Object-Oriented Programming in C++ – The 4 Pillars Explained

Object-Oriented Programming (OOP) is a programming paradigm that uses “objects” – data structures consisting of fields and methods – to design applications and programs. C++ is one of the most popular languages that supports OOP.

There are 4 pillars of OOP:

1. Encapsulation
2. Abstraction
3. Inheritance
4. Polymorphism

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1️⃣ Encapsulation

Definition: Encapsulation is the practice of binding data and the methods that operate on that data into a single unit (class) and restricting direct access to some of the object’s components.

Why Encapsulation?

• It helps keep data safe from outside interference and misuse.
• Ensures better control over class data.

Key Concepts:

• Private members: Not accessible from outside the class.
• Public methods (getters/setters): Used to access and modify private members.

Example:

#include <iostream>
using namespace std;

class BankAccount {
private:
    int balance; // Encapsulated (private)

public:
    BankAccount() {
        balance = 0;
    }

    void deposit(int amount) {
        if (amount > 0)
            balance += amount;
    }

    void withdraw(int amount) {
        if (amount > 0 && balance >= amount)
            balance -= amount;
    }

    int getBalance() {
        return balance;
    }
};

int main() {
    BankAccount acc;
    acc.deposit(500);
    acc.withdraw(100);
    cout << "Current Balance: " << acc.getBalance() << endl;
    return 0;
}

Summary:

Encapsulation = Data hiding + Access control using public methods.

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2️⃣ Abstraction

Definition: Abstraction means showing only the essential features of an object while hiding the unnecessary details.

Why Abstraction?

• It simplifies complex systems by breaking them into smaller, manageable parts.
• Focuses on what an object does instead of how it does it.

Key Concepts:

• Achieved through abstract classes and interfaces (pure virtual functions in C++).
• Pure virtual functions are defined using = 0.

Example:

#include <iostream>
using namespace std;

class Shape {
public:
    virtual void draw() = 0; // Pure virtual function
};

class Circle : public Shape {
public:
    void draw() override {
        cout << "Drawing Circle" << endl;
    }
};

class Rectangle : public Shape {
public:
    void draw() override {
        cout << "Drawing Rectangle" << endl;
    }
};

int main() {
    Shape* s1 = new Circle();
    Shape* s2 = new Rectangle();

    s1->draw();
    s2->draw();

    delete s1;
    delete s2;
    return 0;
}

Summary:

Abstraction = Hiding implementation + Showing interface.

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3️⃣ Inheritance

Definition: Inheritance allows a class (child or derived class) to inherit properties and behavior (methods) from another class (parent or base class).

Why Inheritance?

• Promotes code reusability.
• Establishes an “is-a” relationship.

Types of Inheritance in C++:

• Single
• Multiple
• Multilevel
• Hierarchical
• Hybrid

Example (Single Inheritance):

#include <iostream>
using namespace std;

class Animal {
public:
    void eat() {
        cout << "This animal eats food." << endl;
    }
};

class Dog : public Animal {
public:
    void bark() {
        cout << "The dog barks." << endl;
    }
};

int main() {
    Dog myDog;
    myDog.eat();  // Inherited
    myDog.bark(); // Own function
    return 0;
}

Example (Multiple Inheritance):

class A {
public:
    void showA() {
        cout << "Class A" << endl;
    }
};

class B {
public:
    void showB() {
        cout << "Class B" << endl;
    }
};

class C : public A, public B {
    // Inherits both A and B
};

int main() {
    C obj;
    obj.showA();
    obj.showB();
    return 0;
}

Summary:

Inheritance = Reuse existing code + Establish relationships between classes.

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4️⃣ Polymorphism

Definition: Polymorphism allows functions or methods to behave differently based on the object or data they are operating on. Literally, it means “many forms”.

Why Polymorphism?

• Increases flexibility and scalability.
• Reduces code duplication.

Types of Polymorphism:

• Compile-time (Static) Polymorphism
  • Function Overloading
  • Operator Overloading
• Run-time (Dynamic) Polymorphism
  • Function Overriding using virtual functions

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Compile-time Polymorphism

Function Overloading:

class Print {
public:
    void show(int i) {
        cout << "Integer: " << i << endl;
    }

    void show(string s) {
        cout << "String: " << s << endl;
    }
};

int main() {
    Print obj;
    obj.show(100);
    obj.show("Hello");
    return 0;
}

Operator Overloading:

class Complex {
private:
    int real, imag;
public:
    Complex(int r, int i) : real(r), imag(i) {}

    Complex operator + (const Complex& obj) {
        return Complex(real + obj.real, imag + obj.imag);
    }

    void display() {
        cout << real << " + " << imag << "i" << endl;
    }
};

int main() {
    Complex c1(3, 4), c2(1, 2);
    Complex c3 = c1 + c2;
    c3.display();
    return 0;
}

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Run-time Polymorphism

Function Overriding with Virtual Functions:

class Animal {
public:
    virtual void sound() {
        cout << "Some generic animal sound" << endl;
    }
};

class Cat : public Animal {
public:
    void sound() override {
        cout << "Meow" << endl;
    }
};

int main() {
    Animal* a = new Cat();
    a->sound();  // Calls Cat's version due to virtual function
    delete a;
    return 0;
}

Summary:

Polymorphism = One interface, many implementations.

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The Diamond Problem in C++

What is the Diamond Problem?

The Diamond Problem is a classic ambiguity issue in multiple inheritance, where a derived class inherits from two classes that have a common base class.

Structure of the Diamond:

    A
   / \
  B   C
   \ /
    D

• Class B and class C inherit from class A.
• Class D inherits from both B and C.

The Problem:

When class D tries to access a member of class A, C++ compiler doesn’t know whether to use the A part of B or the A part of C, because both contain their own copies of A.

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Example of the Diamond Problem

#include <iostream>
using namespace std;

class A {
public:
    void display() {
        cout << "Class A" << endl;
    }
};

class B : public A { };

class C : public A { };

class D : public B, public C { };

int main() {
    D obj;
    // obj.display(); //  Error: Ambiguous!
    obj.B::display(); //  Okay: Access through B's A
    obj.C::display(); //  Okay: Access through C's A
    return 0;
}

Explanation:

• Class D has two copies of class A (one from B, one from C).
• If you call obj.display(), the compiler gets confused — which display() function to call?
• This is the Diamond Problem.

To explore the diamond problem and its solution in more detail, do consider reading post: The Diamond Problem in C++ Explained with Real Examples

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Solution: Virtual Inheritance

C++ allows us to solve this using virtual inheritance. This tells the compiler to share a single copy of the base class when it appears multiple times in an inheritance hierarchy.

Modified Example Using Virtual Inheritance:

#include <iostream>
using namespace std;

class A {
public:
    void display() {
        cout << "Class A (via virtual inheritance)" << endl;
    }
};

class B : virtual public A { };

class C : virtual public A { };

class D : public B, public C { };

int main() {
    D obj;
    obj.display(); //  No ambiguity now
    return 0;
}

What Changed:

• We added virtual before public A in B and C.
• Now, class D contains only one shared instance of class A.
• No ambiguity – problem solved!

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Virtual Functions vs Virtual Inheritance

These terms may sound similar, but they solve different problems:

Concept	Purpose	Usage Context
Virtual Functions	Enable dynamic (runtime) polymorphism	Function overriding
Virtual Inheritance	Avoid multiple copies of a base class (diamond problem)	Multiple inheritance

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Virtual Functions Recap

Virtual functions ensure the correct function is called for an object, regardless of the reference type, when using base class pointers or references.

Example:

class Animal {
public:
    virtual void sound() {
        cout << "Generic animal sound" << endl;
    }
};

class Dog : public Animal {
public:
    void sound() override {
        cout << "Bark" << endl;
    }
};

int main() {
    Animal* a = new Dog();
    a->sound();  // Calls Dog's sound(), not Animal's
    delete a;
    return 0;
}

Why use virtual?

Without it, a->sound() would call Animal’s version instead of Dog’s.

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Summary

• The Diamond Problem happens due to ambiguous inheritance paths in multiple inheritance.
• Use virtual inheritance to ensure only one instance of the base class exists.
• Virtual functions provide runtime polymorphism, letting you override functions dynamically.
• Both features are powerful tools in C++, each solving a different class of problem.

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Final Thoughts

Pillar	Purpose	Achieved via
Encapsulation	Hide data, protect from misuse	Access specifiers + Getters/Setters
Abstraction	Show essential, hide details	Abstract classes, interfaces
Inheritance	Reuse code	Derived classes from base classes
Polymorphism	One name, many forms	Overloading, Overriding, Virtual func

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Programming Tasks

1. Encapsulation Task:
   Write a class Student with private members: name, rollNumber, and GPA. Add public methods to set and get each of these values. Include validation for GPA (should be between 0.0 and 4.0).
2. Abstract Shape Class:
   Create an abstract class Shape with a pure virtual function area(). Derive classes Rectangle and Circle from it, and override the area() function appropriately. Test the behavior using base class pointers.
3. Polymorphism via Virtual Functions:
   Design a class Employee with a virtual function getSalary(). Derive classes Manager and Engineer from it, each returning different salary values. Demonstrate runtime polymorphism using base class pointers.
4. Diamond Problem Handling:
   Implement a class hierarchy that causes the diamond problem, and then resolve it using virtual inheritance. Demonstrate that there is only one instance of the base class in the final derived class.