OOP principles overview in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C++ code demonstrating encapsulation?

Consider this C++ class that uses encapsulation to protect its data.

class Box {
private:
    int length;
public:
    void setLength(int len) { length = len; }
    int getLength() { return length; }
};

int main() {
    Box box;
    box.setLength(10);
    std::cout << box.getLength() << std::endl;
    return 0;
}

What will this program print?

C++

#include <iostream>
class Box {
private:
    int length;
public:
    void setLength(int len) { length = len; }
    int getLength() { return length; }
};

int main() {
    Box box;
    box.setLength(10);
    std::cout << box.getLength() << std::endl;
    return 0;
}

A Undefined behavior due to uninitialized variable

B 0

C Compilation error due to private member access

D 10

💡 Hint

Think about how the setter and getter methods control access to the private variable.

The private variable length is set using the public method setLength. Then getLength returns the value 10, so the output is 10.

❓ Predict Output

intermediate

What is the output of this C++ code demonstrating inheritance?

Look at this code with a base class and a derived class:

class Animal {
public:
    void speak() { std::cout << "Animal speaks" << std::endl; }
};

class Dog : public Animal {
public:
    void speak() { std::cout << "Dog barks" << std::endl; }
};

int main() {
    Dog dog;
    dog.speak();
    return 0;
}

What will this program print?

C++

#include <iostream>
class Animal {
public:
    void speak() { std::cout << "Animal speaks" << std::endl; }
};

class Dog : public Animal {
public:
    void speak() { std::cout << "Dog barks" << std::endl; }
};

int main() {
    Dog dog;
    dog.speak();
    return 0;
}

A Dog barks

B Compilation error due to method overriding

C Animal speaks

D Runtime error due to ambiguous call

💡 Hint

Which class's speak method is called on the Dog object?

The Dog class overrides the speak method. Calling dog.speak() calls the Dog version, printing "Dog barks".

❓ Predict Output

advanced

What is the output of this C++ code demonstrating polymorphism with virtual functions?

Consider this code using virtual functions:

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

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

int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

What will this program print?

C++

#include <iostream>
class Base {
public:
    virtual void show() { std::cout << "Base show" << std::endl; }
};

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

int main() {
    Base* ptr = new Derived();
    ptr->show();
    delete ptr;
    return 0;
}

A Derived show

B Base show

C Compilation error due to virtual function

D Runtime error due to invalid pointer

💡 Hint

Virtual functions allow the program to decide which method to call at runtime.

The pointer ptr is of type Base* but points to a Derived object. Because show is virtual, the Derived version is called, printing "Derived show".

❓ Predict Output

advanced

What is the output of this C++ code demonstrating abstraction with pure virtual functions?

Look at this abstract class example:

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

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

int main() {
    Circle c;
    c.draw();
    return 0;
}

What will this program print?

C++

#include <iostream>
class Shape {
public:
    virtual void draw() = 0; // pure virtual function
};

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

int main() {
    Circle c;
    c.draw();
    return 0;
}

A Compilation error due to abstract class instantiation

B Drawing Circle

C Runtime error due to pure virtual function call

D No output, program hangs

💡 Hint

Can you create an object of the abstract class? What about the derived class?

The abstract class Shape cannot be instantiated, but Circle implements draw. Creating a Circle object and calling draw prints "Drawing Circle".

🧠 Conceptual

expert

Which OOP principle is best demonstrated by this C++ code snippet?

Consider this code:

class Logger {
public:
    void log(const std::string& message) {
        std::cout << "Log: " << message << std::endl;
    }
};

class FileLogger : public Logger {
public:
    void log(const std::string& message) {
        // code to write message to a file
        std::cout << "File log: " << message << std::endl;
    }
};

Which OOP principle does this best illustrate?

C++

#include <iostream>
#include <string>
class Logger {
public:
    void log(const std::string& message) {
        std::cout << "Log: " << message << std::endl;
    }
};

class FileLogger : public Logger {
public:
    void log(const std::string& message) {
        // code to write message to a file
        std::cout << "File log: " << message << std::endl;
    }
};

A Encapsulation - hiding data inside classes

B Polymorphism - using the same interface with different behaviors

C Inheritance - reusing code from a base class

D Abstraction - hiding complex implementation details

💡 Hint

Look at how FileLogger relates to Logger.

FileLogger inherits from Logger, reusing and extending its functionality. This is an example of inheritance.

Practice

1. Which of the following is NOT one of the four main principles of Object-Oriented Programming (OOP)?

easy

A. Encapsulation

B. Compilation

C. Polymorphism

D. Inheritance

Solution

1. Step 1: Recall the four main OOP principles

   The four main principles are encapsulation, inheritance, polymorphism, and abstraction.

2. Step 2: Identify the option not in the list

   Compilation is a process of converting code, not an OOP principle.

3. Final Answer:

   Compilation -> Option B

4. Quick Check:

   OOP principles exclude compilation [OK]

Hint: Remember the four OOP pillars: E, I, P, A [OK]

Common Mistakes:

• Confusing compilation with abstraction
• Mixing OOP principles with programming processes

2. Which keyword is used in C++ to create a new class?

easy

A. class

B. struct

C. object

D. define

Solution

1. Step 1: Identify the C++ keyword for defining a class

   In C++, the keyword to define a class is class.

2. Step 2: Check other options

   struct defines a structure, object is not a keyword, and define is for macros.

3. Final Answer:

   class -> Option A

4. Quick Check:

   Use class to define classes [OK]

Hint: Use 'class' keyword to define classes in C++ [OK]

Common Mistakes:

• Confusing struct with class
• Using 'object' as a keyword
• Mixing macros with class definition

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

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

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

int main() {
    Animal* a = new Dog();
    a->sound();
    delete a;
    return 0;
}

medium

A. Runtime error

B. Bark

C. Compilation error

D. Animal sound

Solution

1. Step 1: Understand method call without virtual keyword

   The sound method is not virtual, so the pointer type Animal* calls Animal's method.

2. Step 2: Determine output

   Since sound is not virtual, Animal sound is printed.

3. Final Answer:

   Animal sound -> Option D

4. Quick Check:

   Non-virtual method calls base class method [OK]

Hint: Without virtual, base class method runs [OK]

Common Mistakes:

• Assuming polymorphism without virtual
• Expecting derived class method call
• Ignoring pointer type effect

4. Identify the error in the following C++ code snippet:

class Vehicle {
private:
    int speed;
public:
    void setSpeed(int s) { speed = s; }
};

int main() {
    Vehicle v;
    v.speed = 100;
    return 0;
}

medium

A. setSpeed method is not defined

B. Missing semicolon after class definition

C. Cannot access private member 'speed' directly

D. Vehicle object not created properly

Solution

1. Step 1: Check access to private member

   The member speed is private and cannot be accessed directly outside the class.

2. Step 2: Identify the error in main()

   Trying to assign v.speed = 100; causes an access violation error.

3. Final Answer:

   Cannot access private member 'speed' directly -> Option C

4. Quick Check:

   Private members need public methods for access [OK]

Hint: Private members can't be accessed outside class [OK]

Common Mistakes:

• Trying to access private variables directly
• Ignoring access specifiers
• Assuming public by default

5. You want to design a C++ class hierarchy where a base class Shape defines a method area() but each derived class calculates area differently. Which OOP principle does this best demonstrate?

hard

A. Polymorphism

B. Inheritance

C. Encapsulation

D. Abstraction

Solution

1. Step 1: Understand the scenario of different area calculations

   Each derived class implements area() differently, but shares the same interface.

2. Step 2: Identify the OOP principle

   This is polymorphism: same method name, different behaviors in subclasses.

3. Final Answer:

   Polymorphism -> Option A

4. Quick Check:

   Same method, different behaviors = Polymorphism [OK]

Hint: Different behaviors, same method name = polymorphism [OK]

Common Mistakes:

• Confusing with inheritance or abstraction
• Thinking encapsulation means method overriding