Access specifiers in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the three main access specifiers in C++?

The three main access specifiers in C++ are <strong>public</strong>, <strong>private</strong>, and <strong>protected</strong>. They control the visibility of class members.

beginner

What does the private access specifier do?

Members declared private can only be accessed within the same class. They are hidden from outside code and derived classes.

intermediate

How does protected differ from private?

<code>protected</code> members are like <code>private</code> but can also be accessed by derived (child) classes, not just the class itself.

beginner

What is the default access specifier for class members in C++?

By default, members of a class are private if no access specifier is given.

beginner

What is the default access specifier for struct members in C++?

By default, members of a struct are public if no access specifier is given.

Which access specifier allows members to be accessed from anywhere?

A public

B private

C protected

D internal

public members can be accessed from any part of the program.

If no access specifier is given in a class, what is the default?

A public

B private

C protected

D friend

Class members are private by default.

Which access specifier allows derived classes to access members but hides them from outside?

A public

B static

C private

D protected

protected members are accessible in derived classes but not outside.

What is the default access specifier for members of a struct?

A public

B protected

C private

D none

Struct members are public by default.

Can private members be accessed directly by derived classes?

A Only if friend class

B Yes

C No

D Only if public inheritance

Private members are not accessible by derived classes directly.

Explain the difference between public, private, and protected access specifiers in C++.

Think about who can see or use the class members.

What are the default access levels for class and struct members in C++?

Remember the difference between class and struct.

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