Base and derived classes in C++ - Mini Project: Build & Apply

Base and Derived Classes in C++

📖 Scenario: You are creating a simple program to represent vehicles. You want to show how a basic vehicle can be extended to a specific type of vehicle, like a car, using base and derived classes.

🎯 Goal: Build a C++ program that defines a base class Vehicle and a derived class Car. You will add properties and methods to both classes and display information about a car.

📋 What You'll Learn

Create a base class named Vehicle with a public string variable brand.

Create a derived class named Car that inherits from Vehicle and has a public string variable model.

Add a public method display() in Car that prints the brand and model.

Create an object of Car, set its brand and model, and call display().

💡 Why This Matters

🌍 Real World

Base and derived classes help organize code when many objects share common features but also have unique details, like different types of vehicles.

💼 Career

Understanding inheritance is key for software development jobs that involve object-oriented programming, making code easier to maintain and extend.

1

Create the base class Vehicle

Write a base class called Vehicle with a public string variable named brand.

C++

// Your code here

Hint

Use class Vehicle { public: std::string brand; }; to define the base class.

2

Create the derived class Car

Create a derived class called Car that inherits from Vehicle. Add a public string variable named model.

C++

#include 
#include 

class Vehicle {
public:
    std::string brand;
};

// Your code here

Hint

Use class Car : public Vehicle { public: std::string model; }; to create the derived class.

3

Add display() method to Car

Add a public method named display() inside the Car class. This method should print the brand and model separated by a space.

C++

#include 
#include 

class Vehicle {
public:
    std::string brand;
};

class Car : public Vehicle {
public:
    std::string model;
    // Your code here
};

Hint

Define void display() { std::cout << brand << " " << model << std::endl; } inside Car.

4

Create Car object and display info

In the main() function, create an object named myCar of type Car. Set myCar.brand to "Toyota" and myCar.model to "Corolla". Then call myCar.display() to print the brand and model.

C++

#include 
#include 

class Vehicle {
public:
    std::string brand;
};

class Car : public Vehicle {
public:
    std::string model;
    void display() {
        std::cout << brand << " " << model << std::endl;
    }
};

int main() {
    // Your code here
    return 0;
}

Hint

Create Car myCar;, set myCar.brand = "Toyota"; and myCar.model = "Corolla";, then call myCar.display();.

Practice

1. What is the main purpose of a base class in C++?

easy

A. To store only private data members

B. To create objects directly without inheritance

C. To override functions in derived classes

D. To hold common features shared by derived classes

Solution

1. Step 1: Understand base class role

   A base class is designed to hold common features that multiple derived classes can share.

2. Step 2: Compare options

   To hold common features shared by derived classes correctly states the purpose. Other options describe derived class behavior or incorrect roles.

3. Final Answer:

   To hold common features shared by derived classes -> Option D

4. Quick Check:

   Base class = common features [OK]

Hint: Base class holds shared features for reuse [OK]

Common Mistakes:

• Confusing base class with derived class
• Thinking base class cannot have data members
• Assuming base class is only for private data

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

easy

A. class Car : Vehicle {};

B. class Car : public Vehicle {};

C. class Car extends Vehicle {};

D. class Car inherits Vehicle {};

Solution

1. Step 1: Recall C++ inheritance syntax

   In C++, a derived class inherits from a base class using a colon and access specifier like public.

2. Step 2: Check options

   class Car : public Vehicle {}; uses correct syntax: class Car : public Vehicle {}. Others use incorrect keywords or omit access specifier.

3. Final Answer:

   class Car : public Vehicle {} -> Option B

4. Quick Check:

   Inheritance syntax = colon + access specifier [OK]

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

Common Mistakes:

• Omitting access specifier like public
• Using wrong keywords like inherits or extends
• Forgetting colon before base class name

3. What will be the output of this C++ code?

class Base {
public:
    void show() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void show() { std::cout << "Derived"; }
};
int main() {
    Derived d;
    d.show();
    return 0;
}

medium

A. Derived

B. Base

C. Compilation error

D. No output

Solution

1. Step 1: Understand method overriding in derived class

   The derived class Derived has its own show() method which hides the base class method.

2. Step 2: Check which method is called

   Calling d.show() on a Derived object calls the derived class method, printing "Derived".

3. Final Answer:

   Derived -> Option A

4. Quick Check:

   Derived method overrides base method [OK]

Hint: Derived class method hides base method if same name [OK]

Common Mistakes:

• Assuming base class method runs without virtual keyword
• Expecting compilation error due to method name clash
• Thinking both outputs print

4. Identify the error in this code snippet:

class Base {
public:
    void display() { std::cout << "Base"; }
};
class Derived : public Base {
public:
    void display() { std::cout << "Derived"; }
};
int main() {
    Base* ptr = new Derived();
    ptr->display();
    return 0;
}

medium

A. Missing virtual keyword in base class method

B. Incorrect pointer type for inheritance

C. Derived class method should be static

D. No error, code runs fine

Solution

1. Step 1: Understand polymorphism with base pointers

   Calling display() via a base class pointer calls base method unless it is virtual.

2. Step 2: Check for virtual keyword

   The base class method display() lacks virtual, so ptr->display() calls base version, not derived.

3. Final Answer:

   Missing virtual keyword in base class method -> Option A

4. Quick Check:

   Virtual needed for polymorphic calls [OK]

Hint: Add virtual to base method for polymorphism [OK]

Common Mistakes:

• Thinking pointer type is wrong
• Believing derived method runs without virtual
• Assuming static keyword fixes this

5. Given these classes:

class Animal {
public:
    virtual std::string sound() { return "Unknown"; }
};
class Dog : public Animal {
public:
    std::string sound() override { return "Bark"; }
};
class Cat : public Animal {
public:
    std::string sound() override { return "Meow"; }
};

Which code snippet correctly creates a list of animals and prints their sounds?

hard

A. std::vector<Animal> animals = {Dog(), Cat()}; for(auto& a : animals) std::cout << a.sound() << ' ';

B. std::vector<Dog> animals = {Dog(), Cat()}; for(auto& a : animals) std::cout << a.sound() << ' ';

C. std::vector animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' ';

D. std::vector<Animal> animals = {Dog(), Cat()}; for(auto a : animals) std::cout << a.sound() << ' ';

Solution

1. Step 1: Understand polymorphism with collections

   To preserve derived class behavior, store pointers to base class, not objects by value.

2. Step 2: Analyze options

   std::vector animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' '; uses std::vector<Animal*> with pointers to Dog and Cat, calling sound() polymorphically.

3. Final Answer:

   std::vector<Animal*> animals = {new Dog(), new Cat()}; for(auto a : animals) std::cout << a->sound() << ' '; -> Option C

4. Quick Check:

   Use base class pointers for polymorphic containers [OK]

Hint: Use base class pointers to keep derived behavior [OK]

Common Mistakes:

• Storing objects by value causes slicing
• Using wrong container type for polymorphism
• Forgetting to use pointers or references