Real-world modeling in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class inheritance and method overriding

What is the output of this C++ program modeling animals with inheritance and method overriding?

C++

#include <iostream>
using namespace std;

class Animal {
public:
    virtual void sound() { cout << "Some sound" << endl; }
};

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

class Cat : public Animal {
public:
    void sound() override { cout << "Meow" << endl; }
};

int main() {
    Animal* a1 = new Dog();
    Animal* a2 = new Cat();
    a1->sound();
    a2->sound();
    delete a1;
    delete a2;
    return 0;
}

A

Bark
Meow

B

Some sound
Some sound

C

Bark
Some sound

D

Meow
Bark

💡 Hint

Think about virtual functions and which method is called when using base class pointers.

The base class Animal has a virtual method sound(). Dog and Cat override this method. When calling sound() through Animal pointers to Dog and Cat objects, the overridden methods in Dog and Cat are called, printing "Bark" and "Meow" respectively.

🧠 Conceptual

intermediate

Understanding encapsulation in a bank account model

Which of the following best describes the purpose of encapsulation in the BankAccount class below?

C++

class BankAccount {
private:
    double balance;
public:
    BankAccount(double initial) : balance(initial) {}
    void deposit(double amount) { if(amount > 0) balance += amount; }
    bool withdraw(double amount) {
        if(amount > 0 && amount <= balance) {
            balance -= amount;
            return true;
        }
        return false;
    }
    double getBalance() const { return balance; }
};

A To hide the balance variable and control access through methods

B To make balance a global variable

C To allow direct access to balance from outside the class

D To prevent the class from being inherited

💡 Hint

Think about why balance is private and how deposit and withdraw work.

Encapsulation means hiding internal data and only allowing controlled access. Here, balance is private and can only be changed through deposit and withdraw methods, protecting it from invalid changes.

🔧 Debug

advanced

Identify the runtime error in the vehicle speed simulation

What runtime error will this C++ code produce when simulating vehicle speed changes?

C++

#include <iostream>
using namespace std;

class Vehicle {
public:
    int speed;
    Vehicle() : speed(0) {}
    void accelerate(int amount) {
        speed += amount;
    }
};

int main() {
    Vehicle* v = nullptr;
    v->accelerate(10);
    cout << v->speed << endl;
    return 0;
}

A Logical error: speed remains 0

B Compilation error: cannot call method on pointer

C Output: 10

D Segmentation fault (accessing null pointer)

💡 Hint

What happens if you call a method on a null pointer?

The pointer v is null and calling v->accelerate(10) tries to access memory through a null pointer, causing a segmentation fault at runtime.

📝 Syntax

advanced

Which option correctly defines a struct for a 2D point with a constructor?

Select the option that compiles without errors and correctly defines a struct Point with x and y coordinates and a constructor.

A

struct Point {
    int x, y;
    Point(int a, int b) { x = a; y = b; }
};

B

struct Point {
    int x, y;
    Point(int a, int b) { x: a; y: b; }
};

C

struct Point {
    int x, y;
    Point(int a, int b) : x(a), y(b) {}
};

D

struct Point {
    int x, y;
    Point(int a, int b) -> x(a), y(b) {}
};

💡 Hint

Look for correct constructor initialization syntax in C++.

Option C uses the member initializer list syntax which is correct. Option C uses assignment in constructor body which is valid but less preferred. Option C uses invalid syntax with colons inside constructor body. Option C uses invalid arrow syntax.

🚀 Application

expert

Calculate total area of mixed shapes using polymorphism

Given the classes below modeling shapes, what is the output of the program calculating total area?

C++

#include <iostream>
#include <vector>
#include <memory>
using namespace std;

class Shape {
public:
    virtual double area() const = 0;
    virtual ~Shape() = default;
};

class Rectangle : public Shape {
    double width, height;
public:
    Rectangle(double w, double h) : width(w), height(h) {}
    double area() const override { return width * height; }
};

class Circle : public Shape {
    double radius;
public:
    Circle(double r) : radius(r) {}
    double area() const override { return 3.14159 * radius * radius; }
};

int main() {
    vector<unique_ptr<Shape>> shapes;
    shapes.push_back(make_unique<Rectangle>(3, 4));
    shapes.push_back(make_unique<Circle>(2));
    double total = 0;
    for (const auto& shape : shapes) {
        total += shape->area();
    }
    cout << total << endl;
    return 0;
}

A 25.56636

B 24.56636

C 12.56636

D 18.56636

💡 Hint

Calculate area of rectangle (3*4) and circle (π*2²) and add them.

Rectangle area = 3*4 = 12. Circle area = 3.14159*2² = 12.56636. Total = 24.56636. Polymorphism ensures the correct area() is called for each shape via base class pointers.

Practice

1. What is the main purpose of using classes in real-world modeling in C++?

easy

A. To write faster code without any structure

B. To represent real-world objects with attributes and behaviors

C. To avoid using variables in the program

D. To make the program run without errors

Solution

1. Step 1: Understand the role of classes

   Classes group data (attributes) and functions (methods) to model real-world things.

2. Step 2: Identify the purpose in modeling

   Classes help represent objects with their properties and actions clearly.

3. Final Answer:

   To represent real-world objects with attributes and behaviors -> Option B

4. Quick Check:

   Classes model objects = A [OK]

Hint: Classes group data and actions like real objects [OK]

Common Mistakes:

• Thinking classes only speed up code
• Confusing classes with simple variables
• Believing classes remove all errors

2. Which of the following is the correct way to declare a class named Car in C++?

easy

A. Car class { };

B. class = Car { };

C. class Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class syntax

   The correct syntax starts with the keyword class, followed by the class name and braces.

2. Step 2: Check each option

   class Car { }; uses class Car { }; which is correct. Others have syntax errors.

3. Final Answer:

   class Car { }; -> Option C

4. Quick Check:

   Correct class syntax = B [OK]

Hint: Use 'class Name { };' to declare classes [OK]

Common Mistakes:

• Writing 'Car class' instead of 'class Car'
• Using parentheses after class name
• Missing semicolon after class definition

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

class Dog {
public:
  int age;
  void bark() { std::cout << "Woof!"; }
};

int main() {
  Dog myDog;
  myDog.age = 5;
  std::cout << myDog.age << " ";
  myDog.bark();
  return 0;
}

medium

A. Woof!5

B. Woof! 5

C. 5Woof!

D. 5 Woof!

Solution

1. Step 1: Analyze output statements

   The code prints myDog.age followed by a space, then calls bark() which prints "Woof!".

2. Step 2: Combine outputs

   Output is 5 plus Woof!, so result is "5 Woof!".

3. Final Answer:

   5 Woof! -> Option D

4. Quick Check:

   Print age + space + bark = 5 Woof! [OK]

Hint: Check order and spaces in output statements [OK]

Common Mistakes:

• Assuming space after bark output
• Mixing order of printed values
• Ignoring no newline after prints

4. Identify the error in this C++ class definition:

class Book {
  string title;
  int pages;
public:
  void setTitle(string t) { title = t; }
  void setPages(int p) { pages = p; }
};

medium

A. Missing std:: before string

B. Missing semicolon after class definition

C. Methods should be declared outside the class

D. Private members must be declared after public

Solution

1. Step 1: Check usage of string type

   The code uses string without std:: or using namespace std;.

2. Step 2: Identify missing namespace

   In C++, string is in the std namespace, so std::string is needed unless using directive is present.

3. Final Answer:

   Missing std:: before string -> Option A

4. Quick Check:

   Use std::string or using namespace std [OK]

Hint: Always prefix string with std:: or use namespace std [OK]

Common Mistakes:

• Forgetting std:: prefix for string
• Thinking methods must be outside class
• Believing private must come after public

5. You want to model a Library that holds many Book objects. Which design best represents this real-world relationship in C++?

hard

A. class Library { std::vector<Book> books; };

B. class Library { Book book; };

C. class Library { int bookCount; };

D. class Library { void addBook(Book b); };

Solution

1. Step 1: Understand the relationship

   A library contains many books, so it needs a collection (like a vector) of Book objects.

2. Step 2: Evaluate options

   class Library { std::vector<Book> books; }; uses std::vector<Book> to hold multiple books, matching the real-world model.

3. Final Answer:

   class Library { std::vector<Book> books; }; -> Option A

4. Quick Check:

   Use collection for many objects = D [OK]

Hint: Use vector to hold many objects in a class [OK]

Common Mistakes:

• Using single object instead of collection
• Only storing count without objects
• Defining methods without data members