Data hiding in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of private member access attempt

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

C++

class Box {
private:
    int length;
public:
    Box(int l) : length(l) {}
};

int main() {
    Box b(10);
    // std::cout << b.length << std::endl; // Uncommenting this line
    return 0;
}

A Outputs 0

B Outputs 10

C Runtime error

D Compilation error due to private member access

💡 Hint

Remember that private members cannot be accessed directly outside the class.

The member 'length' is private, so accessing it directly outside the class causes a compilation error.

❓ Predict Output

intermediate

Output when accessing private member via public method

What is the output of this C++ program?

C++

#include <iostream>
class Box {
private:
    int length;
public:
    Box(int l) : length(l) {}
    int getLength() { return length; }
};

int main() {
    Box b(15);
    std::cout << b.getLength() << std::endl;
    return 0;
}

A 15

B Runtime error

C 0

D Compilation error

💡 Hint

Check how the private member is accessed.

The private member 'length' is accessed through the public method getLength(), so it prints 15.

🧠 Conceptual

advanced

Why use private members in a class?

Which of the following is the main reason to declare class members as private in C++?

A To hide internal details and protect data from unauthorized access

B To allow direct access from outside the class

C To make the program run faster

D To allow inheritance from other classes

💡 Hint

Think about what data hiding means.

Private members hide internal details and protect data from being accessed or modified directly from outside the class.

❓ Predict Output

advanced

Output of friend function accessing private member

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

C++

#include <iostream>
class Box {
private:
    int length;
public:
    Box(int l) : length(l) {}
    friend int getLength(Box& b);
};

int getLength(Box& b) {
    return b.length;
}

int main() {
    Box b(20);
    std::cout << getLength(b) << std::endl;
    return 0;
}

A Compilation error due to private member access

B 0

C 20

D Runtime error

💡 Hint

Friend functions can access private members.

The friend function getLength can access the private member length and returns 20.

❓ Predict Output

expert

Value of private member after inheritance and access

What is the output of this C++ program?

C++

#include <iostream>
class Base {
private:
    int secret = 42;
public:
    int getSecret() { return secret; }
};

class Derived : public Base {
public:
    int getSecretFromDerived() {
        // return secret; // Uncommenting this line
        return getSecret();
    }
};

int main() {
    Derived d;
    std::cout << d.getSecretFromDerived() << std::endl;
    return 0;
}

A Compilation error due to private member access in Derived

B 42

C 0

D Runtime error

💡 Hint

Private members are not accessible directly in derived classes.

The Derived class cannot access 'secret' directly because it is private in Base, but it can call Base's public method getSecret().

Practice

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

easy

A. To hide data from outside access and protect it

B. To allow direct access from anywhere

C. To make data global

D. To speed up the program execution

Solution

1. Step 1: Understand the role of private members

   Private members in a class are not accessible from outside the class, which means they are hidden from direct access.

2. Step 2: Recognize the purpose of data hiding

   Data hiding protects important data by restricting access, making the program safer and easier to manage.

3. Final Answer:

   To hide data from outside access and protect it -> Option A

4. Quick Check:

   Data hiding = private members protect data [OK]

Hint: Private means hidden from outside, protecting data [OK]

Common Mistakes:

• Thinking private allows direct access
• Confusing private with public
• Assuming private makes data global

2. Which of the following is the correct syntax to declare a private integer variable named age inside a class?

easy

A. private: int age;

B. int private age;

C. int age private;

D. private int age;

Solution

1. Step 1: Recall C++ access specifier syntax

   In C++, access specifiers like private are followed by a colon and then the members are declared.

2. Step 2: Identify correct declaration

   The correct way is to write private: then declare variables like int age; inside the class.

3. Final Answer:

   private: int age; -> Option A

4. Quick Check:

   Access specifier ends with colon before members [OK]

Hint: Use 'private:' label before variables inside class [OK]

Common Mistakes:

• Writing 'private' before type without colon
• Placing 'private' after variable name
• Using 'int private' which is invalid

3. Consider this code snippet:

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

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

What will happen when this code is compiled?

medium

A. It will print 10

B. It will print 0

C. Compilation error due to accessing private member

D. Runtime error

Solution

1. Step 1: Identify access to private member

   The code tries to access b.length directly in main(), but length is private.

2. Step 2: Understand C++ access rules

   Private members cannot be accessed outside the class, so this causes a compilation error.

3. Final Answer:

   Compilation error due to accessing private member -> Option C

4. Quick Check:

   Private members cause compile error if accessed outside [OK]

Hint: Private members cannot be accessed directly outside class [OK]

Common Mistakes:

• Assuming private members print default value
• Expecting runtime error instead of compile error
• Trying to access private members directly

4. Find the error in this class definition:

class Person {
  int age;
public:
  void setAge(int a) { age = a; }
  int getAge() { return age; }
};

int main() {
  Person p;
  p.age = 25;
  std::cout << p.getAge();
  return 0;
}

medium

A. No error, code runs fine

B. Error: 'age' is private and accessed directly

C. Error: Missing semicolon after class

D. Error: getAge() should be private

Solution

1. Step 1: Check member access in main()

   The code tries to assign p.age = 25; but age is private by default in class.

2. Step 2: Understand default access in classes

   In C++, class members are private by default, so direct access outside class causes error.

3. Final Answer:

   Error: 'age' is private and accessed directly -> Option B

4. Quick Check:

   Private members cannot be accessed directly outside class [OK]

Hint: Class members default to private, cannot access directly [OK]

Common Mistakes:

• Assuming no error without explicit private label
• Thinking getAge() must be private
• Missing semicolon is not the issue here

5. You want to protect a class's data but allow controlled access. Which design correctly applies data hiding and access control?

class Account {
private:
  double balance;
public:
  void deposit(double amount) { if(amount > 0) balance += amount; }
  double getBalance() { return balance; }
};

What is the benefit of this design?

hard

A. Balance is hidden but cannot be updated

B. Balance is public, so anyone can change it freely

C. No way to access balance from outside

D. Balance can be changed only through deposit, preventing invalid values

Solution

1. Step 1: Analyze private data and public methods

   The balance is private, so it cannot be changed directly from outside the class.

2. Step 2: Understand controlled access via methods

   The deposit method allows adding to balance only if amount is positive, preventing invalid changes.

3. Final Answer:

   Balance can be changed only through deposit, preventing invalid values -> Option D

4. Quick Check:

   Private data + public methods = controlled access [OK]

Hint: Use private data with public methods for safe updates [OK]

Common Mistakes:

• Thinking balance is public and freely changeable
• Assuming no access to balance at all
• Believing balance cannot be updated