Access control in inheritance in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class B that inherits publicly from class A.

C++

class B : [1] A {};

Drag options to blanks, or click blank then click option'

Aprotected

Bprivate

Cpublic

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using private or protected inheritance changes access levels unexpectedly.

Using public inheritance means all public members of A remain public in B.

2fill in blank

medium

Complete the code to make class B inherit privately from class A.

C++

class B : [1] A {};

Drag options to blanks, or click blank then click option'

Aprivate

Bpublic

Cprotected

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing private inheritance with private members.

Private inheritance means all public and protected members of A become private in B.

3fill in blank

hard

Fix the error in the code to allow class B to access protected members of class A.

C++

class A {
protected:
    int x;
};

class B : [1] A {
public:
    void setX(int val) { A::x = val; }
};

Drag options to blanks, or click blank then click option'

Apublic

Bprivate

Cprotected

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using private inheritance blocks access to protected members.

Public inheritance allows B to access protected members of A.

4fill in blank

hard

Fill both blanks to declare class B that inherits protectedly from class A and class C that inherits publicly from B.

C++

class B : [1] A {};
class C : [2] B {};

Drag options to blanks, or click blank then click option'

Aprotected

Bprivate

Cpublic

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up protected and public inheritance keywords.

Class B inherits protectedly from A, so A's public and protected members become protected in B.
Class C inherits publicly from B, so B's protected members remain protected in C.

5fill in blank

hard

Fill all three blanks to create a class D that inherits publicly from B and privately from C, where B inherits publicly from A and C inherits protectedly from A.

C++

class B : [1] A {};
class C : [2] A {};
class D : [3] B, private C {};

Drag options to blanks, or click blank then click option'

Apublic

Bprotected

Cprivate

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing inheritance access specifiers and their effects on member visibility.

B inherits publicly from A, so A's public members stay public in B.
C inherits protectedly from A, so A's public and protected members become protected in C.
D inherits publicly from B, so B's public members stay public in D, and privately from C, so C's members become private in D.

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