Classes and objects in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class method call

What is the output of this C++ program?

C++

#include <iostream>
class Counter {
public:
    int count = 0;
    void increment() { count++; }
    void print() { std::cout << count << std::endl; }
};

int main() {
    Counter c;
    c.increment();
    c.increment();
    c.print();
    return 0;
}

A 0

B 1

C 2

D Compilation error

💡 Hint

Think about how many times the increment method is called before printing.

The increment method increases the count by 1 each time. It is called twice, so count becomes 2. Then print outputs 2.

❓ Predict Output

intermediate

Value of object attribute after constructor

What will be the value of obj.x after this program runs?

C++

#include <iostream>
class MyClass {
public:
    int x;
    MyClass(int val) { x = val * 2; }
};

int main() {
    MyClass obj(5);
    std::cout << obj.x << std::endl;
    return 0;
}

A 10

B Compilation error

C 0

D 5

💡 Hint

Look at how the constructor sets the value of x.

The constructor multiplies the input value by 2 and assigns it to x. So 5 * 2 = 10.

🔧 Debug

advanced

Identify the error in this class definition

What error will this code produce when compiled?

C++

#include <iostream>
class Sample {
public:
    int value;
    Sample(int v) {
        value = v
    }
};

int main() {
    Sample s(10);
    std::cout << s.value << std::endl;
    return 0;
}

A Runtime error: uninitialized variable

B Syntax error: missing semicolon after assignment

C Compilation error: missing return statement in main

D No error, outputs 10

💡 Hint

Check punctuation inside the constructor.

The assignment line 'value = v' is missing a semicolon at the end, causing a syntax error.

❓ Predict Output

advanced

Output of method modifying object state

What is the output of this program?

C++

#include <iostream>
class BankAccount {
    int balance;
public:
    BankAccount() : balance(100) {}
    void deposit(int amount) { balance += amount; }
    void withdraw(int amount) { balance -= amount; }
    int getBalance() { return balance; }
};

int main() {
    BankAccount acc;
    acc.deposit(50);
    acc.withdraw(30);
    std::cout << acc.getBalance() << std::endl;
    return 0;
}

A 150

B Compilation error

C 80

D 120

💡 Hint

Calculate balance after deposit and withdrawal.

Starting balance is 100. Deposit 50 makes it 150. Withdraw 30 leaves 120.

🧠 Conceptual

expert

Number of objects created and output

Consider this program. How many objects are created and what is the output?

C++

#include <iostream>
class Item {
public:
    static int count;
    Item() { count++; }
};

int Item::count = 0;

int main() {
    Item a;
    Item b;
    Item c = b;
    std::cout << Item::count << std::endl;
    return 0;
}

A 3 objects created, output: 2

B 2 objects created, output: 2

C 3 objects created, output: 3

D Compilation error

💡 Hint

Copy construction uses the implicit copy constructor, which does not execute the default constructor body.

Three objects are created: a, b, and c (copy of b). The default constructor increments count for a and b. The copy constructor for c does not increment count, so output is 2.

Practice

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

easy

A. To group data and functions that represent a real-world concept

B. To execute a program faster

C. To store only numbers

D. To replace variables with constants

Solution

1. Step 1: Understand what a class represents

   A class groups related data (variables) and functions (methods) to model real-world things.

2. Step 2: Compare options with the definition

   Only To group data and functions that represent a real-world concept correctly describes this purpose; others are unrelated to classes.

3. Final Answer:

   To group data and functions that represent a real-world concept -> Option A

4. Quick Check:

   Class = group data + functions [OK]

Hint: Classes bundle data and actions together [OK]

Common Mistakes:

• Thinking classes only store numbers
• Confusing classes with program speed
• Believing classes replace variables

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

easy

A. class Car { };

B. Car class { };

C. class = Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class declaration syntax

   The correct syntax is: class ClassName { }; with a semicolon at the end.

2. Step 2: Check each option

   class Car { }; matches the correct syntax. Options A, C, and D have syntax errors or wrong order.

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class ClassName { }; syntax [OK]

Hint: Remember: class name follows 'class' keyword, ends with semicolon [OK]

Common Mistakes:

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

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

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

int main() {
  Box box1;
  box1.setLength(10);
  std::cout << box1.getLength();
  return 0;
}

medium

A. length

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand the class methods

   The method setLength sets the length to 10 for box1. The method getLength returns this value.

2. Step 2: Trace the output statement

   std::cout << box1.getLength(); prints the value 10 stored in length.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   setLength(10) then getLength() = 10 [OK]

Hint: Setter sets value, getter returns it [OK]

Common Mistakes:

• Forgetting to call setLength before getLength
• Expecting default value 0 without initialization
• Confusing method names

4. Identify the error in this class definition:

class Person {
  string name;
  int age;
public:
  void setName(string n) { name = n; }
  void setAge(int a) { age = a; }
};

medium

A. Missing semicolon after class definition

B. Missing public: before data members

C. Missing #include <string> header

D. Incorrect function return types

Solution

1. Step 1: Check class syntax

   The class ends with a semicolon, and functions have correct return types (void). Data members are private by default, which is allowed.

2. Step 2: Check for missing includes

   The code uses string but does not include <string> header, which is required for std::string.

3. Final Answer:

   Missing #include <string> header -> Option C

4. Quick Check:

   Use #include <string> for string type [OK]

Hint: Always include headers for standard types [OK]

Common Mistakes:

• Assuming private members need public keyword
• Forgetting to add semicolon after class
• Confusing function return types

5. Given this class:

class Student {
  int id;
  string name;
public:
  Student(int i, string n) : id(i), name(n) {}
  void display() {
    std::cout << "ID: " << id << ", Name: " << name << std::endl;
  }
};

int main() {
  Student s1(101, "Alice");
  s1.display();
  return 0;
}

What concept does this code demonstrate?

hard

A. Inheritance between classes

B. Constructor initializing object data

C. Function overloading

D. Static class members

Solution

1. Step 1: Identify the special function in the class

   The function Student(int i, string n) : id(i), name(n) {} is a constructor initializing the object's data members.

2. Step 2: Check other options

   There is no inheritance, no function overloading, and no static members shown.

3. Final Answer:

   Constructor initializing object data -> Option B

4. Quick Check:

   Constructor sets initial values [OK]

Hint: Look for function named as class for constructor [OK]

Common Mistakes:

• Confusing constructor with regular method
• Thinking this shows inheritance
• Assuming static members without keyword