Why inheritance is used in C++ - See It in Action

Why inheritance is used

📖 Scenario: Imagine you are creating a program for a vehicle rental company. They have different types of vehicles like cars and trucks. Many vehicles share common features like having wheels and an engine, but some have special features too.

🎯 Goal: You will create a simple example using inheritance in C++ to show how common features can be shared and special features can be added. This will help you understand why inheritance is used.

📋 What You'll Learn

Create a base class called Vehicle with a function startEngine()

Create a derived class called Car that inherits from Vehicle

Add a function openTrunk() to the Car class

Create an object of Car and call both startEngine() and openTrunk()

💡 Why This Matters

🌍 Real World

Inheritance is used in software to model real-world relationships, like different types of vehicles sharing common features.

💼 Career

Understanding inheritance is key for software developers to write clean, reusable, and organized code in many programming languages.

1

Create the base class Vehicle

Write a class called Vehicle with a public function startEngine() that prints "Engine started".

C++

# Your code here

Hint

Use class Vehicle { public: void startEngine() { ... } }; and std::cout to print.

2

Create the derived class Car

Create a class called Car that inherits from Vehicle. Add a public function openTrunk() that prints "Trunk opened".

C++

class Vehicle {
public:
    void startEngine() {
        std::cout << "Engine started" << std::endl;
    }
};

# Your code here

Hint

Use class Car : public Vehicle and add void openTrunk() inside.

3

Create a Car object and use its functions

Create an object called myCar of type Car. Call myCar.startEngine() and myCar.openTrunk().

C++

class Vehicle {
public:
    void startEngine() {
        std::cout << "Engine started" << std::endl;
    }
};

class Car : public Vehicle {
public:
    void openTrunk() {
        std::cout << "Trunk opened" << std::endl;
    }
};

# Your code here

Hint

Inside main(), create Car myCar; and call the two functions.

4

Run the program and see the output

Run the program and print the output of calling myCar.startEngine() and myCar.openTrunk().

C++

#include 

class Vehicle {
public:
    void startEngine() {
        std::cout << "Engine started" << std::endl;
    }
};

class Car : public Vehicle {
public:
    void openTrunk() {
        std::cout << "Trunk opened" << std::endl;
    }
};

int main() {
    Car myCar;
    myCar.startEngine();
    myCar.openTrunk();
    return 0;
}

# Your code here

Hint

Check the console output shows both lines exactly.

Practice

1. Why do programmers use inheritance in C++?

easy

A. To create unrelated classes

B. To reuse code from an existing class

C. To make programs run faster

D. To avoid using functions

Solution

1. Step 1: Understand inheritance purpose

   Inheritance allows a new class to use code from an existing class, avoiding repetition.

2. Step 2: Analyze options

   Only To reuse code from an existing class correctly states code reuse. Others are unrelated or incorrect.

3. Final Answer:

   To reuse code from an existing class -> Option B

4. Quick Check:

   Inheritance = Code reuse [OK]

Hint: Inheritance means reusing code from another class [OK]

Common Mistakes:

• Thinking inheritance speeds up program execution
• Confusing inheritance with unrelated class creation
• Believing inheritance removes need for functions

2. Which of the following is the correct syntax to declare a class Car that inherits from class Vehicle in C++?

easy

A. class Car Vehicle {};

B. class Car inherits Vehicle {};

C. class Car extends Vehicle {};

D. class Car : public Vehicle {};

Solution

1. Step 1: Recall C++ inheritance syntax

   In C++, inheritance is declared using a colon and access specifier like public.

2. Step 2: Check each option

   class Car : public Vehicle {}; uses correct syntax. inherits and extends use keywords from other languages. class Car Vehicle {}; is invalid.

3. Final Answer:

   class Car : public Vehicle {}; -> Option D

4. Quick Check:

   C++ inheritance syntax = colon + access specifier [OK]

Hint: Use colon and public keyword for inheritance in C++ [OK]

Common Mistakes:

• Using 'inherits' or 'extends' keywords (not C++ syntax)
• Omitting colon before base class name
• Forgetting access specifier like public

3. Consider this code:

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

int main() {
  Dog d;
  d.sound();
  return 0;
}

What is the output when this program runs?

medium

A. Bark

B. Animal sound

C. Compilation error

D. No output

Solution

1. Step 1: Understand method overriding in inheritance

   The Dog class defines its own sound() method, which overrides Animal's method.

2. Step 2: Check which method is called

   Calling d.sound() uses Dog's version, printing "Bark".

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Overridden method runs = Bark [OK]

Hint: Child class method overrides parent method when called [OK]

Common Mistakes:

• Assuming parent method runs instead of child
• Thinking code causes compilation error
• Expecting no output from method call

4. Find the error in this code snippet:

class Base {
public:
  void show() { std::cout << "Base"; }
};
class Derived : Base {
public:
  void show() { std::cout << "Derived"; }
};

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

medium

A. Derived class cannot have a function named show()

B. Function show() must be virtual

C. No error, code runs fine

D. Missing access specifier before Base in inheritance

Solution

1. Step 1: Verify inheritance syntax

   class Derived : Base is valid syntax; class inheritance defaults to private.

2. Step 2: Trace program execution

   Derived d; d.show(); calls Derived's public show(), printing "Derived". No errors.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   Private inheritance defaults ok here [OK]

Hint: Classes inherit privately by default in C++ [OK]

Common Mistakes:

• Assuming default inheritance is public
• Thinking virtual keyword is mandatory here
• Believing derived class cannot have same function name

5. You want to model a system where Car and Bike share common features like start() and stop(), but also have their own unique features. How should you use inheritance in C++ to design this?

hard

A. Create a base class Vehicle with common features, then inherit Car and Bike from it

B. Make Car inherit from Bike to reuse code

C. Write all features separately in Car and Bike without inheritance

D. Use inheritance only if Car and Bike have identical features

Solution

1. Step 1: Identify common features

   Both Car and Bike share start() and stop(), so these belong in a common base class.

2. Step 2: Design inheritance structure

   Create a base class Vehicle with common features, then inherit Car and Bike from it to add unique features.

3. Final Answer:

   Create a base class Vehicle with common features, then inherit Car and Bike from it -> Option A

4. Quick Check:

   Use base class for shared features [OK]

Hint: Put shared features in base class, inherit for unique parts [OK]

Common Mistakes:

• Inheriting Car from Bike (wrong 'is-a' relationship)
• Duplicating code instead of reusing via inheritance
• Thinking inheritance requires identical features