Access specifiers in C++ - Mini Project: Build & Apply

Access Specifiers in C++

📖 Scenario: You are creating a simple program to understand how different access specifiers work in C++ classes. Access specifiers control which parts of your program can see and use the data inside a class.

🎯 Goal: Build a C++ class with public, private, and protected members, then write code to access these members and see which are accessible.

📋 What You'll Learn

Create a class named Box with three member variables: length (public), width (private), and height (protected).

Add a public member function setWidth to set the private width.

Add a public member function getWidth to get the private width.

Create an object of Box in main and access the length directly.

Use the public functions to set and get the width.

Try to access height directly from main and observe the error (commented out).

💡 Why This Matters

🌍 Real World

Access specifiers help protect important data inside classes and control how other parts of a program can use that data.

💼 Career

Understanding access specifiers is essential for writing safe and maintainable C++ code in software development jobs.

1

Create the Box class with public, private, and protected members

Create a class called Box with three member variables: length as public, width as private, and height as protected. Set their types to int.

C++

class Box {
    // Your code here
};

Hint

Use public:, private:, and protected: labels inside the class to set access levels.

2

Add public functions to set and get the private width

Inside the Box class, add a public member function setWidth that takes an int parameter and sets the private width. Also add a public member function getWidth that returns the private width.

C++

class Box {
public:
    int length;

    // Add setWidth and getWidth functions here

private:
    int width;
protected:
    int height;
};

Hint

Remember, public functions can access private members inside the class.

3

Create a Box object and set length and width

In the main function, create an object called box of type Box. Set box.length to 10 directly. Use box.setWidth(5) to set the width.

C++

int main() {
    // Create Box object and set length and width
    
    // Your code here
    return 0;
}

Hint

Objects access public members directly and private members via public functions.

4

Print length and width, and try to access height

In main, print box.length and box.getWidth(). Also add a commented line trying to access box.height directly (this should cause an error if uncommented).

C++

int main() {
    Box box;
    box.length = 10;
    box.setWidth(5);
    
    // Print length and width
    
    // Try to access height (commented out)
    
    // Your code here
    return 0;
}

Hint

Use cout to print values. Access height directly will cause an error, so keep it commented.

Practice

1. Which access specifier allows members of a class to be accessed from anywhere in the program?

easy

A. protected

B. private

C. public

D. internal

Solution

1. Step 1: Understand access specifiers

   Public members are accessible from any part of the program, unlike private or protected.

2. Step 2: Compare with other specifiers

   Private restricts access to inside the class only; protected allows access in derived classes.

3. Final Answer:

   public -> Option C

4. Quick Check:

   Access anywhere = public [OK]

Hint: Public means accessible everywhere, private means hidden [OK]

Common Mistakes:

• Confusing private with public
• Thinking protected is accessible everywhere
• Assuming internal is a C++ specifier

2. Which of the following is the correct way to declare a private member variable int age inside a class?

easy

A. private int age;

B. private:
int age;

C. int age private;

D. int private age;

Solution

1. Step 1: Recall C++ syntax for access specifiers

   Access specifiers are labels followed by a colon, then members are declared below.

2. Step 2: Identify correct syntax

   private: int age; uses 'private:' followed by 'int age;' which is correct syntax.

3. Final Answer:

   private: int age; -> Option B

4. Quick Check:

   Access specifier label + colon = correct [OK]

Hint: Access specifiers end with colon, members declared below [OK]

Common Mistakes:

• Placing access specifier before type
• Missing colon after access specifier
• Trying to put access specifier after variable

3. What will be the output of this code?

class Test {
private:
    int x = 5;
public:
    int getX() { return x; }
};

int main() {
    Test t;
    // std::cout << t.x << std::endl; // Line A
    std::cout << t.getX() << std::endl;
    return 0;
}

medium

A. 5

B. Compilation error at Line A

C. 0

D. Runtime error

Solution

1. Step 1: Understand private member access

   The variable x is private, so direct access like t.x is not allowed (Line A is commented out).

2. Step 2: Use public getter method

   The method getX() is public and returns x's value, so std::cout << t.getX() prints 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Private hidden, public getter shows 5 [OK]

Hint: Private members need public methods to access outside [OK]

Common Mistakes:

• Trying to access private variable directly
• Expecting 0 as default value
• Confusing compilation and runtime errors

4. Identify the error in this code snippet:

class Base {
protected:
    int value = 10;
};

class Derived : public Base {
public:
    void show() {
        std::cout << value << std::endl;
    }
};

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

medium

A. Error: Cannot access protected member 'value' from main using object

B. Error: Cannot access protected member 'value' inside Derived class

C. Error: Missing semicolon after class Derived

D. No error, code runs and prints 10 twice

Solution

1. Step 1: Check access to protected member inside Derived

   Derived class can access protected members of Base, so d.show() works and prints 10.

2. Step 2: Check access to protected member from main

   Main function cannot access protected members directly via object, so std::cout << d.value causes compilation error.

3. Final Answer:

   Error: Cannot access protected member 'value' from main using object -> Option A

4. Quick Check:

   Protected accessible in subclass, not outside [OK]

Hint: Protected members hidden outside class and subclasses [OK]

Common Mistakes:

• Thinking protected is public
• Assuming main can access protected members
• Missing error on direct access in main

5. You want to design a class where some data is hidden from everyone except derived classes, and some functions are accessible everywhere. Which access specifiers should you use for data and functions respectively?

hard

A. Data: private, Functions: public

B. Data: protected, Functions: private

C. Data: public, Functions: protected

D. Data: protected, Functions: public

Solution

1. Step 1: Determine data access for derived classes only

   Protected data is hidden from outside but accessible in derived classes, fitting the requirement.

2. Step 2: Determine function access for everyone

   Functions accessible everywhere must be public.

3. Final Answer:

   Data: protected, Functions: public -> Option D

4. Quick Check:

   Protected data + public functions = correct design [OK]

Hint: Protected hides data, public shares functions [OK]

Common Mistakes:

• Using private data blocks derived classes
• Making functions private hides them outside
• Confusing public and protected roles