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.