Access specifiers in C++

Introduction

Access specifiers control who can see and use parts of your code. They help keep things safe and organized.

When you want to hide details inside a class so others can't change them by mistake.

When you want to allow some parts of your program to use certain data but keep others away.

When you want to protect important information inside your code from being changed directly.

When you want to organize your code clearly by showing which parts are for inside use and which are for outside use.

Syntax

C++

class ClassName {
public:
  // accessible from anywhere
protected:
  // accessible in this class and derived classes
private:
  // accessible only in this class
};

public: means anyone can use these parts.

protected: means only this class and its children can use these parts.

private: means only this class can use these parts.

Examples

Here, length is public, so anyone can access it.

C++

class Box {
public:
  int length;
};

Here, width is private, so only Box can use it.

C++

class Box {
private:
  int width;
};

Here, height is protected, so Box and its children can use it.

C++

class Box {
protected:
  int height;
};

Sample Program

This program shows how speed is private and cannot be changed directly. Instead, we use public functions to set and show the speed.

C++

#include <iostream>
using namespace std;

class Car {
private:
  int speed;

public:
  void setSpeed(int s) {
    speed = s;
  }
  void showSpeed() {
    cout << "Speed: " << speed << " km/h" << endl;
  }
};

int main() {
  Car myCar;
  // myCar.speed = 100; // Error: speed is private
  myCar.setSpeed(100);
  myCar.showSpeed();
  return 0;
}

Important Notes

Private members cannot be accessed outside the class directly.

Protected members are useful when you want child classes to access some data but keep it hidden from others.

Public members are open to everyone, so use them carefully.

Summary

Access specifiers control who can use parts of a class.

Use private to hide details inside a class.

Use public to allow outside access.

Use protected to allow access in child classes.

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