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().