Abstract classes in C++ - Mini Project: Build & Apply

Abstract Classes in C++

📖 Scenario: Imagine you are creating a simple program to manage different types of vehicles. Each vehicle can start and stop, but the way they start might be different. We will use abstract classes to define a general vehicle and then create specific types of vehicles.

🎯 Goal: Build a C++ program that uses an abstract class called Vehicle with a pure virtual function start(). Then create two classes, Car and Bike, that inherit from Vehicle and implement the start() method. Finally, create objects of Car and Bike and call their start() methods.

📋 What You'll Learn

Create an abstract class Vehicle with a pure virtual function start()

Create a class Car that inherits from Vehicle and implements start()

Create a class Bike that inherits from Vehicle and implements start()

Create objects of Car and Bike

Call the start() method on both objects and print the output

💡 Why This Matters

🌍 Real World

Abstract classes are used in software design to define common interfaces for different objects, like vehicles, shapes, or devices, ensuring they share certain behaviors.

💼 Career

Understanding abstract classes is important for designing flexible and maintainable code in many programming jobs, especially in object-oriented programming.

1

Create the abstract class Vehicle

Write an abstract class called Vehicle with a pure virtual function start() that returns void.

C++

// Your code here

Hint

Use virtual void start() = 0; inside the class to make it abstract.

2

Create the Car class inheriting Vehicle

Create a class called Car that inherits from Vehicle. Implement the start() method to print "Car started".

C++

class Vehicle {
public:
    virtual void start() = 0; // pure virtual function
};

// Your code here

Hint

Use class Car : public Vehicle and implement start() with std::cout.

3

Create the Bike class inheriting Vehicle

Create a class called Bike that inherits from Vehicle. Implement the start() method to print "Bike started".

C++

class Vehicle {
public:
    virtual void start() = 0; // pure virtual function
};

class Car : public Vehicle {
public:
    void start() override {
        std::cout << "Car started" << std::endl;
    }
};

// Your code here

Hint

Similar to Car, create Bike class and implement start().

4

Create objects and call start()

In the main() function, create an object myCar of type Car and an object myBike of type Bike. Call start() on both objects to print their start messages.

C++

class Vehicle {
public:
    virtual void start() = 0; // pure virtual function
};

class Car : public Vehicle {
public:
    void start() override {
        std::cout << "Car started" << std::endl;
    }
};

class Bike : public Vehicle {
public:
    void start() override {
        std::cout << "Bike started" << std::endl;
    }
};

int main() {
    // Your code here
}

Hint

Create objects myCar and myBike and call start() on each.

Practice

1. What is true about an abstract class in C++?

easy

A. It can be instantiated like any other class.

B. It cannot be instantiated directly.

C. It must have no member functions.

D. It cannot have any data members.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes have at least one pure virtual function, so they cannot create objects directly.

2. Step 2: Check other options

   The other options are incorrect because abstract classes can have member functions and data members.

3. Final Answer:

   It cannot be instantiated directly. -> Option B

4. Quick Check:

   Abstract class = no direct objects [OK]

Hint: Abstract classes always have pure virtual functions [OK]

Common Mistakes:

• Thinking abstract classes can create objects
• Believing abstract classes cannot have data members
• Confusing abstract classes with interfaces

2. Which of the following is the correct way to declare a pure virtual function in a C++ abstract class?

easy

A. virtual void func() = 0;

B. void func() = 0;

C. virtual void func();

D. void func() {};

Solution

1. Step 1: Recall pure virtual function syntax

   Pure virtual functions are declared with 'virtual' keyword and '= 0' at the end.

2. Step 2: Analyze options

   virtual void func() = 0; matches the correct syntax. void func() = 0; misses 'virtual'. virtual void func(); is a normal virtual function. void func() {}; is a normal function with a body.

3. Final Answer:

   virtual void func() = 0; -> Option A

4. Quick Check:

   Pure virtual = virtual + '= 0' [OK]

Hint: Pure virtual always ends with '= 0;' [OK]

Common Mistakes:

• Omitting 'virtual' keyword
• Adding function body to pure virtual
• Confusing normal virtual with pure virtual

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

#include <iostream>
class Base {
public:
    virtual void show() = 0;
};
class Derived : public Base {
public:
    void show() { std::cout << "Hello"; }
};
int main() {
    Derived d;
    d.show();
    return 0;
}

medium

A. No output

B. Compilation error

C. Hello

D. Runtime error

Solution

1. Step 1: Check abstract class and derived implementation

   Base has a pure virtual function 'show'. Derived overrides it with a concrete implementation.

2. Step 2: Analyze main function output

   Derived object 'd' calls 'show()', which prints "Hello".

3. Final Answer:

   Hello -> Option C

4. Quick Check:

   Derived implements pure virtual, so prints output [OK]

Hint: Derived must implement pure virtual to avoid errors [OK]

Common Mistakes:

• Expecting compilation error without implementation
• Thinking pure virtual functions run by themselves
• Confusing output with no output

4. Identify the error in this code snippet:

#include <iostream>
class Abstract {
public:
    virtual void display() = 0;
};

class Concrete : public Abstract {
public:
    void display() { std::cout << "Hi"; }
};

int main() {
    Abstract a;
    Concrete c;
    c.display();
    return 0;
}

medium

A. Cannot instantiate abstract class 'Abstract' directly.

B. Missing implementation of display() in Concrete.

C. Syntax error in class declaration.

D. No error, code runs fine.

Solution

1. Step 1: Check instantiation of abstract class

   Abstract class 'Abstract' has a pure virtual function, so cannot create object 'a'.

2. Step 2: Verify other parts

   Concrete implements display(), so no error there. Syntax is correct.

3. Final Answer:

   Cannot instantiate abstract class 'Abstract' directly. -> Option A

4. Quick Check:

   Abstract objects not allowed [OK]

Hint: Never create objects of abstract classes [OK]

Common Mistakes:

• Trying to instantiate abstract class
• Ignoring pure virtual function rules
• Assuming implementation missing in derived

5. You want to design a shape system where every shape must provide its area calculation. Which is the best way to enforce this using abstract classes in C++?

hard

A. Create a class Shape with no functions and add area() only in derived classes.

B. Create a class Shape with a normal function double area() that returns 0, and override it in derived classes.

C. Create a class Shape with a virtual function double area() with empty body.

D. Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area().

Solution

1. Step 1: Understand requirement for enforcing area() implementation

   Using a pure virtual function in an abstract class forces all derived classes to implement area().

2. Step 2: Evaluate options

   Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area(). uses pure virtual function, which is the best way. Create a class Shape with a normal function double area() that returns 0, and override it in derived classes. returns 0 by default, which may hide missing implementations. Create a class Shape with no functions and add area() only in derived classes. does not enforce area() in base. Create a class Shape with a virtual function double area() with empty body. has virtual but not pure, so no enforcement.

3. Final Answer:

   Create an abstract class Shape with a pure virtual function double area() = 0; and derive specific shapes implementing area(). -> Option D

4. Quick Check:

   Pure virtual enforces implementation [OK]

Hint: Use pure virtual to force derived class methods [OK]

Common Mistakes:

• Using normal virtual instead of pure virtual
• Not enforcing method in base class
• Returning default values hiding missing overrides