Data hiding in C++ - Mini Project: Build & Apply

Data Hiding in C++

📖 Scenario: Imagine you are creating a simple bank account system. You want to keep the account balance safe so that it cannot be changed directly from outside the account. This is called data hiding.

🎯 Goal: You will create a class called BankAccount that hides the balance inside it. You will add a way to check the balance safely and a way to add money to the account.

📋 What You'll Learn

Create a class called BankAccount with a private variable balance of type double.

Add a public method getBalance() that returns the current balance.

Add a public method deposit(double amount) that adds money to the balance.

Use data hiding to prevent direct access to balance from outside the class.

💡 Why This Matters

🌍 Real World

Data hiding is used in real bank systems to protect account balances from accidental or unauthorized changes.

💼 Career

Understanding data hiding is important for writing secure and reliable code in software development jobs.

1

Create the BankAccount class with a private balance

Create a class called BankAccount with a private variable balance of type double initialized to 0.0.

C++

class BankAccount {
private:
    double balance = 0.0;  // Your code here
};

Hint

Use the private: keyword to hide the balance variable inside the class.

2

Add a public method to get the balance

Add a public method called getBalance() that returns the value of balance. Use public: to make the method accessible outside the class.

C++

class BankAccount {
private:
    double balance = 0.0;
public:
    double getBalance() {
        // Your code here
    }
};

Hint

The getBalance() method should return the private balance variable.

3

Add a public method to deposit money

Add a public method called deposit that takes a double amount parameter and adds it to balance.

C++

class BankAccount {
private:
    double balance = 0.0;
public:
    double getBalance() {
        return balance;
    }
    void deposit(double amount) {
        // Your code here
    }
};

Hint

The deposit method should add the amount to the private balance.

4

Create an object and test the methods

Create an object called myAccount of class BankAccount. Use deposit(100.0) to add money and then print the balance using getBalance().

C++

class BankAccount {
private:
    double balance = 0.0;
public:
    double getBalance() {
        return balance;
    }
    void deposit(double amount) {
        balance += amount;
    }
};

int main() {
    // Your code here
}

Hint

Use std::cout to print the balance returned by getBalance().

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