymorphism

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6️⃣ Interfaces in C++

C++ does not have a separate interface keyword like Java.

But we create interface using:

👉 Abstract class with all pure virtual functions.

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🔹 Example of Interface

class Printable {
public:
    virtual void print() = 0;
};

class Document : public Printable {
public:
    void print() {
        cout << "Printing document\n";
    }
};

Here:
Printable works like an interface.

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🔹 Difference: Abstract Class vs Interface (C++ Context)

Abstract Class	Interface
May contain normal functions	Only pure virtual
May contain variables	Typically no data
Partial abstraction	Full abstraction