Inheritance in C++

# CHAPTER 17

Inheritance in C++ #

1. Introduction #

2. Learning Objectives #

• Define Base (Parent) and Derived (Child) classes.

• Use the protected access modifier.

• Understand Single, Multilevel, and Multiple Inheritance.

• See how Constructors behave during inheritance.

3. Syntax of Inheritance #

#include <iostream>
using namespace std;

// Base Class (Parent)
class Animal {
  public:
    void eat() {
        cout << "I can eat!" << endl;
    }
};

// Derived Class (Child)
class Dog : public Animal {
  public:
    void bark() {
        cout << "Woof woof!" << endl;
    }
};

int main() {
    Dog myDog;
    myDog.bark(); // Its own method
    myDog.eat();  // Inherited method from Animal!
    
    return 0;
}

4. The protected Access Modifier #

class Entity {
  protected:
    int health; // Hidden from main(), but visible to children
};

class Player : public Entity {
  public:
    void takeDamage(int dmg) {
        health -= dmg; // Allowed! Because health is protected.
    }
};

5. Types of Inheritance #

1. Single Inheritance: One child inherits from one parent. (e.g., Dog inherits from Animal).

2. Multilevel Inheritance: A child inherits from a parent, and then another child inherits from that child.

class Vehicle {};
class Car : public Vehicle {};
class SportsCar : public Car {}; // Inherits from Car and Vehicle

3. Multiple Inheritance: A child inherits from *more than one* parent simultaneously. (C++ supports this, but Java/C# do not because it can get messy!).

class Printer {
  public: void print() { cout << "Printing..." << endl; }
};

class Scanner {
  public: void scan() { cout << "Scanning..." << endl; }
};

// Inherits from TWO parents!
class MultiFunctionPrinter : public Printer, public Scanner {}; 

int main() {
    MultiFunctionPrinter mfp;
    mfp.print();
    mfp.scan();
    return 0;
}

6. Constructors and Inheritance #

class Base {
  public:
    Base() { cout << "Base created\n"; }
    ~Base() { cout << "Base destroyed\n"; }
};

class Derived : public Base {
  public:
    Derived() { cout << "Derived created\n"; }
    ~Derived() { cout << "Derived destroyed\n"; }
};

int main() {
    Derived d;
    return 0;
}

Output:

Base created
Derived created
Derived destroyed
Base destroyed

7. Calling a Parameterized Base Constructor #

class Animal {
  public:
    Animal(int age) { /*...*/ }
};

class Dog : public Animal {
  public:
    // Calling the Base constructor in the initializer list
    Dog(int age) : Animal(age) {
        cout << "Dog created";
    }
};

8. Memory-Level Explanation #

9. Common Mistakes #

• Private Inheritance by Accident: If you forget public in class Dog : Animal, it defaults to private inheritance! This means eat() becomes private in Dog and cannot be called from main(). Always write class Dog : public Animal.

• The Diamond Problem: In multiple inheritance, if B and C inherit from A, and D inherits from both B and C, D gets two copies of A's data! (Solved using virtual inheritance).

10. Exercises #

1. 1. Create a base class Shape with protected width and height. Create a derived class Rectangle with a getArea() method.

1. 2. Demonstrate Multilevel inheritance by creating Grandparent, Parent, and Child classes.

11. MCQ Quiz with Answers #

12. Interview Questions #

• Q: Explain the "Diamond Problem" in Multiple Inheritance and how to solve it.

• Q: What is the difference between public, protected, and private inheritance?

• Q: Why do Java and C# not support Multiple Inheritance with classes, while C++ does?

13. Summary #

14. Next Chapter Recommendation #