Access control in inheritance in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The three main access specifiers are <strong>public</strong>, <strong>protected</strong>, and <strong>private</strong>. They control how base class members are accessible in derived classes.

beginner

How does <code>public</code> inheritance affect access to base class members?

With <code>public</code> inheritance, <code>public</code> members of the base class remain <code>public</code> in the derived class, and <code>protected</code> members remain <code>protected</code>. <code>private</code> members are not accessible directly.

intermediate

What happens to base class members when inherited with <code>protected</code> access?

When inherited as <code>protected</code>, both <code>public</code> and <code>protected</code> members of the base class become <code>protected</code> in the derived class. <code>private</code> members remain inaccessible.

intermediate

Explain the effect of <code>private</code> inheritance on base class members.

With <code>private</code> inheritance, all <code>public</code> and <code>protected</code> members of the base class become <code>private</code> in the derived class. <code>private</code> members remain inaccessible.

beginner

Can a derived class access the <code>private</code> members of its base class directly?

No, <code>private</code> members of the base class are not accessible directly by the derived class, regardless of the type of inheritance.

In public inheritance, what access level do public base class members have in the derived class?

A public

B protected

C private

D inaccessible

In public inheritance, public members stay public in the derived class.

Which inheritance type makes all inherited base class members private in the derived class?

A public

B protected

C private

D friend

Private inheritance converts all inherited public and protected members to private.

Can a derived class access the private members of its base class directly?

A Yes, always

B No, never

C Only with public inheritance

D Only with protected inheritance

Private members are never accessible directly by derived classes.

In protected inheritance, what happens to public members of the base class?

A They become inaccessible

B They remain public

C They become private

D They become protected

Public members become protected in protected inheritance.

Which access specifier allows derived classes to access base class members but prevents outside access?

A protected

B friend

C private

D public

Protected members are accessible to derived classes but not outside code.

Describe how public, protected, and private inheritance affect the accessibility of base class members in a derived class.

Think about how each inheritance type changes the access level of base class members.

Explain why private members of a base class are not accessible in derived classes, regardless of inheritance type.

Consider the purpose of private access in object-oriented design.

Practice

1. In C++, if a class Derived inherits from a class Base using public inheritance, what happens to the public members of Base in Derived?

easy

A. They are not accessible in Derived.

B. They remain public in Derived.

C. They become private in Derived.

D. They become protected in Derived.

Solution

1. Step 1: Understand public inheritance effect

   In public inheritance, the access levels of the base class members remain the same in the derived class.

2. Step 2: Apply to public members

   Public members of the base class stay public in the derived class, so they can be accessed from outside through derived objects.

3. Final Answer:

   They remain public in Derived. -> Option B

4. Quick Check:

   Public members stay public with public inheritance [OK]

Hint: Public inheritance keeps public members public [OK]

Common Mistakes:

• Thinking public members become protected or private
• Confusing inheritance types
• Assuming private members become accessible

2. Which of the following is the correct syntax for declaring a class Derived that inherits privately from a class Base in C++?

easy

A. class Derived : private Base {};

B. class Derived : public Base {};

C. class Derived : protected Base {};

D. class Derived private Base {};

Solution

1. Step 1: Recall private inheritance syntax

   Private inheritance is declared using the keyword private before the base class name.

2. Step 2: Match syntax options

   class Derived : private Base {}; uses private correctly: class Derived : private Base {};

3. Final Answer:

   class Derived : private Base {}; -> Option A

4. Quick Check:

   Private inheritance uses 'private' keyword [OK]

Hint: Use 'private' keyword for private inheritance [OK]

Common Mistakes:

• Omitting inheritance access specifier
• Using 'public' or 'protected' instead of 'private'
• Confusing default inheritance for classes

3. Consider the following code:

class Base {
public:
    int x;
protected:
    int y;
private:
    int z;
};

class Derived : protected Base {
public:
    void func() {
        x = 1;
        y = 2;
        // z = 3; // Line A
    }
};

int main() {
    Derived d;
    // d.x = 5; // Line B
    return 0;
}

Which statements are true?

medium

A. Line A causes a compile error; Line B causes a compile error.

B. Line A is fine; Line B causes a compile error.

C. Line A causes a compile error; Line B is fine.

D. Both Line A and Line B are fine.

Solution

1. Step 1: Analyze access inside Derived::func()

   Private member z of Base is not accessible in Derived, so Line A causes a compile error.

2. Step 2: Analyze access from main()

   Derived inherits Base protectedly, so Base's public x becomes protected in Derived. Hence, d.x is inaccessible outside Derived, so Line B causes a compile error.

3. Final Answer:

   Line A causes a compile error; Line B causes a compile error. -> Option A

4. Quick Check:

   Private hidden; protected inheritance hides public as protected [OK]

Hint: Private members always inaccessible; protected inheritance hides public [OK]

Common Mistakes:

• Assuming private members accessible in derived
• Thinking public members stay public with protected inheritance
• Ignoring access from outside derived class

4. Given the code below, identify the error and fix it:

class Base {
private:
    int secret;
public:
    Base() : secret(42) {}
};

class Derived : public Base {
public:
    void reveal() {
        std::cout << secret << std::endl;
    }
};

medium

A. No error; code works fine.

B. Error: 'secret' is private in Base; fix by inheriting Base privately.

C. Error: 'secret' is private in Base; fix by using virtual inheritance.

D. Error: 'secret' is private in Base; fix by making 'secret' protected.

Solution

1. Step 1: Identify access error

   Member secret is private in Base, so Derived cannot access it directly in reveal().

2. Step 2: Fix access by changing access specifier

   Changing secret to protected allows Derived to access it directly.

3. Final Answer:

   Error: 'secret' is private in Base; fix by making 'secret' protected. -> Option D

4. Quick Check:

   Private members hidden; protected allows derived access [OK]

Hint: Private members inaccessible; use protected for derived access [OK]

Common Mistakes:

• Trying to access private members directly
• Using friend function incorrectly
• Changing inheritance type unnecessarily

5. You have a base class Base with a public member int a; and a protected member int b;. You want to create a derived class Derived that hides a from outside users but allows access to b inside Derived and its subclasses. Which inheritance type should you use?

hard

A. Public inheritance

B. Private inheritance

C. Protected inheritance

D. No inheritance; copy members manually

Solution

1. Step 1: Understand access changes with inheritance types

   Public inheritance keeps a public, so it remains accessible outside. Private inheritance hides a but also hides b from further derived classes.

2. Step 2: Choose inheritance that hides a but keeps b accessible in Derived and subclasses

   Protected inheritance makes public members protected (hiding a from outside) and keeps protected members protected (allowing access to b in Derived and subclasses).

3. Final Answer:

   Protected inheritance -> Option C

4. Quick Check:

   Protected inheritance hides public, keeps protected accessible [OK]

Hint: Protected inheritance hides public, keeps protected accessible [OK]

Common Mistakes:

• Choosing public inheritance which keeps public members public
• Choosing private inheritance which hides protected members too
• Thinking manual copying is needed